A Note on a Revox A700 Repair

We have a Revox A700 reel-to-reel tape recorder in our home. We sort of ended up with it, in a story I won’t tell here.

This thing is rather amazing as a device. It came out at the end of 1973, I think, and was madly ambitious as a consumer product. It had all sorts of fancy things: a quartz-clocked capstan motor, servo-controlled spool motors whose speed is continually adjusted based on feedback from tape tension sensors, an optical tape-present sensor, true VU meters, two custom dedicated integrated circuits (in a consumer tape recorder! in 1973!), one for timing adjustment and one for the control logic. It supports three tape speeds (3.75, 7.5, and 15 inches per second) and has a high-quality audio preamp and headphone amp. Its service manual is over 200 pages long. It weighs a massive 24 kg, draws over 100 watts even when not playing anything, and gets very hot.

(For those unfamiliar, Revox was the consumer brand of the Swiss studio electronics company Studer, named after its founder Willi Studer. Revox tape recorders were similar internally to Studers but with more limited I/O and track counts. But when the A700 came out it was in some ways more advanced than any Studer, with much of the electronic stuff turning up in studio gear only later.)

When an A700 is working, it can sound fantastic, although using one is a nutty endeavour at the best of times. I very seldom switch ours on, and then only to play tapes of playlists chosen with my kids that have been recorded to tape from Spotify on my phone. It’s quite calming to have those reels slowly rotating in the background as the music plays, but there’s really no other purpose to it now. It is a massive, heavy, beautifully designed, expensively constructed device that can still perform its job perfectly well and yet is totally obsolete.

The A700 also isn’t all that reliable. It’s a bit like a miniature version of having a classic Ferrari. Ours has had a succession of repairs over the last few years: blown suppression caps (a bit of a fire hazard, replace all the RIFA capacitors), blown motor start/run capacitor (symptom: take-up spool rotates gently backwards instead of insistently forwards: the spool motors are AC motors and rely on big capacitors to achieve momentum), blown something-I-didn’t-understand-and-have-now-forgotten on one of the control boards. Fortunately we haven’t yet had a blown integrated circuit. The failures we’ve had have all been in generic parts, not unobtainable custom ones. Although there are still rather a lot of generic parts in this machine.

In the past when something has broken on this machine I’ve called out a professional to look at it, so it has been expensive as well as impractical. But with the coronavirus going on, when our Revox stopped in the middle of a tape, lit up all the buttons, and refused to start again, it didn’t feel like a good time to be trying to call out an experienced senior repair fellow. And yet, with the coronavirus going on and our horizons shrinking fast, I suddenly really wanted the old Revox not to have broken down. And so these six paragraphs of introduction lead to the much briefer story:

Fuse F6

This is where the magic of the old-school forum-based pre-social-network World Wide Web comes in. I open the back of the Revox and find that fuse F6 on the panel next to the distribution board has blown. I search for “revox a700 fuse f6 blown” and I find this magnificent post:

“This is a blown capacitor 2200µF (FRAKO), C24 plus blown rectifier, D6 on Board.1.067.160 or 161.

“Replace all 4 2200µF (FRAKO capacitors) on this board plus rectifier D6 (by a stronger one e.g. B100C1000).

“Regards, Mart.”

So specific! The capacitor and rectifier numbers don’t actually match up with those on our board, but with the help of a service manual I can see which ones they’re referring to.

This is the power supply board from the A700. The Frako capacitors are, apparently, famous for failing in short-circuit and blowing up other things on the board. This one seems to have only blown up the adjacent rectifier.

I don’t much enjoy soldering, but I de-soldered and tested the four big capacitors and found that the second one (the smallest, a 2200µF 16V cap) had failed. I ordered some replacement caps and rectifiers (the rectifiers are the round black things beneath the big capacitors: the green rectangular chunk is also a rectifier, but I hoped I wouldn’t have to deal with that). I also got replacements for the three smaller Frako capacitors on the board, which unfortunately I could only get with radial wiring: two wires into the bottom of the capacitor rather than one at each end.

When the replacements arrived, I soldered in the new capacitors, but I hesitated over the rectifiers. I’d tested the three round ones with the diode tester on my multimeter and they all showed the right forward voltage. I would really prefer not to have to change those, as they’re quite fiddly. So I put the board back in, slotted in a new fuse, turned on the power… and the fuse blew again.

Gritting my teeth, I desoldered and replaced the three round rectifiers, something that took me longer than I would like to admit, then put the board back and tried again, and it works!

The original 2200µF capacitors are rated at 40V, 16V, and 2x 25V respectively. I ordered replacements at similar ratings, and only afterwards realised I could have bought and fitted four higher-rated caps for 40V or above (they’re smaller nowadays anyway) and they would have fitted fine. The higher voltage-rated ones also have better temperature tolerance, with a 105 degree max instead of 85. But I’m new to this kind of thing.

Anyway: that’s all. I really just wanted to record my thanks to Mart, the author of the reply I quoted above, and also to Kurt, who asked the question. This exchange is from 2014, which is recent enough to give hope that the searchable, stable, forum-style Web might still be alive somewhere.

Notes on the Minolta SR-1 (Model B)

Minolta SR-1 model A (left) and B (right) from the back

A few months ago I published an epic about the first version of the Minolta SR-1 camera: Notes on the Minolta SR-1 (Model A). Released in 1959, that was the first in a series of 35mm film SLRs that Minolta branded the SR-1, a line that continued until 1971. This post will be about the model B: pointing out some differences from the model A within the camera, then some remarks about replacing the mirror and the leather cover.

The revisions have unofficial names that various sources seem to accept:

  • Model A (1959): a cut-down version of the SR-2 from 1958, with the fastest shutter speed (1/1000 sec) removed
  • Model B (1960): made the speed selection dial simpler to use: with a model A you have to lift the dial to turn it, but with a model B you can just flick it around
  • Model C (1961): introduced a fully-automatic aperture, so that the aperture returned to fully-open (ready for focusing) immediately after taking the shot, rather than only when the film was wound on
  • Model D (1962): added a mount for a clip-on light meter on the front of the camera
  • Model E (1963): moved the film counter from left to right of the top of the body (as seen from behind), restyled the advance lever, and introduced a larger mirror
  • Model V (1965): redesigned the body to a squarer shape, and made various minor functional changes. Unlike these other model letters, the Model V designation was official
  • SR-1s (1967): reintroduced the 1/1000 sec fastest shutter speed

Although these look like incremental updates, it turns out that quite a bit changed within the camera from one model to the next.

I think the logic is: The SR-1 was always the label for Minolta’s “entry-level” 35mm SLR, and it was always a cut-down version of some “premium” model. But as they kept releasing new premium models, they also kept updating what the SR-1 was a cut-down version of, so that they could use the same manufacturing process for both cameras.

  • Model A (1959): cut-down version of the SR-2 (1958)
  • Model B (1960): cut-down version of the SR-3 (1960), which added a light meter attachment on the front (not yet included in the SR-1)
  • Model C (1961): cut-down version of the SR-3 (1961), which was revised that year to introduce the fully-automatic aperture (the SR-1 has that feature but still omits the light meter attachment)
  • Model D (1962): inherits the light meter attachment from the SR-3, effectively replacing that model; the SR-3 is dropped, and the premium model becomes the new SR-7 with built-in light meter
  • Model E (1963): cut-down version of the SR-7 (1962), omitting the built-in light meter and using the attachment instead
  • Model V (1965): cut-down version of the SR-7 Model V (1965), omitting the built-in light meter and using the attachment instead
  • SR-1s (1967): cut-down version of the SR-7 Model V; finally has the same feature set as 1961’s high-end SR-3

So the changes on the inside reflected Minolta’s current camera design even when they made little difference to the external working of the camera.

Anyway, here’s my survey. The same disclaimer as before applies: I don’t know what I’m doing, just learning as I go, so please don’t follow anything I say and ruin your own camera. Once again, I bought two examples of this camera (both non-working this time) and dug in.

Survey of features

Refer to my earlier article for a description of the SR-1 model A. I can only see three external changes in the model B.

Model A (above) and B (below)

The most significant one is the speed selection dial, which is slightly smaller, turns without being lifted first (and does so with a very satisfying click), and lists the speeds in the opposite order, which I believe means switching from the same order as used by Nikon to that of Pentax and Canon.

Apart from that, the viewfinder now has a bayonet attachment fitting on it, and has changed from black to silver; and the tripod mount on the base of the camera is a little thicker and stronger-looking. Oh, and one of the screws on the bottom has moved a few millimetres.

There is one other change that’s only visible when the camera doesn’t have a lens attached: the aperture lever within the lens mount now has a curved cutout and travels along a circular section rather than in a straight line.

The camera has the same dimensions as its predecessor and is about 10g lighter, a negligible difference.

What’s under the top cover

So the outside is almost unchanged, with only a few near-cosmetic updates. On the inside, the first surprise comes when the top cover is lifted:

Light meter attachment block, on the inside of the camera only

The attachment block for the external light meter, a feature that wouldn’t appear in the SR-1 until two years later, is already there! It’s on the left, beneath the SR-1 wording. It even has a corner shaved off so that the original SR-1 cover still fits over it.

The rest of the top is more familiar. The rear cover release, rewind knob, and exposure counter on the left are all unchanged. The winding gear and base plate on the right are essentially the same. Sadly the shutter button plate, the bit that actually gets pressed down by the shutter button and that was so convenient to be able to access when testing, has gone into hiding behind the light meter mount.

(I haven’t dug in under the winding base plate, but I suspect they may have removed the cutout and pin in one of the gears, which prevented double exposures by holding up the shutter button until winding is complete, since there is now a mechanism for this in the base of the camera.)

The speed dial of course has been redesigned, but it uses the same arrangement of levers, and the slow-speed gear (both the driving axes and the gear itself, in the bottom of the camera) is the same.

There are a couple of nice improvements around the viewfinder and prism. The first is that the prism no longer has a foam seal glued directly onto its silvering, so it doesn’t suffer from the corrosion that afflicted both of my model A cameras.

The second is the addition of three height-and-level adjustment screws for the focusing screen beneath the prism. One of these can be seen in the picture below, just in front of the viewfinder eyepiece mount. (The others are at left and right of the front edge of the box.)

These screws allow correction of any focus mismatch between the viewfinder and film plane, by slightly raising or lowering the viewfinder focusing screen so that the distance from there, via the mirror, to the rear of the lens matches that from the film plane.

Although this seems nice to have, an awkward corollary is that both of my model B cameras arrived with the viewfinder focus slightly out (when checked using a ground glass placed in the film plane), whereas the model A cameras were both spot on. However, it is extremely handy if you ever need to replace the mirror, since a replacement might not be of identical thickness.

What’s under the bottom cover

This is where it gets peculiar. The only major user-visible change was the design of the speed dial, and the mechanism for that is on the top of the camera, so we’ve seen it already. What can have changed down here?

SR-1 model B (above) and A (below)

Almost everything! The model A (the one at the bottom, with SR 2 stamped on its base plate) is all linear motions, sliders and fairly simple levers driven from a single big rotary wheel on the left. The model B (above, with SR C on its base plate) is all exotic custom pivoted forms; everything rotates instead of sliding. Correspondingly, there is far more use made of angular wire springs rather than wound linear ones. The shutter springs and their adjusters are unchanged, and so is the motor drive attachment on the right (which is useless, as no motor drive was ever released) but I think every other part is different.

This really boggled my mind when I saw it. In software development terms, I had expected this camera to be a “consolidation release” of bugfixes and maintenance improvements. Instead it’s been rewritten in Javascript.

There are some peculiar decisions here, as well. To use another software term, there are several violations of the principle of separation of concerns:

  • See that screw beneath the bottom of the tripod mount, the one with a wire spring wrapped around it? Well, the only thing keeping the spring wrapped around it is… the tripod mount itself. Remove the tripod mount and the spring just pops off. If you remove the tripod mount for access to something else and then fire the shutter, the spring flies across the room. It’s an important spring as well, as it drives the aperture lever.
  • See the flat copper spring attached to the base plate under the top of the tripod mount? That latches the aperture open. The only thing holding it on to the base of the camera is one of the screws designed to attach the slow-speed gear, on the other side of the base, and the receiving thread for that screw is in the slow-speed gear block. Remove the slow-speed gear, for cleaning say, and you no longer have any way to hold this spring in place and can no longer latch the aperture open. And if you try to fire the shutter, the other spring flies across the room again, because you also had to remove the tripod mount to get at the slow-speed gear fixing screws.

Surely these should not have passed code review.

The angled wire springs can sometimes end up in the wrong place, as well. An example is just about visible in the picture above: left of the tripod mount, the end of a small wire spring pokes up next to the shutter trip lever. That spring is erroneously pulling the shutter trip lever downwards (in the plane of this picture), when it’s supposed to be sprung upwards. With the spring in this position the shutter doesn’t wait for the mirror to open before it fires, so your photo gets cut off by the mirror. The fix is to poke the spring back under the lever, but how did it get here in the first place, and could it happen again?

The camera that had that problem also had a mis-adjustment of its new double-exposure prevention mechanism. This is a little combination of levers and plates over on the top left of the picture above, next to where the shutter axis pokes out of the bottom plate. A sprung plate fits into a notch in the shutter axis; the winding action pushes it aside, so the shutter can be pressed, and when the shutter fires it springs back again. The mechanism has an adjustable eccentric, and if its adjustment screw is wrongly set, it doesn’t push aside far enough, preventing the shutter from being pressed at all. (When I bought this camera, it was described as not working because of a stuck shutter button.)

So even though the overall design is more elegant, these changes don’t seem to be all good. A couple of outcomes are more promising: the winding lever is a bit smoother to turn because it doesn’t have to push these long-travel linear sprung sliders, and the flash sync contact mechanism looks simpler than that in the model A, although I haven’t so much as tested this feature on any of these cameras.

If I had to guess why these changes were made (and I do have to guess: I have no idea) I’d say it’s because Minolta were looking ahead to the following year’s revision of the SR-3 and the corresponding SR-1 model C, with their fully-automatic aperture, a mechanism that presumably could never be driven from the model A’s linear sliders. But I don’t own a model C.

Replacing a mirror

One of these cameras arrived with a very tarnished mirror:

It looks particularly bad in this photo because I’m focusing on the mirror surface, which of course is not in focus when you actually use the viewfinder. In practice the mirror was still just about usable, if cloudy.

On the off-chance that the silvering was just marked, or at least that some of the marks were just marks and not actual damage, I tried a series of increasingly aggressive cleaning solutions: water, alcohol, acetone, silver cleaner. Surprisingly, none of them made it any worse, but none of them helped either. I decided to replace the mirror.

SLR mirrors are silvered on the front, unlike a typical mirror, which has the silver on the back with a coat of paint behind it and the glass in front to protect it. Front silvering avoids a ghost image from light reflecting off the front of the glass, such as you’ll see if you look into your bathroom mirror at 45 degrees from the perpendicular.

SLR mirrors come in all sorts of sizes, so you can’t easily buy a matching replacement (this one is 34x25mm, with 3mm or so snipped off the front corners; newer cameras have larger mirrors). They’re also thin (this one is 1.37mm) and the thickness matters when focusing, although this camera’s adjustable focusing screen means we only need it to be “almost” right.

I found a supplier of individual small front-silvered optical mirrors (Knight Optical), though I would have to cut one down to size. But I also read in a forum somewhere that you can make a front-silvered mirror from the mirror in a cheap make-up compact, by flipping it around and using solvent to remove the paint from the back and reveal the silvered surface. That sounded intriguing.

I bought a glass-cutting tool and a couple of double-sided make-up mirrors from Tiger for £1 each to experiment with. These are made of two thin (1.2mm) round mirrors back-to-back, with a cardboard spacer in between, in a metal frame. One of the mirrors is magnifying, so no use to us, but the other is flat.

The mirrors are backed by white paint, which doesn’t dissolve in paint thinner but does in acetone. Soaking and gentle rubbing with cotton wool took a while, but worked, and left a pleasingly shiny silvered (well, aluminised) surface. The picture above shows one of the magnifying mirrors, which I used to test the solvent, along with two of my attempts at cutting out the right shape from the non-magnifying mirror. I measured, marked, cut, and sanded these mirrors (use protective eyewear and a dust mask! … if you can find one) before removing the paint.

The old mirror in the camera is stuck down onto its flap with three points of glue, whose positions are helpfully marked by raised spots on the back of the flap. Prising the mirror gently away with a small screwdriver was enough to separate it from the flap.

Below is the old mirror, on the left, next to the new, on the right, reflecting a lamp. This time I’ve focused on the lamp rather than the mirror surface. You can see that the old one, though cloudy, works a lot better than you might expect, but the new one is still much clearer.

You can take a perfectly good picture with a cloudy viewfinder, and a scratch or mark on your SLR mirror certainly isn’t the end of the world. It mostly just looks bad on the camera when you look at it with the lens off.

I attached the new mirror at the same gluing points as the old, re-fitted it, then adjusted the viewfinder focus, and the result is really very nice. I’ll be interested to see how, if at all, it weathers the years.

Replacing the leather cover

Replacing the mirror wasn’t a complete success. I seriously messed up part of this adventure: I ripped the leather cover beyond repair when opening the front to get to the mirror box. The leather on this camera is the same as that on the model A, that is, thin and easily torn. In my post about the model A I said that the texture on the leather was good at hiding small tears: that’s true, but there are limits.

The leather on this camera was particularly well stuck down, and I soon discovered that, although you can loosen the glue with a squirt of isopropyl alcohol, that also softens the leather and makes it much more liable to tear.

So as well as replacing the mirror, I set out to replace the leather cover. I already had a sheet of pre-glued “lizard ocher” leather from Aki-Asahi, which I had ordered, just in case, at the same time as the fabric I used to replace the model A shutter curtain. Aki-Asahi sell a lot of pre-cut leathers for particular cameras, but they don’t have one for the SR-1.

I measured up and made a template for the SR-1 covers, both front and back, then cut them out with a Stanley knife. An SVG file of the template can be found here. Loaded into Inkscape and printed with a properly configured printer, it should come out with the correct dimensions. The hardest part by far is cutting the circular hole on the back for the film speed reminder dial.

I don’t entirely approve of putting an inauthentic new cover on a fine old camera like this, and it’s a bit on the cheesy side as well, but I have to confess I quite like the result anyway. Since I had already replaced the mirror in this one with a home-made version, I probably have to think of it as a somewhat personalised object already.

This camera also had a non-working slow-speed gear, which just needed cleaning, and a loose mirror box and viewfinder, due to having somehow lost one of the screws attaching the mirror box to the bottom plate. With these fixed, it’s working again, as is the other camera which came to me with a stuck shutter. In both cases the faster shutter speeds still need a bit of adjustment, and I haven’t yet run any film through them.

On macOS “notarization”

I’ve spent altogether too long, at various moments in the past year or so, trying to understand the code-signing, runtime entitlements, and “notarization” requirements that are now involved when packaging software for Apple macOS 10.15 Catalina. (I put notarization in quotes because it doesn’t carry the word’s general meaning; it appears to be an Apple coinage.)

In particular I’ve had difficulty understanding how one should package plugins — shared libraries that are distributed separately from their host application, possibly by different authors, and that are loaded from a general library path on disc rather than from within the host application’s bundle. In my case I’m dealing mostly with Vamp plugins, and the main host for them is Sonic Visualiser, or technically, its Piper helper program.

Catalina requires that applications (outside of the App Store, which I’m not considering here) be notarized before it will allow ordinary users to run them, but a notarized host application can’t always load a non-notarized plugin, the tools typically used to notarize applications don’t work for individual plugin binaries, and documentation relating to plugins has been slow in appearing. Complicating matters is the fact that notarization requirements are suspended for binaries built or downloaded before a certain date, so a host will often load old plugins but refuse new ones. As a non-native Apple developer, I find this situation… trying.

Anyway, this week I realised I had some misconceptions about how notarization actually worked, and once those were cleared up, the rest became obvious. Or obvious-ish.

(Everything here has been covered in other places before now, e.g. Apple docs, KVRaudio, Glyphs plugin documentation. But I want to write this as a conceptual note anyway.)

What notarization does

Here’s what happens when you notarize something:

  • Your computer sends a pack of executable binaries off to Apple’s servers. This may be an application bundle, or just a zip file with binaries in it.
  • Apple’s servers unpack it and pick out all of the binaries (executables, libraries etc) it contains. They scan them individually for malware and for each one (assuming it is clean) they file a cryptographic hash of the binary alongside a flag saying “yeah, nice” in a database somewhere, before returning a success code to you.

Later, when someone else wants to run your application bundle or load your plugin or whatever:

  • The user’s computer calculates locally the same cryptographic hashes of the binaries involved, then contacts Apple’s servers to ask “are these all right?”
  • If the server’s database has a record of the hashes and says they’re clean, the server returns “aye” and everything goes ahead. If not, the user gets an error dialog (blah cannot be opened) and the action is rejected.

Simple. But I found it hard to see what was going on, partly because the documentation mostly refers to processes and tools rather than principles, and partly because there are so many other complicating factors to do with code-signing, identity, authentication, developer IDs, runtimes, and packaging — I’ll survey those in a moment.

For me, though, the moment of truth came when I realised that none of the above has anything to do with the release flow of your software.

The documentation describes it as an ordered process: sign, then notarize, then publish. There are good reasons for that. The main one is that there is an optional step (the “stapler”) that re-signs your package between notarization and publication, so that users’ computers can skip ahead and know that it’s OK without having to contact Apple at all. But the only critical requirement is that Apple’s servers know about your binary before your users ask to run it. You could, in fact, package your software, release the package, then notarize it afterwards, and (assuming it passes the notarization checks) it should work just the same.

Notarizing plugins

A plugin (in this context) is just a single shared library, a single binary file that gets copied into some folder beneath $HOME/Library and loaded by the host application from there.

None of the notarization tools can handle individual binary files directly, so for a while I thought it wasn’t possible to notarize plugins at all. But that is just a limitation of the client tools: if you can get the binary to the server, the server will handle it the same as any other binary. And the client tools do support zip files, so first sign your plugin binary, and then:

$ zip blah.zip myplugin.dylib
adding: myplugin.dylib (deflated 65%)
$ xcrun altool --notarize-app -f blah.zip --primary-bundle-id org.example.myplugin -u 'my@appleid.example.org' -p @keychain:altool
No errors uploading 'blah.zip'.

(See the Apple docs for an explanation of the authentication arguments here.)

[Edit, 2020-02-17: John Daniel chides me for using the “zip” utility, pointing out that Apple recommend against it because of its poor handling of file metadata. Use Apple’s own “ditto” utility to create zip files instead.]

Wait for notarization to complete, using the request API to check progress as appropriate, and when it’s finished,

$ spctl -a -v -t install myplugin.dylib
myplugin.dylib: accepted
source=Notarized Developer ID

The above incantation seems to be how you test the notarization status of a single file: pretend it’s an installer (-t install), because once again the client tool doesn’t support this use case even though the service does. Note, though, that it is the dylib that is notarized, not the zip file, which was just a container for transport.

A Glossary of Everything Else

Signing — guaranteeing the integrity of a binary with your identity in a cryptographically secure way. Carried out by the codesign utility. Everything about the contemporary macOS release process, including notarization, expects that your binaries have been signed first, using your Apple Developer ID key.

Developer ID — a code-signing key that you can obtain from Apple once you are a paid-up member of the Apple Developer Program. That costs a hundred US dollars a year. Without it you can’t package programs for other people to run them, except if they disable security measures on their computers first.

Entitlements — annotations you can make when signing a thing, to indicate which permissions, exemptions, or restrictions you would like it to have. Examples include permissions such as audio recording, exemptions such as the JIT exemption for the hardened runtime, or restrictions such as sandboxing (q.v.).

Hardened runtime — an alternative runtime library that includes restrictions on various security-sensitive things. Enabled not by an entitlement, but by providing the --options runtime flag when signing the binary. Works fine for most programs. The documentation suggests that you can’t send a binary for notarization unless it uses the hardened runtime; that doesn’t appear to be true at the moment, but it seems reasonable to use it anyway. Note that a host that uses the hardened runtime needs to have the com.apple.security.cs.disable-library-validation entitlement set if it is to load third-party plugins. (That case appears to have an inelegant failure mode — the host crashes with an untrappable signal 9 following a kernel EXC_BAD_ACCESS exception.)

Stapler — a mechanism for annotating a bundle or package, after notarization, so that users’ computers can tell it has been notarized without having to contact Apple’s servers to ask. Carried out by xcrun stapler. It doesn’t appear (?) to be possible to staple a single plugin binary, only complex organisms like app bundles.

Quarantine — an extended filesystem attribute attached to files that have been downloaded from the internet. Shown by the ls command with the -l@ flags, can be removed with the xattr command. The restrictions on running packaged code (to do with signing, notarization etc) apply only when it is quarantined.

Sandboxing — a far more intrusive change to the way your application is run, that is disabled by default and that has nothing to do with any of the above except to fill up one’s brain with conceptually similar notions. A sandboxed application is one that is prevented from making any filesystem access except as authorised explicitly by the user through certain standard UI mechanisms. Sandboxing is an entitlement, so it does require that the application is signed, but it’s independent of the hardened runtime or notarization. Sandboxing is required for distribution in the App Store.

MIREX 2019 submissions

For the 2019 edition of MIREX, the Music Information Retrieval Evaluation eXchange, we at the Centre for Digital Music once again submitted a set of Vamp audio analysis plugins for evaluation. This is the seventh year in a row in which we’ve done so, and the fourth in which no completely new plugin has been added to the lineup. Although these methods are therefore getting more and more out-of-date, they do provide a potentially useful baseline for other submissions, a sanity check on the evaluation itself, and some historical colour.

Every year I write up the outcomes in a blog post. Like last year, I’m rather late writing this one. That’s partly because the official results page is still lacking a couple of categories, and says “More results are coming” at the top — I’m beginning to think they might not be, and decided not to wait any longer. (MIREX is volunteer-run, so this is just a remark, not a complaint.)

You can find my writeups of past years here: 2018, 2017, 2016, 2015, 2014, and 2013.

Structural Segmentation

Again no results have been published for this task. Last year I speculated that ours might have been the only entry, and since we submit the same one every year, there’s no point in re-running it if nobody else enters. Pity, this ought to be an interesting category.

Multiple Fundamental Frequency Estimation and Tracking

A rebound! Two years ago there were 14 entries here, last year only three: this year we’re back up to 12, including our two (both consisting of the Silvet plugin, in “live” and standard modes).

This category is famously difficult and I think still invites interesting approaches. An impressive submission from Anton Runov (linked abstract is worth reading) uses an approach based on visual object detection using the spectrogram as an image. Treating a spectrogram as an image is typical enough, but this particular method is new to me (having little exposure to rapid object detection algorithms). The code for this has been published, in C++ under the AGPL — I tried it, it seems like good code, builds cleanly, worked for me. Nice job.

Another interesting set of submissions achieving similar performance is that from Steiner, Jalalvand, and Birkholz (abstract also well worth a read) using “echo state networks”. An ESN appears to be like a recurrent neural network in which only the output weights are trained, input and internal weights remaining random.

Our own submissions are some way behind these methods, but there’s plenty of room for improvement ahead of them as well: I think the best submissions from 2017’s bumper crop still performed a little better than any from this year, and perfection is still well out of reach. (At least among labs that submit things to MIREX. Who knows what Google are up to by now.)

Results pages are here and here.

Audio Onset Detection

No results have (yet?) been published for this task.

Audio Beat Tracking

Another quiet year, with Sebastian Böck’s repeat submission still ahead. Results are here and here.

Audio Tempo Estimation

No results are yet available for this one either.

We made a tiny change to the submission protocol for our plugin this year (as foreshadowed in my post last year, I changed the calculation of the second estimate to be double instead of half of the first, in cases where the first estimate was below an arbitrary 100bpm) and I was curious what difference it made. I’ll update this if I notice any results having been published.

Audio Key Detection

We actually submitted a “new” plugin for this category: a version of the QM Key Detector containing a fix to chromagram initialisation provided by Daniel Schürmann, working in the Mixxx project. We submitted both “old” (same as last year) and “new” (with fix) versions, and saw significantly better results from the fixed version in all five test sets. So thank you, Daniel.

The most interesting submission, from Jiang, Xia, and Carlton, actually seems to be a presentation of a new(ish?) crowd-annotated dataset, used to train a key detection CRNN. It gets good results, with the rather critical caveat that the crowd-sourced training dataset could overlap with the MIREX test data. It’s not clear from the abstract whether the dataset is publicly available — I think it may be accessible via a developer API from the company (Hooktheory) that put it together.

Results are here.

Audio Chord Estimation

Last year was busy, this year isn’t: it sees only one submission besides ours, a straightforward CNN from the MIR Lab at National Taiwan University, whose performance is roughly comparable to our own Chordino. Results here.

 

Notes on the Minolta SR-1 (Model A)

The SR-1 is a model of 35mm film SLR camera made by Minolta (Chiyoda Kogaku) from 1959 onwards. It was the second SLR that Minolta released. As was typical at the time, their first was a more expensive model, the SR-2 in 1958, which they then cut down for a  cheaper alternative by removing the fastest shutter setting1. The resulting SR-1 remained in production for the next 12 years, but with incremental changes, which mean that the designation SR-1 actually covers at least five different models.

This first SR-1, unofficially the SR-1 model 1 or model A, is — in its style — surely the finest looking 35mm SLR camera ever made.

In 1962 Minolta spoiled its good looks by adding an industrial-looking bracket on the front for an optional light meter, and in 1965 they redesigned the body in an altogether squarer style. Later models are far more common, at least in the UK, and there is not so much information online about the earlier ones.

Early SR-1s still say SR 2 beneath the bottom plate.

This article hopes to remedy that. Read on for everything I could learn about the workings of the SR-1 model A.

Given the similarity of the two cameras, this probably almost all applies to the SR-2 as well, although actual SR-2s are expensive enough that I’m not sure I would dare to open one.

Standard disclaimer

This is all just the findings of an amateur poking about and making notes. Please don’t follow anything I say and ruin your lovely camera.

This will be a very long article, and unlike my previous post, there’s no story arc for the contemporary reader — I did solve the problem that set me off on this chase, but I did it in a way that would have been unexceptional 50 years ago, and old hands will just tut at my ignorance. The expected audience for this post is pretty much me, re-reading it the next time I need to fix something.

Background

I bought an SR-1 (the one shown above) from a charity shop in non-working condition, thinking it would be nice to get to grips with the workings of a camera with no electrical components at all.

Its second shutter curtain ran very slowly and made a sort of zipper noise, and the shutter mechanism seized up after firing and winding it a few times. I later found other things wrong with it: the slow shutter speeds (1/4 second and slower) didn’t work, and there were various cracks and tears in the second shutter curtain, which needed to be replaced.

I tried to find a maintenance manual for it. I couldn’t find any freely available for download, so I looked around for copies for sale.

I tried a company that sells paper copies of old manuals; they listed two repair manuals for the SR-1, identified as “older version” and “newer version”. I emailed asking whether this referred to the version of the camera or of the manual, and they replied that they couldn’t tell me because they had lost the older one: it existed only on microfiche, to be digitised on demand, but their reader had broken down before anyone had ordered it.

I ordered the version of the manual they could supply; it turns out to cover the later boxy revision that shares a basic body style with the SR-7 model. It is gorgeous to look at, being a negative scan from microfiche of a manual with hand drawn diagrams:

I would happily hang the first of those pages on my wall. But that’s about all I can do with it, as there were a great many changes between the model I have and the one shown here. Fundamental differences in the picture above include:

  • In the picture, both sets of shutter curtain barrels and the whole timing gear are attached to the sides of the mirror box. In the earlier model, all of this stuff was attached to the diecast camera body, and the mirror box had the mirror mechanism and nothing else.
  • In the picture, the second shutter curtain and its tapes are guided directly around the first-curtain barrels. In the earlier model, the second curtain had separate guide pins and never touched the first-curtain barrels.

There are also many omissions, for example the page shown above is the only appearance in the manual of the slow-speed gear (parts numbered 2301-2327), and it’s drawn from below as a single unit so you can’t see any of the details.

Given the lack of an accurate manual, before diving in I bought a second camera of the same model in unknown condition to use as a reference. It was cheaper and in a worse state on the outside, and I had to beat out a dent in the top cover with a small hammer wrapped in a cloth before the winding lever turned smoothly, but otherwise it turned out to be in really good working order.

All of the photos that follow are of the first SR-1 I bought. Most are snaps taken beneath a task light during disassembly. In most cases I have used the same names for the parts as appear in the maintenance manual for the later SR-12.

Survey of features

The Minolta SR-1 model A is a mechanical single-lens reflex camera with pentaprism viewfinder, taking 35mm film and using interchangeable lenses with Minolta’s bayonet lens mount. It has a self-timer, a thread for a mechanical remote in the shutter button, and flash X and FP sync terminals. There is no light meter, no battery, and nothing electrical inside except the wires and contacts used for the flash sync.

It has a semi-automatic aperture (when used with Minolta lenses) and automatic mirror return. That is, focusing is done at fully-open aperture and then the lens stops down automatically to its selected aperture to take the photo; after taking, it doesn’t re-open until you wind on the film. The mirror however does return to its lowered position as soon as the photo is taken, so you can see through the viewfinder again immediately, just not at full aperture. As in any similar camera of the age, this is achieved through cunning arrangements of gears, springs, and levers.

The camera uses a horizontal-travel cloth focal-plane shutter with speeds from 1/500 to 1 second, selected using a dial that must be lifted up in order to turn it. There are three timing mechanisms: the slowest speeds use a governor mechanism with a clockwork escapement, intermediate speeds use the same governor but bypass the escapement, and fast speeds offset the second shutter by a measured distance from the first.

The SR-1 is made almost entirely of metal, weighing about 700g without a lens. It’s not that easy to take apart and work on, as the design is not particularly modular and it uses a large number of different slot-headed screws, but at least there’s no risk of shearing off any threaded pieces of plastic.

P7120012.jpg

In use, the camera is a little wide and heavy, not that easy to grip, and the pointy end of the metal winding lever can put a dent in your forehead. It’s prettier than it is ergonomic. The shutter and mirror are loud (being totally undamped) but with a satisfyingly crisp sound, and not heavy to operate. The viewfinder is bright and easy to focus with.

How to tell a Model A apart from later models

Referring to this page for the model types, the most obvious distinguishing feature is the speed selection dial.

If the speeds are unevenly spaced on the dial (as in the picture to the right), and you have to lift up the dial to turn it, then it’s a model A.

If the speeds are equally spaced and the dial turns directly with a satisfying click, but there is no mount point for a light meter on the front of the camera, then it’s a model B or C. These also have a chrome ring on the viewfinder eyepiece, where the A has a black one. I don’t know how to tell the difference between B and C from the outside.

If it has a mount point for a light meter on the front, then it’s a model D or later.

Top and bottom covers

The bottom cover is simply held on by two screws.

The top cover is more involved. These pieces must be removed before it can be lifted off:

PB290369.jpg

  • The rewind knob can be unscrewed by hand (open the film door and put a screwdriver across the film engagement fork to hold it still while unscrewing it), but there is a circular plate beneath it that ideally needs a tool such as a spanning wrench to unscrew.
  • The eyepiece is in three parts. The black ring is used to hold on the optional flash cold-shoe, and can be unscrewed by hand. The glass eyepiece in a chromed ring also “simply” unscrews, but I found it maddeningly hard to do — some grippy rubber material may help. The sealing ring then lifts off.
  • To remove the speed selection knob, loosen (but not so far as to remove) the three small grub screws set into its perimeter at regular intervals, then lift it off.
  • The shutter button surround can be unscrewed by hand, and the shutter button then lifts out. The brass ring holding the winding lever in place has notches for unscrewing using a spanning wrench again; once it’s gone, the lever lifts off.
  • Finally three screws hold the cover on.

Don’t mess with the exposure counter window: it’s attached to the top cover.

You can put the winding lever and its brass ring back on after removing the cover — not a bad idea for testing things. No need to put on the shutter button, as all it does is push down a small plate that is now accessible directly just behind the winding gear.

What’s under the top cover

sr1-top.pngThe viewfinder pentaprism is in the middle, beneath a black plastic cover, with the eyepiece next to it.

The parts on the right are organised onto two separate chunky metal plates, the speed change base plate and the winding base plate. In both cases the user-accessible controls are attached on top of the plate, and the gears that do the work are hidden beneath it.

The two plates’ mechanisms are separate, but are connected underneath by a pair of gears in a stack attached to the shutter button plate. You can just see a few of the teeth of one of these gears in the picture, above the exposure counter pulley. When the shutter button is pressed, the first thing that happens is that these gears are detached from one another, disconnecting the two plates, so that the winding gear is not involved in the shutter release action.

Neither plate is easy to remove and replace. The winding plate is easy to remove, but it takes some trial and error to replace it with the right orientations for the gears beneath. The speed change plate can’t be removed without detaching the lever at the top of the slow-motion axis, which I think then can’t be re-attached at the correct angle without access to the lever at the other end of the axis, which is down where the slow speed gear lives, under the baffle at the bottom of the mirror box.

What’s under the bottom cover

sr1-bottom

In this picture the tripod mount has been removed (it is screwed into the three larger holes arranged in a triangle in the middle), and so has the slow-speed gear within the camera, leaving its two attachment holes empty of screws.

Note that the ratchet gears and the attachment screws for their pawls (the little hooks that hold them in place) are threaded backwards — turn right (clockwise) to loosen, left to tighten. This makes sense visually for the pawls because turning left pushes the pawl against the ratchet through friction from the screw, which is what you want to happen. For the ratchets it just has to do with the direction the shutter curtains move.

I didn’t label the flash sync contact mounting block or contacts, which the green and white wires are soldered to. The contacts are attached to a small plastic block which is screwed to the shutter spring base plate at its left end, and directly to the diecast camera body at its right.

Front plate, lens mount, and leather wrap

To get at the mirror box, shutter curtains, shutter springs (but not their tensioning ratchet gears, which we saw above) and barrels, and self-timer, it’s necessary to go in through the front. This means peeling off the leather cover.

First remove the self-timer furniture from the front — the lever cover unscrews with a spanning wrench or similar, and the knurled knob can be unscrewed with small pliers — as well as the lens release knob at the top of the lens mount, which also just unscrews. The top cover of the camera overlaps the lens mount, so it has to come off as well before the front plate can be removed.

The leather is thin in comparison to later cameras, tears easily, and is well stuck down with both glue and tape. It’s not that easy to lift, and it wouldn’t stick back down again without new adhesive, but I did find the texture quite good at hiding mistakes afterwards.

The best bet seemed to be to go in with a small sharp flat-headed screwdriver where the leather meets the lens mount, dig in and lift, and keep chipping away underneath and gradually pulling the cover out as you go. The leather is stuck so tightly up against the lens mount that I was convinced at first that it must have been nosed in under it, but no, it does just sit on top.

The front plate consists of the entire lens mount and two metal “wings” either side of it, beneath the leather, one of which (in picture) has the self-timer attached to its back. The plate is held on by four screws, one at the top and bottom of each wing. Remove these and the front plate then lifts off.

(The two “inner” screws visible in the picture, close to the self-timer lever/switch attach points, hold the self-timer on to the front plate. The self-timer isn’t attached to anything behind it, only to the front plate, so these don’t need to be unscrewed to remove the front.)

When replacing the front plate, put it gently in place first and then press the shutter button a couple of times to make sure the self-timer lever settles into place in the cutout of the shutter release shaft (see picture under the self-timer section below) before trying to fasten it down. Don’t tighten the screws without being sure the self-timer lever is properly settled, or you might damage it.

Shutter speed selection

speed-control-settings

The speed control knob on top of the camera rotates a collar with a pin in it that pokes down through a hole in the plate beneath. The position of the pin is visible from the top of the collar, so the current speed can be read off even though the knob has been removed. The sketch to the right shows the speed corresponding to each position of the pin.

Under the speed control knob is a stack of three eccentric discs which rotate along with the speed setting, serving to set the positions of these three sprung control arms:

sr1-speed-baseThe shutter lever is engaged at fast speeds. It is sprung so as to push against the high-speed lever which is attached to the first-curtain gear, and it has a hook that keeps the second-curtain gear in place. At the appropriate point in the rotation of the first-curtain gear, it gets pushed aside, releasing the second curtain. The screw on top of the lever adjusts an eccentric which controls its sensitivity and thus the shutter spacing.

The slow motion axis is engaged at the slowest speeds. It communicates to the slow speed gear in the base of the camera that its escapement mechanism should not be bypassed.

The slow speed control shaft is used for intermediate and slow speeds. It communicates to the slow speed gear the extent of deflection to use and therefore how long to delay for.

Speed setting Shutter lever position Slow motion axis position Slow speed control shaft extent
B Engaged
500 Engaged
250 Engaged
125 Engaged
60 1/8 approx
X 1/8 approx
30 1/4 approx
15 1/2 approx
8 Engaged 1/8
4 Engaged 1/4
2 Engaged 1/2
1 Engaged Furthest extent

Slow speed gear

The slow-speed gear or governor is a self-contained module that lives beneath a black-painted metal baffle at the bottom of the mirror box. You can see it at the bottom of the picture above, which shows the landscape with the mirror box removed.

You don’t need to remove the whole mirror box to have a look at it — you can just take out the baffle. But I found I had to remove the pentaprism and focusing screen from the top of the mirror box before I could get a screwdriver in vertically enough to undo the two screws at the front that hold the baffle in place.

Slow-speed gear module

The purpose of the slow-speed gear is to allow a lever to rotate a certain extent and then be returned by a spring to its original position, at a speed controlled by an escapement mechanism. In the picture on the right, the lever that is being controlled is engaged with the sprung wheel on the left side, and the escapement consists of the star gear and pallet on the right side. The inertia of the pallet determines the speed at which the wheel is allowed to return to the position set by its spring. An additional lever on top of the module optionally pushes the pallet away entirely, removing this control and allowing the mechanism to run much faster.

Two vertical shafts, visible in the picture at the top of this section, connect the slow-speed gear to the speed control mechanism above. These are referred to as the slow motion axis and the slow speed control shaft. The slow motion axis (the right-hand one of the two in the picture) has the job of pushing the lever on top of the slow-speed gear that deactivates the escapement mechanism for faster speeds. The slow speed control shaft (on the left) is the one that is controlled by the slow-speed gear, being pushed out to an extent fixed by the speed setting and then allowed to return when the shutter is fired. See the following section for more about how these are set for the different shutter speeds.

In my camera the slow-speed gear initially didn’t work, it just got stuck. I removed it, air-dusted and brushed it, and applied a pinpoint of sewing machine oil to the outside ends of the bearings, which was enough to get it moving again. (I believe one doesn’t oil the gears themselves, in a clockwork mechanism.) The escapement runs correctly now, but the intermediate shutter speeds with the escapement disengaged are a bit too fast, so perhaps oiling the bearings was a bad idea too. Though I’m puzzled by how the mechanism is ever expected to time anything accurately in the case when it doesn’t have the escapement engaged. Comments welcome.

Winding gear

Beneath the winding base plate

The base plate which the winding lever attaches to is, unsurprisingly, the winding base plate. The winding lever itself is fixed to the top of a sprung rotating cylinder with a stay at the return point, and this drives the gearing below the plate. I believe the only purpose of the spring in the cylinder is to make the lever flip back when you let go of it.

Winding the lever turns the film advance spool, and also turns the first and second curtain gears, pulling the shutters across to this end of the camera against the pull of the shutter springs at the other end.

Since the shutter curtain gears must spin back again when the shutter is fired, there has to be a mechanism to decouple them from the winding gear while the shutter button is pressed. This consists of a stack of two linked gears, shown in the picture.

The gear sitting at the front of the picture has a hole in it, and it goes on top of the slightly smaller gear just left of centre in the picture — which gets pushed down when the larger gear is added, so that it engages with the winding gear on the right.

This smaller gear has a pin that fits the hole in the larger, so that the two gears normally turn together. The smaller gear is driven from the winding gear, and the larger then drives the gears on the left. When the shutter button is pressed, the smaller gear is lowered and detaches from the larger, leaving the larger one free to spin without any connection to the winding gear.

The small gear also has a cutout in the bottom (not visible here) which fits over the head of a screw jutting from the camera body. The gear can only be lowered when this cutout is aligned over the screw. That’s what prevents the shutter button from being pressed before the film has been fully wound on. It also prevents the winding lever from turning while the button is pressed.

In hindsight there was no good reason other than ignorance and curiosity for me to remove this plate. There were no faults here, the actual fixes I ended up making didn’t require it, and it took some trial and error to fit again. To do so, make sure the small gear is oriented so that the button can be pressed, then fit the larger one over it, push the plate over, and screw down — but it took me several tries before I got the angle of rotation just right so that the shutter could be wound and fired again without jamming because the cutout in the lower gear hadn’t been properly aligned over the screw head.

(It’s also necessary to make sure the disc at the other end of the winding gear, on the bottom of the camera, is at the correct point in its phase of rotation before replacing the winding plate. That is, the pin that pushes the winding charge plate needs to be at roughly the angle shown in the picture at “What’s under the bottom cover” above.)

Shutter firing order

  1. Shutter release shaft

    The shutter release shaft is pressed down, either by the shutter button or by the self-timer lever.

  2. At the top of the camera, lowering the shutter release shaft disengages the winding gear from the transmission and makes space for the fast-speed shutter lever to move in against the first curtain gear if an appropriate speed is selected.
  3. At the bottom of the camera, the pointed end of the shutter release shaft pushes aside a lever that releases the aperture slider at the base of the lens mount, allowing the aperture on the lens to spring closed.
    This picture shows the aperture slider in its cocked position, with the various levers around it. (Cocking the slider is done by the thing I have labelled the winding charge plate, which pushes it along during winding until it latches as below.)
  4. sr1-bottom-detailThe travel of the aperture slider trips a catch I have labelled the mirror lever catch, which holds back the mirror lever on the side of the mirror box. This releases the mirror, which is sprung so as to flip into the upward position.
  5. The gears that control the first and second shutter curtains: first on top, second beneath

    At the end of its travel, the mirror lever strikes a lever I have labelled the shutter trip lever, which I believe is connected to the transmission shaft (this is one part I haven’t actually uncovered). This releases the first curtain gear, firing the first shutter curtain and releasing the slow speed control shaft to begin the slow timer if appropriate.

  6. The first curtain gear spins back to its un-cocked position as the first shutter moves. If a fast speed is selected, then the high-speed lever attached to it pushes aside the shutter lever at the appropriate moment, releasing the second-curtain gear and firing the second shutter curtain. Otherwise it has to wait until the slow speed control shaft has finished its travel.
  7. Mirror kick gear

    At the other end of the second curtain, there is a gear at the bottom of the curtain spring barrel, which rotates the mirror kick gear next to the base of the mirror box. At the end of its rotation a pin on it knocks the mirror hook aside, reversing the direction of spring for the mirror operation lever and pulling the mirror back into lowered position.

Most of these steps are triggered by the completion of the prior step, so guaranteeing that the timing works out. For example, the shutter can’t normally open before the mirror has flipped up, because it is triggered by the mirror lever itself at the end of its travel. But there are some minor timing-related weaknesses:

  • There is no means of synchronisation with the lens aperture itself, only with the aperture release slider. If the aperture takes too long to close, it may be still closing when the shutter begins to travel. This can happen with some lenses whose aperture blades tend to get oily. Yes I mean you, Minolta W.Rokkor-HG 35mm f2.8.
  • It is just possible to press and release the shutter button so quickly that the winding gear (disengaged in step 2) has re-engaged before step 5 is reached, preventing the transmission gears from turning and leaving the mirror flipped up but the shutter still closed. Pressing the button again will fire the shutter and complete the process.
  • The winding gear is re-engaged at the end of the first shutter’s travel in step 6; it doesn’t wait for the second shutter. So there is nothing preventing you from winding the film on in the middle of a long exposure.

Some of the working of this depends on the angle at which the eccentric control that I’ve labelled the mirror lever catch eccentric is set. This is partly hidden by the tripod mount and is shipped with its adjustment screw secured with a blob of shellac to prevent it from coming loose. If this is wrongly set, then either the mirror lever won’t hook up on winding (so the mirror will remain up and the viewfinder will be useless) or the mirror lever won’t reach the shutter trip lever (so the shutter button will close the aperture and flip the mirror, but not fire the shutter).

Pentaprism and viewfinder

The pentaprism (see the middle of the “under the top cover” pic earlier) is held on by two springs hooked over screws on the sides of the mirror box. Unhook the top ends of these, and you can lift off the plastic cover and pentaprism. Then three screws around the edges release the focusing screen.

(The manual I have suggests that you can take the whole thing off by unscrewing the screws without unhooking the springs first — that’s definitely not true for this model. If you try it, you’ll find the screws very hard to put back in.)

I don’t have a photo of any of these bits, as I forgot immediately picked them up with a cloth and put them in a box to avoid getting fingerprints on them. I should have taken some, because both of my cameras showed a typical problem: corrosion of the mirror coating of the pentaprism where the dust seals were glued on to it. The result is a rough dark line across the bottom of the image as you look into the viewfinder.

As far as I can learn, there is no way for an ordinary person to re-silver a mirrored prism like this. I left the less seriously damaged prism from the two cameras alone, and in the other one I scraped off the thin corroded strip of silver (making that part transparent) and then wrapped a strip of aluminised plastic, cut from the inner wrapper of a pack of tea-bags, around it to add a bit of reflection. There is no problem so serious that it cannot be mitigated by a good cup of tea. The result is much less intrusive in the viewfinder than it was, making the bottom part of the image vaguely accurate in colour rather than solid black, but it isn’t possible to see detail there and it wouldn’t work at all for damage in the middle of the frame.

Exposure counter

This is nudged along by a hook drawn by a pulley connected to the winding gear on the other end of the camera. The counter is sprung so as to return to zero, but it is on a ratchet and is latched under pressure from the camera back while the back is closed. When the back of the camera is opened, the counter springs to zero.

The thin aluminium surround on top of the counter is easily bent and I suspect could snap, so it seems like a good idea to unscrew it when working on the camera for any length of time. There are a couple of hazards, though.

The first is that the surround, or the washer beneath it, holds the counter down on its axis. If you remove it and the counter gets lifted up too far, there are two different rotations in which it can be pushed back down — the correct rotation, or 180° out from it — and under pressure of its spring it will prefer the latter. So if you find your counter is resetting to somewhere around 20 instead of to zero, that’s why: you need to lift it, rotate half a turn against the spring, and push it down again.

The other little problem is that I found this the most bizarrely troublesome screw to put back in in the entire camera. I don’t know why, it just doesn’t want to sit right.

Mirror box

There doesn’t seem to be a great deal to say about the mirror box, from my standpoint of ignorance, except to marvel at the lever and spring mechanism, which I imagine to be essentially the same as in all SLRs. The box is attached with four screws, two at the top by the eyepiece and two at the bottom (whose locations are marked in the “under the bottom cover” picture further up).

The two metal shafts also visible in the central picture above are the slow speed control shaft (top) and slow motion axis (bottom) of the slow-speed gear.

Note that after removing the mirror box you can no longer fire the shutter in the normal way, because the mirror lever on the side of the box is what triggers the shutter release. To fire the shutter, push the shutter trip lever on the bottom of the camera yourself while pressing the shutter button.

Flash sync contacts

Attached to one side of the mirror box is the little electrical contact board shown to the right, which provides the contacts for the flash X- and FP-sync terminals on the side of the lens mount.

The wires are soldered here and to a switch block on the base of the camera, passing through a small hole in the diecast body in between. This is a surprisingly frustrating arrangement, as the board gets in the way and can’t be removed without un-soldering and subsequently re-soldering it.

I tried to work without removing the board, then broke one of the wires near the contact point, gave up and removed it, didn’t have enough wire left to strip and re-solder, and so had to replace the wire as well. I found the data cores of a USB cable to be a good replacement, with the proper insulation colours (green and white). Of course you have to feed the wires back through the hole before you can solder the ends on, so you’re waving your hot iron rather close to the camera. I am not a confident solderer and had hoped to avoid soldering with this project, but no such luck.

This is the only part of the job that I haven’t tested — I see no reason why it shouldn’t work after being rewired, but I don’t know for sure. If it doesn’t, I think I would probably rather not know.

Shutter curtains and barrels

The cloth focal-plane shutter consists of two opaque fabric curtains with tapes attached to revolving barrels at both ends. With the shutter cocked, the first curtain covers the film. When the shutter is fired, the first curtain is pulled aside to expose the film, and then the second curtain follows it to cover up the film again.

The barrels at the rewinding end of the camera are metal with a central spring, and it is these springs that power the motion of the shutters and consequently everything else about the shutter firing sequence. I’ve read that it’s common for the springs (or the grease they are packed with) to seize up through age or disuse, but this wasn’t a problem in either of the cameras I have.

The picture on the right shows the spring barrels with the shutter cocked. The first shutter’s barrel is on the left. Its cloth is glued to this barrel, and it has tapes glued to the un-sprung barrel at the other end. The second shutter is the other way around: its cloth is glued to the barrel at the other end, with tapes attached to this one.

The spring barrels can be accessed by removing the cover plate on the bottom of the camera. This is fiddly and means losing the spring tension, so having to “re-program” the shutters afterwards by re-tensioning, but it’s necessary to do this as well as remove the mirror box if you want to actually take out either shutter curtain.

Here’s the other end, with the un-sprung barrels (seen from the top). These ones are plastic sleeves surrounding metal shafts, held in place by two screws that go all the way through the barrel. You can just about see one of those screws in this photo, where I’ve pulled the (old, cracked and torn) shutter cloth away from the barrel. The holes in the cloth are useful to insert a screwdriver and unscrew the screws to slide the central shaft out of the top of the camera and extract the barrel.

This picture shows the central shaft of that second curtain barrel, and the screws that attach the barrel to it.

The second shutter curtain also passes around two guide pins, which keep it away from the first curtain barrels. These are just to the “film side” of the first curtain barrels at either end. The pin at the spring end can be removed along with the spring barrels, but I couldn’t discover how to remove the other pin, and since the second curtain passes behind it, that means the only way I know to remove that curtain is to un-glue it from its barrels at one end or the other. (The tape end is easier.)

The curtains themselves are made of an inelastic fabric with some sort of opaque black paint-like finish. They don’t appear to have a separate coating layer.

Where the tapes meet the curtain cloth there is a rigid metal lath, formed from a thin strip folded in half around the end of the cloth and crimped and glued down. The tapes pass through slits in the ends of the lath, and are sewn to the cloth.

A detail of the original second shutter curtain is shown to the right.

I needed to replace the cracked and torn second curtain, so I ordered new cloth, tapes, and glue from Aki-Asahi. The cloth (silk with an opaque rubber coating on one side) is more flexible than the old and feels very high quality, though it’s also a little more elastic even when cut with the warp. I cut it to size, glued and sewed the tapes down at the corners, then prised the lath off the old curtain and wrapped, glued, and crimped it with pliers. Here it is being glued on to the plastic barrel, waiting for the glue to dry:

The result is visibly amateur in comparison to the original. I used a little too much glue, the lath is bumpy after being forcibly prised from the old one, and I left the curtain not lying completely flat when not under tension. Fortunately it’s not at all obvious when installed, and the shutter does actually run well, but I’d hope to do better next time.

Tuning the shutter speeds

Tricky this. After replacing a shutter you have to adjust everything. The tensions of the first and second curtain spring barrels affect the timings of all shutter speeds, and have to be both sufficiently accurate and sufficiently similar to one another. Then the slow speeds depend on those tensions and on how well the slow-speed gear runs, and the fast speeds depend also on the adjustment screw on the shutter lever.

I started out by setting the speed dial to B and adjusting the shutter spring tensions, using the ratchets at the bottom, until the shutter curtains seemed to be moving at similar-ish speeds to those in my reference camera. (They are supposed to traverse the width of the film negative in 1/60 second, about 13ms.)

I found the escapement-timed slow speeds were OK, but the intermediate speeds, using the slow speed gear without the escapement, ran too fast. I have no idea whether anything can be done about that.

Then I adjusted the screw on the shutter lever so as to get it to a good middle point. If this screw is turned too far one way, it makes the fastest speed (1/500) behave like bulb mode — the button opens the shutter and doesn’t close it until you let go. If it’s turned too far the other way, then the slowest of the fast speeds (1/125) does the same thing. There isn’t an enormous range in between, and I don’t know how much difference this control makes, so I just picked a point somewhere in the middle.

Then I happen to have a CRT TV among my apparently increasing collection of obsolete stuff, so I was able to use Rick Oleson’s method to test the fast speeds, using the spring ratchets to fix inconsistencies between the two curtains’ speeds (shown by the bright region being wider at one end than the other) or gross timing errors (bright region taking the wrong proportion of the whole field).

Self-timer

The self-timer is a cute little clockwork module that’s fun to operate and watch outside the camera. The arm winds the spring, the slider button releases it, and when it comes back to a point close to the origin, a lever pokes down the shutter release shaft to fire the shutter.

The picture on the right shows how the module sits within the camera and the position of its lever over the shutter release shaft, although in practice the module is attached to the front plate and is installed and removed with it.

The self-timer of course uses an escapement mechanism. I believe that, as in the slow-speed gear, the gears themselves should not be oiled, but a pinpoint of light oil on the outside ends of the bearings may be acceptable. Corrections welcome.

Dust and light seals

There are not that many of these — nothing in the film area and no mirror damper — and those there were in my camera had largely crumbled into a red dust which took quite a bit of cleaning out.

The main seals are a big wide one within the curved base of the lens mount, edge seals along the bottom edge behind the front cover, and edging around the top of the back of the mirror box. A 3mm thickness open-cell foam from Camera Light Seals seems like a fair replacement.

What the problems were with my camera, and their solutions

At the top of this article I listed the problems I had with this camera:

Its second shutter curtain ran very slowly and made a sort of zipper noise, and the shutter mechanism seized up after firing and winding it a few times. I later found other things wrong with it: the slow shutter speeds (1/4 second and slower) didn’t work, and there were various cracks and tears in the second shutter curtain, which needed to be replaced.

I’ll close with a breakdown of those, with what I think were their causes and the solutions:

Second shutter curtain ran very slowly and made a sort of zipper noise

Caused by friction in the running of the second shutter curtain gear, the lower of the two stacked gears in the picture to the right.

I didn’t get as far as working out how to remove these gears entirely for cleaning, but air-dusting them and squirting the underside of the lower one with light oil (then mopping up the excess) fixed this problem surprisingly quickly, for now at least.

Shutter mechanism seized up after firing and winding it a few times

I’m fairly sure this was just a consequence of the above. The shutter spring has to have enough energy to return the gears to their “home” positions; the friction was enough to prevent them from getting all the way there.

The slow shutter speeds (1/4 second and slower) didn’t work

Fixed by cleaning the slow-speed gear module and applying a tiny quantity of sewing-machine oil to the outer ends of its bearings.

Cracks and tears in the second shutter curtain

Fixed by replacing the second shutter curtain!

This goes something like the following. (I’m writing this from memory — wonder if I’m forgetting anything):

  1. Remove the eyepiece, top controls, top and bottom covers and the front plate; put the winding lever back on
  2. Unhook and remove the pentaprism, undo the screws holding on the focusing screen, remove the focusing screen
  3. Remove the baffle from the bottom of the mirror box (I can’t actually remember whether this is essential, but it’s fairly easy so might as well)
  4. Remove the tripod mount from the base, unscrew the fixing screws at bottom and top of the mirror box, and delicately ease out the mirror box (first unhooking the mirror lever from below, and removing the little metal screen to its left if it gets in the way)
  5. Detach the lever from the top of the slow motion axis and remove the speed control base plate
  6. Remove the two control shafts for the slow-speed gear
  7. Remove the four screws holding in the two shutter blinds at top and bottom of the shutter by the film window and detach the blinds
  8. Make a note of the positions of the screws threaded through the barrels at the un-sprung ends of the first and second shutter curtains when the shutter is not cocked, so as to replace them at the right rotations again later
  9. Loosen the pawls beneath the shutter spring barrels (losing their spring tension), undo the ratchet gears around the barrel ends, unscrew the retaining screws for the covering plate, and remove the plate with the mirror kick gear and guide pin
  10. Unscrew the two screws threaded through the barrel at the un-sprung end of the first shutter (the shutter whose barrels are closer to the film plane), pull out the central shaft for that barrel, and remove the whole of the first shutter curtain with both barrels still attached
  11. Unscrew the two screws threaded through the barrel at the un-sprung end of the second shutter, by poking a screwdriver through the holes in the second shutter curtain, and pull out the central shaft for that barrel. Observe that you can’t just remove the curtain because it runs behind a guide pin for which access is not obvious
  12. Detach the second curtain’s tapes from the spring barrel by lifting them away with a sharp tool, and remove the second curtain and both of its barrels
  13. Detach the second curtain from the unsprung barrel as well by lifting it away
  14. Obtain some shutter cloth and cut a rectangle of it to match the original, cutting along the warp of the cloth so as to have the least stretchy direction along the length of the shutter. (I think the rubbery coating should go on the lens side rather than the film side, though I didn’t find an authoritative source)
  15. Obtain some shutter tape and cut two lengths to match the originals; I think these can be a bit too long without problems, but not too short
  16. Prise the metal lath away from the original shutter curtain
  17. Glue the tapes to the corners of the shutter cloth using a contact adhesive (I had the Japanese Super-X glue) and allow to dry for a while
  18. Sew a small strong square of black cotton through each of the tapes’ contact areas (I did it by hand but you could do it with a machine if you knew how)
  19. Apply some glue to the very edge of the cloth, push the lath from the old shutter over the edge of the cloth, square it up carefully, and crimp it tight with pliers
  20. Glue the end of the curtain cloth to the un-sprung barrel following the positioning of the previous curtain, and let dry
  21. Re-fit and screw into place the un-sprung barrel of the second curtain, following the positioning noted before removing it earlier
  22. Make sure the second curtain is routed behind its guide pin, but don’t accidentally route it behind the nearby shaft from the shutter gears! I stupidly did this the first time and then had to un-glue it and fit it again
  23. Glue the tapes at the other end of the second curtain onto the spring barrel, matching the original positioning and being very careful not to get glue in the spring
  24. Re-fit and screw into place the un-sprung barrel of the first curtain, following the positioning noted before removing it earlier
  25. Introduce the two sprung barrels and the other guide pin for the second curtain into the appropriate holes in their bottom plate through their metal collars, ensure the mirror kick gear has the right orientation (as in the picture in the “Shutter firing order” section above, when the shutter is un-cocked) and screw down the plate
  26. Tighten the spring ratchets enough to pull the curtains tight and check their positioning and overlap (they should overlap by about the thickness of the lath)
  27. Test winding and firing the shutters — you’ll have to hold back the shutter trip lever yourself (see note in “Mirror box” above and picture in “Shutter firing order”) and there is no control over timing, but the shutters should work at this point
  28. Replace the blinds at top and bottom of the shutter area
  29. Replace the two control shafts for the slow-speed gear. The slow speed control shaft engages with the slot in the wheel, and the slow motion axis sits next to the lever end (see picture in “Slow speed gear” above)
  30. Replace the speed control base plate, put the lever back on the slow motion axis, and arrange it so that when it is pushed out by the appropriate cam on the speed control base, it pushes the lever on the slow speed gear so as to deactivate the escapement. Tighten the set screws on this lever
  31. Wind the shutter a little bit so as to move the pin on the mirror kick gear out of the way, then put the mirror box back in (may take some patience to get the mirror lever through the base properly) and screw down
  32. Replace the baffle at the bottom of the mirror box
  33. Replace the front plate
  34. Tune the shutter speeds
  35. Replace the focusing screen and pentaprism, hook the cover back on
  36. Remove the winding lever again, replace the top and bottom covers, re-attach the top controls and eyepiece.

Here’s a photo taken with the camera after I replaced the shutter. It’s not a very exciting photo, but just to indicate that it does work:

grasses

(Ilford FP4+ film, developed using Ilford DD/X.)

And that is the end of this post. Phew.

The repaired SR-1 (left) and its stunt double (right)

1. The most cynical example of this came from Pentax, whose cut-down S1a model simply had the label for the 1/1000 shutter speed removed from the dial – the setting itself still worked fine.

2. One exception is that I refer to the “first-curtain gear” and “second-curtain gear” when referring to the roles of the large transmission gears in the timing order. In the manual these gears are called the “change speed gear” and “transmission gear” respectively, and the names “first-curtain gear” and “second-curtain gear” are used to refer to the small gears at the ends of the curtain shafts that are connected to them.

Repairing a Minolta XG-9 camera

This is the story of how I repaired, broke, and repaired again a Minolta XG-9 35mm SLR film camera.

It will be long, and of niche appeal, but I’m writing it up in case anyone else finds it as useful as I would have done before I started. Here’s the plot summary:

  • The camera’s shutter was sometimes sticking open when fired in Auto mode
  • I set out to fix this by cleaning the electrical contacts of the film-speed selection switch, beneath the camera’s top plate
  • That appears to have been the correct fix, but when reassembling the camera, I broke a fragile plastic part which holds the power switch in place, rendering the camera useless
  • I modelled a replacement part using 3D modelling software, had it 3D printed, and replaced the part in the camera
  • The replacement part is good enough to use, but could probably have been better; I’ve published the model for it, and would appreciate any ideas

If that sounds in any way interesting, read on. This was my first attempt at repairing a camera and my first experience of 3D modelling and printing, so it was certainly interesting to me.

The original problem: a sticking shutter

OLYMPUS DIGITAL CAMERA

The camera is one that I wrote a happy post about last year (“A film camera“). It was made in 1980 or so, but I bought it in 2018.

It worked well, except that the shutter would sometimes stick open when fired in Auto mode, and the only way to close it was to switch out of Auto. When stuck open, it often let in enough light to ruin both the previous and following photos as well as the current one. I was keen to fix it, especially as these cameras are designed so that the light meter only works when in Auto mode.

The first hint I found online was here, in a photo.net post from 2003 by “rokkor fan” who writes: “I had this once on a XG-1, and it was as a result of the mirror slap jarring the circuitry under the shutter release. I had a friend clean the circuits and it worked a treat after that.” No more details there, but a poor electrical contact sounds promising.

There’s a 238-page service manual available for these cameras (thank you Benoît Suaudeau) and it has an electrical troubleshooting section starting at page 174 that says:

Annotation 2019-07-04 204232“AUTO… curtain is kept open” looks like our problem, and “ASA contact defective” seems worth checking. ASA refers to the film-speed selection switch, which is on the top of the camera. It makes sense that a lost contact from that switch would only affect Auto mode, since Auto needs to know the film speed to decide how long to leave the shutter open.

There’s a YouTube video here, by Florian Buschmann, which shows how to take the lid off. The top cover has three axes poking out of it, one for the rewind knob and power switch, one for the shutter button and film speed knob, and one for the winding lever. The components attached to the tops of these can all be unscrewed from their axes and removed, leaving a plastic top-plate attached via four small screws, two at front and two at the back towards the middle of the camera. Here’s the camera with the top off:

Minolta XG-9 with top plate removed

The film-speed switch is beneath the large black nubbin poking out of the top about 3/4 of the way across from the left. The smaller nubbin at the left side is the power switch mechanism and rewind axis, which is the part I was just about to break while reassembling the camera.OLYMPUS DIGITAL CAMERA

Unscrewing the parts atop the film-speed switch reveals a brush mechanism with a flat contact plate. I cleaned the plate with switch cleaner (it was quite grubby) and made sure the brushes were sticking out enough on the switch side, then reassembled the top of the camera.

After reassembly I tested the shutter in Auto mode a few times, and it didn’t stick. Flushed with success, I set out to load a film — and that’s when the power switch came loose. While screwing on the metal collar that keeps the power switch in place, I had managed to shear off the entire threaded top of the plastic part that holds it down. A problem.

The broken part

Minolta part 2006-3309-2, Rewinding axis receiverThe picture on the right shows the part I had broken. It’s intact in this photo, because this was taken during one of my initial attempts to glue it back together again — which always failed when force was applied to screw the threaded collar back on the top.

The 40-year-old plastic is quite brittle, and as you can see, I also broke one of its little lugs just unscrewing it from the body. And I am a reasonably delicate person.

Without this part, it’s impossible to use the power switch: it is left floating, switching between on, off, and self-timer modes at random. Having failed to glue it, my options seemed to be:

  1. Write off the camera. I wasn’t going to do that yet.
  2. Figure out some ingenious bodge to keep using the power switch even though the official mechanism didn’t work. Well, I did sort of do this using a piece of thread attached to the switch mechanism, but I didn’t like that very much.
  3. Find a replacement part. For an easily-broken part in a relatively inexpensive 40-year-old camera from a company that stopped making cameras over a decade ago, that seemed unlikely, and my first searches came up with nothing.
  4. Buy a non-working camera, sold for parts, and plunder it for this part. Yes, but what if it just broke in the same way again? That would be unbearable. Or if this part was already broken in the other camera?
  5. Make a new part. This takes time and money and might not work, but it puts the outcome in my own hands and hopefully teaches me something new.

My first assumption, knowing very little about 3D printing, was that this component was too fiddly to be produced that way — especially the rather fine threaded bit at the top. But reading about different 3D processes, it seemed faintly possible that a nylon SLS printer, with 0.1mm resolution, might just be able to produce a viable part, especially as the thread was required to screw into a metal collar (which could perhaps do a bit of self-tapping) rather than a plastic one.

Modelling the part

Here’s this component in the service manual, identified as “Rewinding axis receiver”. It comes in two variations with codes 2006-3309-02 and 2006-3309-04. I think mine is a 2006-3309-02.Part in maintenance manual

First I needed a 3D model.

I measured it with a vernier caliper and some close-up photos. The thread seems to be 0.5mm x 7mm and the three screw-fixing lugs are spaced at 0°, 140°, and 220°. A hole through the middle admits a metal axle of 4mm diameter. There is a cutout in the collar at the bottom, into which the mechanism to release the film door when the axle is lifted fits. There’s also a slot in the midriff which a metal clip pokes into, to meet a notch in the axle that snaps it into its usual lowered position.

Knowing nothing about 3D modelling software, I asked my kids for advice first. Their suggestion was Blender, but I’m a bit afraid of Blender. I did try Wings3D, another free application (written in Erlang using wxWidgets — interesting!) but I got stuck on how to model a screw thread, and decided I should probably start with a trial version of something expensive with extensive documentation.

I came across Autodesk’s Fusion 360 while searching for screw thread tutorials, when I found a page that basically just said “use the thread tool”, which seemed about right. Fusion 360 is a very expensive subscription product with cloud-storage lock-in, but it has a one-year trial period for “hobbyist” use. That’s me!

Annotation 2019-07-05 203934.png

I really enjoyed using Fusion 360, and I waxed lyrical about it here on Twitter. Much the appeal has to do with good interactive feedback, but the core thing is that it’s built on a very nice 2D sketching program: the expectation seems to be that you sketch in 2D and then extrude into 3D, which I find a lot simpler than trying to design in 3D. But I don’t know how other software of this kind works. Anyway, I successfully built a nice-looking model, though with a thread that I suspected wasn’t really possible to print.

3D printing

I exported the model as an STL file and sent it for nylon/polyamide SLS printing at i.materialise. I have to admit that I picked this company because they had the most anonymous, automatic-looking order page, and I felt embarrassed about having a real person look over my impossible design. I didn’t notice at first that i.materialise are actually in Belgium, and it’s slightly crazy to send your model from London to Belgium for printing when there are companies in London (digits2widgets, 3DPRINTUK) doing the same thing, but by that point I was sort-of committed.

Because of the minimum order price, I ordered three different pieces. Two were from my best attempt at the model, which I asked for once in SLS (laser) process and once in MJF (HP’s ink-based) process. The third was an SLS print with slightly different dimensions for some things I wasn’t sure of. The pieces took about a week to arrive, and the price was quite high — the nominal charge for printing each item was £11.15, but that was quoted before VAT and shipping, and the eventual total was very close to £50.

Here are the results. The black part on the left is the broken original, the white one is the SLS of the variant model, and the grey one on the right is the MJF. (The colour doesn’t matter, as the part is not visible from outside the camera.)

3D printed Minolta parts

Receiving these was a really exciting moment: a true marvel that it was actually possible to design and build this neat little component with no worthwhile expertise on my part!

Of the two processes, the MJF version came out a bit fatter than the SLS — the holes are smaller, the walls are thicker. I think the SLS copy is more true to the design size. Although the texture of the printed nylon is unpolished (they both feel a bit like paper) and looks almost crumbly, both of them feel very solid, are harder and less flexible than I expected, and seem pretty tough.

The threads are indeed pretty sketchy. The SLS one is a little too narrow, without enough depth to the thread: the collar doesn’t tighten well and can easily jump a thread if pushed. The MJF has even less visible thread, but it is a bit too fat, and it is hard to fit the collar onto it at all. In this situation that would probably be a good thing, but the thicker walls of the MJF part interfered with the film-door release disc underneath the part, so it was an SLS copy that I ended up fitting. Here it is in the camera.

3D printed Minolta part fitted in XG-9 camera

I think that the thread, although weak, may hold well enough. It isn’t the last line of defence against the power switch falling off: the rewind knob on top of it, held on with a metal-on-metal screw fitting, is there to prevent that. This thread just needs to stop the power switch from lifting and losing its position while loading or rewinding film. But it would be nice to have done better, if it were possible to do so. I’d like to know if there is some technique for making threads more “printable” at this kind of scale.

Conclusion

This feels like a successful outcome, and at least the original problem is fixed. It would obviously be better not to have broken this part at all — and although it was my fault, I did spend some time mentally railing at Minolta for using a plastic thread here in the first place. Perhaps the main lesson is just that old plastic is fragile. But I enjoyed the process and am happy with the result, which is after all a camera that works better than it previously did.

I’ve published the 3D model, in a Github repository at cannam/minolta-2006-3309-2, in case it is of use to anyone else. If you’ve any suggestions for how this could have been done better, I’d like to hear! Of course this does show a big limitation of using Fusion 360 to do the modelling: the main file in that repo is in a proprietary format and probably useless to any other tool. I’ve included a couple of exports as well, including the STL file.