Canon T90: Fixing dropouts on the control wheel

[Andreas Thaler]

A more annoying than serious problem with the Canon T90 is the control wheel on the handle, which turns in notches that sometimes no longer make contact.

You then have to turn the wheel one or two positions further in order to be able to set a shutter speed or an aperture value, for example.

I suspect that this is caused by contact problems due to contamination.

I had already dealt with this problem before, but didn't know how to remove the adjustment wheel to clean its contacts. A tight cable harness prevented removal.

Canon T90: recommendations and notes

Unless you need 4000 speed? there is very little you can’t do as well or better with the better build, more reliable, lighter, cheaper and more numerous other FD cameras. No PC port either. I see

www.photrio.com

Today I know how to do it thanks to Thomas Tomosy


Position on the T90

Here I destructively removed the assembly with trigger button, control wheel and spot metering button during a section.

At the time I didn't know how to deal with the very tightly routed connection cables and had cut them for this purpose.

The assembly

Top front: trigger button
Behind: control wheel
Right: spot metering button

View from below

The arrow points to the contact plate that rotates with the wheel.

Above this are the wipers that switch with the contacts.

Contact errors can occur here due to suspected contamination.

Rear view

The contact plate is not sealed to the outside.

This allows dust and dirt to accumulate. Moisture can also penetrate and lead to corrosion.

Unfortunately, the contacts cannot be reached from the outside with electronic cleaning spray.

---

The project will start soon.

Stay tuned

 

[Andreas Thaler]

I recently was able to buy a bundle of four T90s at a very reasonable price.

According to the seller, one of them is working (whatever that means), the other three are probably broken/dirty/corroded due to leaked batteries.

Ideal for this project, because then we can check the T90s not only for the control wheel to work, but also for all the usual problems.

 

[Andreas Thaler]

We‘ll start tomorrow morning (CET)

 

[Andreas Thaler]

I have chosen these two T90s from the collection of T90s and other cameras.


Condition

• Both cameras are dirty,
• one of them has a lot of abrasion on the housing,
• the LCDs can be activated after inserting batteries.
• The mirror magnets do not trigger, an arrow in the displays flashes.
• The control wheels only change the display in a few positions when I turn them.
• The other T90 has less abrasion on the outside.
• The shutters and mirror boxes are clean and there is no visible damage.
• The battery chambers are clean.

The control wheels have practically no function; the display on the LCD only changes at a few click-in positions.


So we have two ideal candidates for our purpose!

I assume that both the mirror magnets and the control wheels are dirty or even corroded. The cameras were obviously stored unprotected in a basement or attic.

I will start by cleaning the mirror magnets and then continue with the control wheels.

 

[Andreas Thaler]

Removing and cleaning the mirror magnet

To solve the problem with the mirror magnet not triggering, I dismantle it to clean its three contact surfaces with electronic cleaner.

Contrary to other opinions, I do not believe that this well-known and common problem with the T90 is caused by a change/decrease in the magnetic properties. I assume that it is dirt.

I only show the essential stages here, I have compiled details on removal and cleaning of the mirror magnet here:

Canon T90: Cleaning the mirror magnet

I have a T90 here whose mirror magnet no longer works. The usual symptom of a quiet clicking sound and a flashing arrow in the display occurs. The recommended cure of removing the front cover and activating the - easily accessible - magnet with a magnetized screwdriver tip works with patience...

www.photrio.com

---

I was careless here and tore the self-adhesive leatherette on the bottom of the camera when I removed it.

At worst, this is a cosmetic problem ...

… as are the fine creases on the handle's cover after it was removed.

In both cases, the plastic is no longer completely elastic to withstand strain.

The battery compartment is removed.

The bayonet screw connection is loose. I'm tightening the screws.

Both contact surfaces of the magnet have a coating.

The contact surface of the counterpart is also not clean.

I clean all contact surfaces with electronic cleaner.

The magnet is back in its place.

The battery holder is inserted, the main switch is on - and the mirror is triggered again. The shutter also works.

Good news, this means that the two shutter magnets are also OK, which I could not reach for cleaning because they are installed too deep.

The first problem is solved

 

[forest bagger]

Good work, Andreas!

And now it comes to fixing the dropouts of the control wheel?

 

[Andreas Thaler]

Unfortunately …

… I made a grave mistake when I tried to remove the assembly with the control wheel.

I followed Thomas Tomosy's excellent instructions1, but the thing just wouldn't come off the camera.

Until I used the screwdriver as a lever and damaged the assembly.

I was able to install and solder a replacement, but the T90 responded with „bc“ on the display.

There was nothing I could do.

When I then read Thomas's instructions again, the text said „Remove the top cover“.

By then it was too late

This mistake could have been avoided.

The T90 has now joined the ranks of spare parts donors.

---

1Thomas Tomosy, Camera Maintenance & Repair, Book 1, Buffalo: Amherst, 1999.

 

[Andreas Thaler]

The second attempt is a success

I immediately implement my previous experience and dismantled the top cover of the second T90 which also has a faulty control wheel and stuck mirror magnet.

Now nothing should stand in the way of removing the control wheel with its assembly unit.

The cable harness leading to the assembly must be removed from its guide first so that the cables have sufficient space when the assembly is removed.

Three screws need to be loosened.

And the assembly can be easily removed from the handle.

Here is the contact plate of the control wheel with the associated wipers.

Before I start cleaning the contacts, I insert the battery holder and check whether the unit is still correctly electrically connected even after disassembly.

Everything works, the displays respond to inputs.

The four screws for the bayonet are also loose on this T90. I tighten the screws.

Use two sprays of electronic cleaner and then move the control wheel and all adjustment elements on the assembly. This should solve contact problems.

The control wheel is working again. Each click sets a value when turned.

Interestingly, the function had already improved without electronic cleaner just by disassembly.

Very mysterious.

I take this opportunity to clean the two soft plastic buttons with distilled water.

The flexible circuit board is clean, no dirt or dust visible.

 

[Andreas Thaler]

The foam seals are fine, no decay.

A bit of work for the dust brush on the bottom edges of the top cover.

The assembly with the control wheel and the top cover are reassembled.

I put the cable harness back into its guide and fix the cables with a drop of Pliobond, which can be removed if necessary.

The violet electrolytic capacitor shows no electrolyte leakage and looks fine.

I removed some white contamination next to it with electronics cleaner.

The T90 is back in shape.

The mirror triggers and the control wheel works in almost all positions. I'll move it a few more times to improve the contacts further.

There's still some cleaning to be done here, but that can wait until tomorrow.

This T90 has focusing screen C on board.

I think this is the first time I've seen this type

Matte/Fresnel field with clear matte center spot. Especially recommended for macro and telephoto photography, this screen enables the entire field of view to be seen without distraction. The lens is in focus when the subject can be clearly seen.

T90 Focusing Screens

 

[Andreas Thaler]

Conclusion

• As suspected the control wheel fails due to contact problems probably caused by contamination, which can be solved by cleaning.
• Why the control wheel started working better just by removing it and without cleaning its contacts is a mystery to me. I didn't see any problems with soldering joints either.
• After working on three T90s in total, it has now been proven that the mirror magnet is not blocked by magnetic problems, but by contamination of its contact surfaces. The problem can therefore be cured by cleaning.
• You can't solve problems with force, especially not when repairing a camera. I feel sorry for the T90, which I could have got going again. But there's another experience and spare parts left.

+++

All information provided without guarantee and use at your own risk.

 

[Andreas Thaler]

Good work, Andreas!

And now it comes to fixing the dropouts of the control wheel?

Yes, that was the plan. But unfortunately it only worked with the second T90, see report

 

[Andreas Thaler]

After turning the control wheel back and forth a few times, all click positions work perfectly.

Maybe I won't install the front cover at all.

The T90 will look even better that way

 

[koraks]

The T90 has now joined the ranks of spare parts donors.

That's a pity. In your place, I would have re-assembled the camera and stored it away for later diagnosis. After all, there's a good chance of finding the defect since the cause is known, and it's a matter of tracking down the electrical fault. Not much chance of getting to that point once the camera is torn apart into little bits and pieces, of course. Maybe something to keep in mind for the next time? You're in the lucky possession of a number of these cameras and given their scarcity, it seems worthwhile to maximize the odds of getting them back to work again.

 

[Andreas Thaler]

That's a pity. In your place, I would have re-assembled the camera and stored it away for later diagnosis. After all, there's a good chance of finding the defect since the cause is known, and it's a matter of tracking down the electrical fault. Not much chance of getting to that point once the camera is torn apart into little bits and pieces, of course. Maybe something to keep in mind for the next time? You're in the lucky possession of a number of these cameras and given their scarcity, it seems worthwhile to maximize the odds of getting them back to work again.

The T90s are not rare, at least here in Austria, but one that doesn't have a fault is rare indeed.

I have two more ideas about this example: a ground contact in the assembly unit that might not close or the replacement assembly unit has a fault.

I'll know more about the latter when I work on the two acid corpses, then I can look at the wiring in the assembly unit, there's only mechanics in there.

Nothing goes to waste in my box, most things are documented.

But I've also gotten into the habit of not running into every wall anymore. In some cases I give it up in favor of other projects.

And if the respect for a camera is no longer there, it is pointless for me to continue working on it.

 

[koraks]

But I've also gotten into the habit of not running into every wall anymore. In some cases I give it up in favor of other projects.

I understand, of course, however...

And if the respect for a camera is no longer there, it is pointless for me to continue working on it.

...it comes and goes. Put it away and you'll find that a year or so down the road, you feel differently about it and you're willing to take on the puzzle that seemed pointless not so long ago.

Just my €0.02 of course - these are your projects, not mine!

 

[Andreas Thaler]

Just my €0.02 of course - these are your projects, not mine!

But you are welcome to make it your project. Would you like to have it? I would be happy to send it to you. There are plenty of other projects here.

 

[koraks]

That's super kind and generous of you, but there are too many other things I want/need to do first, so it would end up in a box and I might only dig it up two years down the road. That doesn't feel quite right to me!

 

[Andreas Thaler]

In such cases, I consider the presumed effort versus the chance of success.

After thinking about it, the ground contact cannot be the problem, so the assembly with the control wheel remains.

There, six fine cables establish the connection to the camera electronics. All I can do specifically is redo the soldering points and check the wiring in the assembly with another, presumably intact one, for any abnormalities. To do this, I have to remove the assembly from another T90. I still have three in the box whose shutter magnets don't work, so they are de facto hopeless cases.

If that doesn't work, I'll be left with the electronics of the T90 on my own. Without meaningful circuit diagrams, SMD construction, ICs, many of them inaccessible. Maybe I've already caused a short circuit and destroyed the electronics, who knows.

That's why I draw the line here, even though I naturally want to solve every case.

In any case, I'm glad that I can fix the most common errors on the T90, apart from the inactive shutter magnets.

This means there is a good chance of getting more patients fit again.

 

[Andreas Thaler]

Unfortunately …

… I made a grave mistake when I tried to remove the assembly with the control wheel.

I followed Thomas Tomosy's excellent instructions1, but the thing just wouldn't come off the camera.

View attachment 374917

Until I used the screwdriver as a lever and damaged the assembly.

I was able to install and solder a replacement, but the T90 responded with „bc“ on the display.

View attachment 374883

There was nothing I could do.

When I then read Thomas's instructions again, the text said „Remove the top cover“.

By then it was too late

View attachment 374882

This mistake could have been avoided.

The T90 has now joined the ranks of spare parts donors.

---

1Thomas Tomosy, Camera Maintenance & Repair, Book 1, Buffalo: Amherst, 1999.

Today I looked at this T90 again and replaced the assembly with the control wheel and shutter release.

But unfortunately it didn't work, „bc“ appeared again on the LCD and I couldn't find any faults on the circuit board.

That's the end of the line, especially since the Canon service manual for the T90 doesn't contain any troubleshooting instructions.

So I'm giving up on this model for good.