Canon T90: Cleaning the mirror magnet

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Andreas Thaler

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Now I was able to verify that contamination is the cause of the problem with the mirror magnet.

Larry Lyells, himself:



Canon T90 Repair Guide

From the Society of Photo-Technologists Journal
learncamerarepair.com


This means that the common fix with the magnetized screwdriver is no longer necessary, as this does not solve the problem or does not solve it permanently.

The sustainable solution is to remove the magnet and clean its three contact surfaces.


With this I close the topic.


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moggi1964

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Andreas, I cleaned the contact surfaces on my T90 but it didn't prevent the issue when the camera had sat for a week. I have found a solution that has been working well:

I dropped a 2x2mm neodymium magnet (0.10 kg pull force) onto one of the contact surfaces and so far the camera hasn't had a flickering drive arrow or failed to fire.
 

Taylor Nankervis

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The T90 was my fifth camera, a masterful, if daring design, a real tour de force and conversation starter for its time. It introduced me to what Canon was tinkering away with that would later be the Canon EOS system, of which the T90 pioneered a great many design features.
 
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Andreas Thaler

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That's interesting, because I got several T90s up and running by cleaning the magnet.

I also find your solution very interesting.

Do you have any photos of it? How did you manage to attach the additional magnet?

Or did you only temporarily magnetize the original magnet with the neodymium one?
 
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Andreas Thaler

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Yes, the T90 can still be overwhelming today with its many functions.

When the camera was introduced in the photography magazines in the mid-1980s, I at least couldn't keep up with what was suddenly technically possible.

The Minolta (Maxxum) 7000 in 1985 offered a foretaste.

That was a huge leap away from previous cameras, which were comparatively straightforward.
 

Taylor Nankervis

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My only readily available recollection of one problem was battery life with the T90; this may have related to my use of lithium batteries at the time, and the camera's drive on H for rapid shooting of what my interest back then in the mid-1980s was — bird watching. Still, keeping the T90 fed with batteries (x4 AA) was cheaper than keeping today's power drive booster E1 (8 x AA!) for the Canon EOS 1N fresh and live doing the same thing!
 

moggi1964

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I'll get a photo soon so you can see. The magnet stayed attached to the metal contact and will I guess continue to magnetise for quite some time.
 
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Andreas Thaler

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moggi1964 said:
I'll get a photo soon so you can see. The magnet stayed attached to the metal contact and will I guess continue to magnetise for quite some time.
Click to expand...

As far as I understand, it is a permanent magnet whose magnetic field is neutralized by the flow of electricity and thus opens.

But that doesn't match what I observed when removing it.

As soon as the magnet is removed from its holder, the mirror folds upwards.

So the magnet opens. But no current can flow because the battery part was removed beforehand, any capacitors are discharged over time and the backup battery has other tasks and would not last long to supply power to the magnet.

So, as always, it is more complicated than expected
 

moggi1964

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The little bit of debris on the battery is glue from an earlier attempt.

 

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ic-racer

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Coils like this usually run on DC. Hooking them to an appropriate battery or power supply should make them stick to metal.

One way to separate good from bad is to see how strong they are with a given voltage and test the resistance of the coil between good and bad ones.

Yes difficult, but not impossible to repair these by re-windiding or finding the break in the wire.

 
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Andreas Thaler

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Here I had a mirror magnet for the T90 with a broken winding. Fixing it requires more than a steady hand.

I'll summarize what I think I know:

On the left are the connections, i.e. when current flows through the winding (coil), a magnetic field is built up that counteracts that of a permanent magnet (armature) and cancels it out. Then the armature is released and with it a switching contact.

So it is an electromagnet like in the video you show.

The question is whether the current flows through both windings or whether - as with a transformer - there is induction that transfers voltage from one coil to the other via a magnetic field, possibly with a voltage difference. There are nice formulas for this in electrical engineering.

How it all fits together, whether it happens like this or differently, I can't say.

I only know from experience that the armature is released as soon as the electromagnet with its two coils is removed from the camera chassis. That could mean that the permanent magnet is attached to the chassis.

Maybe someone here can say more about it.
 
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Andreas Thaler

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In-depth information about the technology of the Canon T90 can be found on this site (Italian), which I have already presented here:

La Canon T90

La canon T90, l'ultima ammiraglia FD. Febbraio 1986, Canon lancia sul mercato la T90: era estremamente innovativa e nel cont...
sistemacanonfd.blogspot.com

Regarding the magnets, see these comments (Google Translate):



La Canon T90

La canon T90, l'ultima ammiraglia FD. Febbraio 1986, Canon lancia sul mercato la T90: era estremamente innovativa e nel cont...
sistemacanonfd-blogspot-com.translate.goog

(Google Translate)


Based on this explanation, I understand that both coils are attached to a permanent magnet that holds the armature as a switching contact.

If current flows through the coils, a magnetic field is built up that cancels out the magnetic field of the permanent magnet and thus releases the armature.

However, this does not explain my observation that the armature is released even when I remove the unit with the coils from the chassis (mirror folds up).

This can only mean that the permanent magnet is attached to the chassis (see my comments above).


In any case, for the repair only reliable solutions come into consideration if the mirror does not trigger (flashing arrow in the T90 display).
  • So far I have been successful with cleaning the contact surfaces of the armature and permanent magnet (?),
  • @moggi1964 shows a solution with additional magnets,
  • and the fix with a magnetized screwdriver that you run around the unit is the well-known one.
 
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moggi1964

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This is wonderful work, Andreas. Following your various journeys inside cameras is fascinating and truly enjoyable.
 
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