Canon T90: Replacing the mirror magnet

[Andreas Thaler]

The mirror of one of my T90s didn't want to move.

The symptom is well known: When you press the shutter button, an arrow flashes in the display and you hear a quiet ticking sound.

Otherwise the camera remains still.

I opened the camera and tapped the mirror magnet with a magnetized screwdriver to trigger it. This is the usual fix for this problem

But that didn't help.

I pushed the anchor away with a spring hook that I threaded into the housing via the mirror magnet.

Now the shutter clicked.

Interesting!

What irritated me, however, was the fact that there was no quiet ticking sound when you pressed the shutter button, as is usual with this problem.

I concluded that the coils of the mirror magnet were not receiving any voltage.

I examined how the mirror magnet is attached to a removed mirror box from a spare part T90.

Surprisingly simple: With a screw and some glue, presumably to hold it in place to prevent the screw from loosening due to vibrations.

It should be possible to replace the mirror magnet without dismantling the camera.

And actually, it worked

(The arrow points to the mirror magnet, which is already detached from its attachment.)

In order to loosen the screw with the screwdriver, the battery compartment must be removed. Otherwise the angle is too large for a (normal) screwdriver and it cannot grasp the screw head slots.

The mirror magnet can be removed from its holding plate after loosening a fastening screw.

The anchor remains in the housing and is attached to a lever of the mirror mechanism.

I cut the power supply cables with the side cutters.

And the wire actually came off on the right coil.

No current flow through the coil, no magnetic field that separates the armature from the permanent magnet.

I cleaned the contact surfaces of the replacement magnet (from an abandoned T90) ...

... and the contact surface of the armature with electronics cleaner.

The replacement magnet is already sitting on its holding plate in the housing ...

... and is fixed by its retaining screw.

Connection to the power supply by soldering the cable ends together and applying pieces of shrink tubing to stabilize and insulate the connection points.

And the shutter released again


Unfortunately, this T90 also has problems with the shutter magnets, which I cannot fix because they are installed in the housing inaccessible to me.

Also the method with the rubber mallet (hitting the camera base to activate the magnets didn't help.

But that doesn't matter.

Spare parts are always welcome - and from now on a defective mirror magnet ore one which cannot be activated with the magnetized tip of a screwdriver no longer has to be a fate

This T90 service day ended 3:1 for the error devil

Three T90s with a locking shutter magnet problem go into the spare parts box until a solution is found here too.

After the check, a total of eleven T90s have the green dot for “passed”.

A red sign that says “sticky” means that the damper that has become sticky must be removed from the shutter to prevent the shutter blades from jamming one day.

What is pleasing is that the diabolical Shutter Magnets locking error (“EEE”, “HELP”) has not yet occurred with my long-term stock.

Regular movement of the shutter may actually be the best way to prevent this.

+++

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

 

[Andreas Thaler]

Shutter magnets

In the SPT Journal/Contribution to the Canon T90 you can find this information on page 31:

Release magnet MG2

Unsolder the release magnet wires
(orange and black) from the top of
the flex, Fig, 28. Then apply around
3V between the release magnet
wires ( + to orange, - to black). The
mirror should release. If not, the
problem is the release magnet. If
the LCD drive-mode arrow flickers
(release probably erratic or fails
after the camera has been sitting),
the problem may be a dirty release magnet
interface.

(BLUE in quote = own emphasis)


That would also explain why the two magnets don't work.

By the way, there is a fourth hybrid magnet that works in the aperture control.

The article in the SPT Journal presents the technology of the T90 on 36 pages.

Extensive explanations of the circuit and detailed troubleshooting tips.

Much more informative than Canon's service manual. At least in this regard.

After looking through Canon's service manual and the SPT article mentioned, I currently see no way to access the shutter magnets without damaging the camera.

SPT describes around 35 steps for removing the mirror box, a large part of which involves unsoldering cables. Given the density of installation with flexible printed circuit boards and cable harnesses, this is a tricky undertaking; you have to keep track of everything and then reassemble it.

The question then arises as to what should be done with the two magnets on site.

The windings around the coils made of the finest wire are soldered to the control electronics installed above them.

The two magnets are integrated into the locking mechanism, the anchors are obviously firmly connected to a lever in the mechanism.

Disconnecting it and then reassembling it is tricky again.

And the question of how the anchors can be demagnetized in a controlled/reproducible manner is still open.

I'm not saying that all this is impossible, but it needs to be well prepared.

Documentation during dismantling, practicing soldering work on a abandoned T90, finding a way to remove the anchors and then readjusting the magnetization/demagnetization.

Even if the magnets and the anchors are to be exchanged for two functioning units, the removal and installation work remains, with the removal of the donor camera also being necessary.

Canon, you've built a fortress there

+++

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

 

[nusproizvodjac]

I'm sorry for necroposting, but this thread seems appropriate.

I started working on the T90. The battery holder was atrocious, but I managed to clean it. The camera released once, and stopped. I could hear the faint clicking inside of it, and the cursor was blinking the whole time, no HELP or EEE message. The viewfinder LEDs would go with the shutter button pressed.

Here's what I noticed so far. The release magnet was clean, and its resistance was around 66ohm, so well within specs. Upon removal, the magnet sticks something mettalic to itself, and with +6V applied it relases the object.

With the magnet removed, I can still hear clicking somewhere in the camera, and If I can deduce correctly, the release magnet moves the armature away, but it does not release the curtains. I can move the armature of the release magnet by hand, so it's not stuck.

What's the next course of action? Thanks in advance!

 

[Andreas Thaler]

Cleaning the release magnet should resolve the flashing arrow on the LCD and the faint clicking sound.

If the shutter doesn't fire, it could be due to the two shutter magnets, which also need cleaning, and/or the sticky damper inside the shutter that holds the shutter curtains in place. "EEE/HELP" may appear in the viewfinder and „EEE“ on the LCD.

But these aren't the only possibilities.

I suggest you take the long route.

Here is my current recommendation for the T90, including service and repair information:

Canon T90: Guide for service and repair work

After four years of intensive service and repair of SLRs from the 1980s, I'd like to summarize my experiences with the Canon T90. The T90 was released in 1986 and is considered arguably the most powerful manual focus 35mm camera ever produced. Canon no longer offers service and repairs; in my...

www.photrio.com

I would approach troubleshooting in this order:

1. SPT Journal, which contains detailed troubleshooting instructions.
2. C & C Guide, which offers even more information, particularly regarding the error codes EEE/HELP and bc.
3. My supplementary documentation, which is linked there.

If you want to access the shutter magnets, you’ll need to remove the mirror box.

The only potential complications can arise when opening the frontflex connector. That’s why I’d recommend leaving it closed; I’ve developed a method for doing this, but you have to be careful not to damage the FPC, see link above or

Canon T90: Solution found for the stubborn front flex connector, service and repairs on the T90 are no longer a gamble

It's happened to me several times now that I've serviced a T90, only to find the work was in vain because of unsolvable contact problems in the front flex connector. The front flex connector is located on the front release side, where the red self-timer LED is located. Three flexible circuit...

www.photrio.com

Otherwise, it involves a lot of soldering and removing parts.

 

[Andreas Thaler]

Since the problem can have several causes, I’m sorry I can’t provide any more specific solutions. However, with the documentation provided, you should be able to make progress.

Please submit a report on your work

 

[nusproizvodjac]

You sir are a gold mine! I'll get to it over the next couple of days, I'm still waiting on my bench power supply to arrive, it's too big of a hassle with the batteries and holders.

 

[lumoStoria]

Since I already read several reports on the replacement of the solenoids of Canon A series cameras I'm wondering whether the "dying" of these parts is more common for Canon cameras than for other manufacturers. I'm not quite sure why they "die" since (from a naive point of view) there shouldn't be any physical wear. Is it because the wire is too thin for the currents occuring (like a classical light bulb dying after ~1000hours)? Is it the heat produced during operation so that the lacquer insulation burns off, resulting in partial short circuits? Is it an interaction between the magnetic field and the wire that results in small movements of the wire that breaks it over time?

Any information on this?

 

[Andreas Thaler]

You sir are a gold mine! I'll get to it over the next couple of days, I'm still waiting on my bench power supply to arrive, it's too big of a hassle with the batteries and holders.

Thanks

I've written a lot (too much) about this, and it is time to summarize it. But if it helps with your own work, it makes sense.

When connecting the T90 to a laboratory power supply, note the yellow cable in the battery compartment, which has a reset function activated by a switch when the battery is replaced. This reset is then performed manually by connecting the cable to ground. You can find more information on this in the SPT Journal.

 

[Andreas Thaler]

Since I already read several reports on the replacement of the solenoids of Canon A series cameras I'm wondering whether the "dying" of these parts is more common for Canon cameras than for other manufacturers. I'm not quite sure why they "die" since (from a naive point of view) there shouldn't be any physical wear. Is it because the wire is too thin for the currents occuring (like a classical light bulb dying after ~1000hours)? Is it the heat produced during operation so that the lacquer insulation burns off, resulting in partial short circuits? Is it an interaction between the magnetic field and the wire that results in small movements of the wire that breaks it over time?

Any information on this?

No, nothing is dying there from my point of view; it's usually contamination of the contact surfaces in the solenoid. Unless, of course, the solenoid coils are overloaded due to a short circuit—which could be caused by moisture, for example.

 

[lumoStoria]

You sir are a gold mine! I'll get to it over the next couple of days, I'm still waiting on my bench power supply to arrive, it's too big of a hassle with the batteries and holders.

I strongly advocate the use of a bench power supply when doing repairs of electro-magnetic SLRs. For the Canon A series I built myself a professional looking battery adapter :

Beside from saving you the costs of new batteries, a bench power supply has two major advantages:

• You can set a current limit so that damages of the camera can be prevented (or, at least, are not severe) when you short circuit anything accidentally. For most SLRs, a current limit of 100mA should be sufficient.
• You can see the current drawn by your camera at all times (in the picture above, the current is 0.015A = 15mA). This can help to identify errors. Service manuals and journal articles sometimes contain expected current values for different operation modes. For example, a Canon A-1 should draw about 29mA when metering, 39mA when the shutter is open, and 60mA at battery check.

(In the picture above, the camera draws 15mA while switched on but completely inactive. Despite several attempts, I never found out what caused this "battery drain". Rest in peace, poor Canon.)

 

[Andreas Thaler]

For most SLRs, a current limit of 100mA should be sufficient.

When motors are installed, like in the T90, significantly more current flows, so the motor connections in these SLRs are dimensioned accordingly.

If you set your A-1—which isn't the subject here—to the MA motor, you'll probably reach that range at the drive.