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.