Canon T90: Aperture issue resolved, shutter disassembled and cleaned, alternative method of removing the mirror box has proven itself again

One of the T90s I recently had on my desk, see


is exhibiting a problem with the aperture.

Under the same lighting, the aperture opens wider at one point, then narrows at another.

I suspect a dirty contact track for the aperture control inside on the mirror box and/or a dirty aperture magnet.




Arched aperture contact track.




Aperture magnet Mg1 (green arrow) and release magnet Mg2 (magenta arrow).


Since I chose this T90 for my photography work and want to stay in training, I'll start repairing it soon. To do this, I'll remove the mirror box without opening the front connector to avoid contact problems during reassembly, see


Stay tuned!

The regular division of the contact path on the aperture control indicates that an analog signal is being converted into a digital one.

As the wiper moves along the contact path, it generates a voltage pulse (ON/OFF) whose duration depends on the distance traveled.

This pulse controls a digital counter, and the T90 uses this to determine how wide the aperture is open and can close it accordingly.

That's my simplified assumption.

Removing the mirror box with the front connector closed, servicing the shutter

When removing and installing the T90's mirror box, problems can occur with the front connector, which connects three flexible circuit boards. Contact issues can render the camera inoperable, rendering all previous repairs useless.

Instead of opening the front connector, I've developed an alternative method for removing the mirror box that leaves the front connector closed. This avoids contact problems during reassembly.

See details here:


I used this method again today with success.

To access the aperture magnet for cleaning, the mirror box has to be removed.

I also use this opportunity to clean the release magnet, which no longer works (flashing arrow on the display).

Both magnets are installed in the same place.




The battery compartment and covers have been removed.




The mirror box is unfolded and not separated from the camera. The front flex connector on the release side remains closed.

Here, the T90 is already connected to my lab power supply to test the charge motor coupling. I use this to test whether the shutter, mirror, and aperture are cocked properly after reinstalling the mirror box.




The shutter is a surprise.

The second damper has also become sticky; I had already removed the first one during the initial repair session.

So I have to disassemble and clean the shutter again.




Due to the sticking and operation of the shutter, one of the rear curtains’ slats is bent.

There's nothing that can be done to straighten it; the slats are too flexible, but I have a replacement.




The T90's vertical focal plane shutter is easy to disassemble, unlike other SLRs.




Happy is who has original spare parts




The complete rear shutter curtain (first curtain) as a spare part from a discontinued T90.

I clean the sticky parts and assemble the shutter with the new curtain.




Both black worm screws for tensioning the curtains are intact.

They break easily on the T90, so their condition should also be checked during a shutter service like today.


Cleaning the magnets



Both magnets are located at the rear, bottom left of the mirror box.

Here, on the folded-away mirror box, you can see the release magnet with its two copper coils at the top.




The release magnet in closeup.




After loosening its retaining screw …




… I can clean the contact surfaces of the magnet with benzine.

You can also remove the release magnet for cleaning from the outside, see





The aperture magnet is located directly above the release magnet.

You can remove it for cleaning, see

Post in thread 'Canon T90: Service and repair using practical examples'
https://www.photrio.com/forum/threa...-using-practical-examples.213586/post-2898803

but I don’t want to do that because the unfolded mirror box is unwieldy to work on.



So I bend the tip of a cleaning swab with pliers to reach the contact surfaces of the magnet. I clean again with benzine.




It looks worse than it is …

I also clean the arched aperture contact track on the flush plate with electronic cleaner.

The corresponding wiper under the cover above receives a spray.




This cover became loose.






The cover is quickly reattached.




Reassembly.




Today I have enough space to work






First test.

Cleaning the magnets was successful. The T90 is firing again, and the apertures are forming evenly again. I check this in high-speed mode.


Performance on the camera tester





1/4000 second as the shortest shutter speed.






1/60 second.






Program mode.


Very nice results.

The shutter is a bit on the slower side, but it exposes the image evenly, which is crucial.

The automatic exposure is also OK.

Conclusion

• With the T90, the four magnets are the prime suspects when problems arise. EEE/HELP, no shutter release/a flashing arrow on the display, or an erratic aperture control can often be traced back to dirty magnets.
• I now recommend always disassembling the shutter and removing both sticky dampers. It's obvious that otherwise, the delicate slats can suffer irreparable damage.
• Keeping the front connector closed avoids the risk of contact problems, which in the worst case scenario are impossible to resolve.

+++

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

Erratum

I replaced the front (second) curtain.