Canon T90: Tutorial for DIY service and repair; excursus technical details

[Andreas Thaler]

Interestingly, the LED lights up at just 1 volt of forward voltage. I'm familiar with values of 2 volts and higher, but I'm no expert.

The structure of the LED is also interesting.

I wonder if this isn't a light bulb with a filament?

 

[Andreas Thaler]

Interaction between mirror, aperture control and FDn lenses

To better understand the interaction between mirror, aperture control and lens, I take a look at the removed mirror box (not complete) of a T90.

Front view, the mirror is cocked and ready to fire.

Aperture control, rewind site.

Mirror mechanism, release side.

View from above.

View from below.

I use the probe to pull the armature away from the release electromagnet.

This corresponds to the current flow through the coil on the magnet when it is triggered, which releases the armature.

The mirror has triggered.

I tension the mirror.

The mirror is folded down and ready to be released again.

Now I attach a FDn 50/1.8 to the mirror box.

The aperture ring is in the automatic position (A).

The mirror is still tensioned.

Front view, with the mirror folded down.

The subject can be viewed through the viewfinder (no longer present here).

View of the aperture control.

Using the probe, I release the aperture via a lever (white arrow).

The wiper moves down over the flush plate (magenta arrow).

The aperture control locking pawl is not engaged yet (blue arrow).

Now I release the mirror.

The locking pawl engages.

The aperture is closed to the minimum value (f = 22).

 

[Andreas Thaler]

Now I set aperture f = 4 manually.

I release the aperture.

The wiper moves a little over the flush plate.

The locking pawl has not engaged.

Now I trigger the mirror, the locking pawl engages.

The lens is stopped down to f = 4.


My understanding of the sequence in automatic mode (A) is

• that, when the T90 is triggered, the aperture first closes.
• To do this, the aperture electromagnet is triggered electronically, releases a lever which clears the wiper moving down the flush plate.
• I triggered this lever manually with the probe.
• If the aperture is larger, the wiper moves less downwards.
• If the aperture is smaller, the wiper moves more downwards.
• The locking pawl secures the wiper in the respective position as soon as the mirror is released via the release magnet.
• In short, the aperture magnet starts the closing of the aperture and the release magnet stops it at the right time, depending on the aperture value.
• The gold-plated contact track on the flush plate, together with the movable wiper, reports the respective aperture position to the control electronics of the T90.
• The whole process is synchronized and takes place in milliseconds.

The manual mode with aperture pre-selection on the lens should only differ in that the aperture control clutch on the lens controls the position of the wiper on the flush plate mechanically.

 

[Andreas Thaler]

Here are the FDn coupling elements in the mirror box and on the lens:

Yellow: stop down lever in the mirror box and corresponding stop down clutch on the FDn lens.

Turquoise: aperture control lever in the mirror box and corresponding aperture control clutch on the FDn lens.

I attached the bayonet ring of a T90 to the lens to simulate the position of the aperture when the lens is attached.


The stop-down lever in the mirror box (yellow) always moves fully toward the release side when the shutter is triggered.

The aperture control lever in the mirror box (tourqoise) determines the aperture setting and has two modes:

1. In automatic mode (A on the aperture ring of the lens), its movement is electronically controlled by the T90. It moves less for large apertures and further down for small apertures.
2. In manual mode (aperture selected on the lens), the aperture control clutch on the lens moves to the appropriate position, thus setting the aperture.

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That all is my assumption, unfortunately I have not found any description of these sequences and correlations in my technical documents.

 

[Andreas Thaler]

Aside from general interest, these insights are important for troubleshooting a problem on the T90 featured in this thread.

The aperture only closes to f/8 in both automatic and manual mode.

When I remove the mirror box, I now know that I have to inspect the aperture control lever, its mechanics, and its controls.

 

[Andreas Thaler]

The position of the two electromagnets on the mirror box of the T90 (rewind side):

• release magnet (turquoise)
• aperture magnet (magenta, partly hidden)

 

[Andreas Thaler]

More on this soon in my repair report.


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