Canon T90: release magnet, EEE/HELP/bc, C & C as saviors, mirror shock absorbers and cover, front flex connector acting crazy/alternative?, victory

[Andreas Thaler]

Having suffered setbacks with my recent T90 projects, my pride was bruised

So I made another attempt and pulled the next T90 out of the repair box.

Today I had to succeed!

Both mirror shock absorbers are missing along with their mountings, the cover is no longer where it should be.

Battery electrolyte in the battery compartment …

… and on the tripod thread.

The ground cable has come loose from its soldering point due to the electrolyte.

Since it was also contaminated inside, I replaced it,

The T90 is alive, but it doesn't fire, and the arrow on the LCD is flashing.

Clearly, the release magnet Mg2 is dirty and needs to be cleaned.

I'm using a new method to get better access to the hybrid magnet screw.

After removing the two retaining screws on the DC/DC board, the screwdriver can be positioned at a right angle.

This allows the torque to be transmitted to the magnet’s screw more effectively than previously at an oblique angle.

Done


I clean the magnet with benzine and reinstall it.

The shutter now releases …

… but immediately afterwards my old friend appears.

The C & C Troubleshooting guide for the T90 has a solution for exactly this error scenario:

LCD displays EEE, finder Help after mirror cycles.
Shutter operates for one cycle before EEE.
Power, control dial, lightmeter, release normal.

Both shutter magnets are electrically triggered directly via the shutter connector. The position of the shutter switches (SW18, 19) is then determined with a multimeter.

The measurements confirm what is seen: The first shutter curtain has triggered, but the second has not.

Here you can see the switches after removing the shutter. SW18 is open (first shutter curtain) …

… SW19 is closed (second shutter curtain).

This blocks the camera, which it communicates with EEE/HELP.

 

[Andreas Thaler]

The plastic adjusting screws for the shutter curtains are intact.

I clean the two shutter magnets with benzine.

The sticky damper is removed from the shutter.

I replace the cover and the two mirror shock absorbers from a T90 for spare parts.

Spare T90: The cover has already been removed.

I'm still dismounting the dampers.

The spare parts.

Assembled

I check the mirror mechanism and the aperture control.

Everything's OK.

A clean battery compartment comes from the spare parts stock.

Cleaning foam, the most efficient method to clean housing parts.

A picture from hell!

But this error message is fine; it appears when the aperture slider is operated and the lens is in A(utomatic) mode.

After assembly, the T90 works. Only the LCD backlight won't turn on and the viewfinder LEDs won't turn off.

Then follows an hours-long battle with the front flex connector, which connects three circuit boards by pressure.

The connector goes crazy and makes a motor run, EEE appears, then bc, then the T90 is dead, then back on again. When I put the front panel on, it starts all over again, and so on.

Only when I tightened the two retaining screws on the connector's pressure plate to the correct ratio did the haunting stop.

Now also the LCD backlight works, and the LEDs can be turned off.

Madness - but victory!

 

[Andreas Thaler]

On the camera tester: 1/4000 second

1/60 s

Automatic exposure in program mode.


Everything is fine, the shutter is just a bit on the slow side.

 

[Andreas Thaler]

Conclusion

• You must not give up and must hunt down the dreaded EEE/HELP and bc.
• Technical instructions (see also my other T90 project reports) help with this or make the hunt possible in the first place.
• The front flex connector truly comes from hell. It doesn't tolerate any deviation when tightening it’s pressure plate. But it has to be overcome when you disassemble the T90 for service and repair.

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This concludes my coverage of the T90.

I hope I've given you the courage to take on this magnificent diva.

After a longer break, I'll be focusing on the Canon A-1, the T90's predecessor. It's just as impressive, and there's excellent technical documentation available for it.

Good luck with your T90 repair projects!


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

 

[Andreas Thaler]

Access to the mirror box mechanism and the shutter with the front flex connector closed

The front flex connector proved to be a key point when removing the mirror box and shutter.

When reassembling the T90, contact problems can arise here, undoing all the previous repair work.

Therefore, I looked for a way to remove the mirror box and shutter with the front flex connector closed.

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All pictures of a T90 for spare parts:

Front flex connector closed with pressure plate.

Beneath the pressure plate, three circuit board components are sandwiched together.

A large portion of the T90's data communication passes through this connection.

Between the gold-plated contact rows are elastic connector strips that, after decades of attachment to the contacts, have become slightly stuck.

I've always removed them when opening the connector. I think it's a good idea to only remove them on one side to minimize disruption to the entire assembly.

I was able to remove the mirror box to this position with the front flex connector closed.

The mirror box is still connected to the housing via two circuit boards.

This also makes the shutter underneath accessible which can be removed for service.

The dismantling effort is reduced.

There is also no need to desolder the cables on the front flex connector.

The AE unit on the side of the mirror box and the auto-diaphragm unit below it can be completely removed for service work.

See also

Post in thread 'Canon T90: Service and repair using practical examples'
https://www.photrio.com/forum/threa...-using-practical-examples.213586/post-2898959

Post in thread 'Canon T90: Service and repair using practical examples'
https://www.photrio.com/forum/threa...-using-practical-examples.213586/post-2898950

The retaining screws of the shutter are also accessible (the third screw at the rewind side of the shutter is directly accessible).

With patience and sensitivity, the shutter can be removed as usual.

It just catches slightly on the plastic plate beneath the front flex connector, which would otherwise be unscrewed.

Installation should also be possible this way.

Here, you have to pay attention to the flexible circuit board strip that connects the shutter to the main board.

I had rotated the shutter twice to get to a position where I could open it. In doing so, the circuit board tore and a circuit track was severed.

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Conclusion

Is this method a realistic alternative to opening the front flex connector when disassembling the T90?

Does it eliminate the risk of potential contact problems?

• If it's about working on the mirror box, cleaning the shutter magnets, or checking the condition of the adjustment screws on the two shutter curtains (the screws are made of plastic and can break. The T90's shutter can be adjusted using these screws even when the shutter is installed. You just need to know their condition.), then yes.
• If the shutter needs to be disassembled to remove the sticky damper, then I'm undecided. Since the shutter is attached to a strip of circuit board, its movement is limited. Working on it isn't impossible, but it does require some preparation to ensure it's securely fixed. In any case, damage to the circuit board must be avoided.
• The shutter should also be able to be reinserted. Despite the friction point with a cover plate.
• This method puts stress on the board connections. Anyway I couldn't find any damage except for the area around the shutter.
• Whether it works will only be shown by a brave attempt with a T90, which is to be repaired in this way

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

 

[moggi1964]

Thank you.