Canon T90: systematic troubleshooting, DC/DC converter replaced, one LED digit missing, sticky shutter, EEE again, summary

Andreas Thaler

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Checking a Canon T90 according to the troubleshooting instructions by Larry Lyells in the SPT Journal.

Canon T90 Repair Guide

From the Society of Photo-Technologists Journal
learncamerarepair.com




No sign of life.






Battery compartment removed, measuring the input voltage on the DC/DC converter.






Front and top cover removed for further measurements.






A double diode needs to be tested. To do this, I'm desoldering it with hot air.








The diodes are ok.






Soldered in and back in place.




The DC/DC converter, which generates different voltages from a 6-volt input (4 x AA batteries), has incorrect voltage readings.

There are obviously cold solder joints on the circuit board that I can't repair.

The converter is defective; I have a replacement in my stash.
 
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koraks

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Andreas Thaler said:
here are obviously cold solder joints
Click to expand...

It's hard to make out on the photo, but it looks like that part of the PCB overheated. Something downstream might have shorted out, causing overload on the DC/DC converter. That's just a guess though.
Either way, I'd be cautious replacing it since you might just end up frying the next one, too.
 
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Andreas Thaler

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DC/DC converter. Front and back




After the replacement, the T90 is back to life. All functions are working.




One of the seven-segment displays in the finder is missing a digit. I would have a replacement LED.




As with all T90s, the sticky shutter is also present here.




There is still some work to be done
 
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Andreas Thaler

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Yes, a transistor may have been overloaded; it doesn't look good. However, I desoldered it at high power. It's safest to replace the entire assembly.
 

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Andreas Thaler said:
However, I desoldered it at high power.
Click to expand...

Not sure what that means.

Andreas Thaler said:
It's safest to replace the entire assembly.
Click to expand...

What I meant is that the DC converter supplies power to other parts of the camera. The failure of the DC converter may be due to failure elsewhere. Monitor power draw on the converter closely.
 
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Andreas Thaler

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I'll monitor this and measure the current flowing through the converter. Voltages too, which are measured via the back of the circuit board.
 

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I now see you added a photo to #4 that I didn't see before. That switching transistor is fried, obviously, and it's not due to desoldering. The question is why it broke.
 
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Andreas Thaler

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Current and temperature measurements



I measured the following currents in the T90 (without film, connected to the laboratory power supply; in brackets the reference values according to the SPT Journal):
  • Main switch OFF: 7,8 uA (7,0 uA)
  • Main switch (LCD) ON: 44,5 uA (430 uA)
  • Trigger switch (lightmeter, LEDs and LCD illumination) ON: 109 mA (190 mA)
  • Serial shooting in motor mode HIGH: 600 mA (650 - 700 mA during film advance after exposure)
Additionally, I measured the surface temperature of the connectors on the DC/DC converter board with an IR thermometer.

The temperature remained constant even during serial operation.

I conclude that the problem was with the DC/DC converter and that replacing the board resolved it.


Possibilities to replace the viewfinder LEDs

A digit in the viewfinder display (a bar on one of the seven-segment LEDs) has failed (see the thread). This means that the displayed values can still be read.

I've looked into possible replacement options.






Position of the board with the LEDs.








Unit diassembled (on a spare parts mirror box).






The LED unit is attached to the FD board, which is soldered to the mainboard (blue arrow). It can be removed.




Alternatively, only the LED array could be separated from its circuit board. This can be done using hot air.

However, soldering it back on is a delicate process; the connections would have to be tinned and then re-soldered with hot air, out of sight.


Conclusion
  • Both options are risky.
  • Replacing the FD board with the LED array also involves replacing an IC that presumably controls the LEDs. I don't know the board versions or whether they're compatible, so it would be an experiment.
  • Soldering and re-soldering the LED array is tricky, and if it's not done correctly, it could render the connection unusable.
  • I'll therefore leave everything as it is. The failure of one digit doesn't justify the risk.
  • It remains to be seen whether the failure is related to the previously defective DC/DC converter (possible overload). I'll continue to monitor this.
 
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Andreas Thaler

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Contacts on command dial



(Picture from another T90 repair project.)


The adjustment wheel (command dial) on the grip near the shutter release does not change the values on the LCD in click increments.

This means that the corresponding gold-plated contacts on a disc in the grip must be cleaned.

This is not a big effort; see

Post in thread 'Canon T90: Address unresolved problems; broken plastic parts in Copal S shutter, epoxy, EEE/HELP/bc errors, troubleshooting partially successful'
https://www.photrio.com/forum/threa...ting-partially-successful.213404/post-2892278
 
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Andreas Thaler

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Unfortunately, this project was also unsuccessful

I was able to disassemble the shutter, clean it, and adjust it on the camera tester.

After assembling the T90, the charge motor started briefly as soon as I applied voltage, followed by an EEE on the display.



By lifting the mirror box and operating the mirror switch (SW13) I was able to start the camera for a moment.

The SPT Journal has a dedicated troubleshooting instructions for this.



And so I was able to locate the error. It appears to be a contact problem in the front flex connector under a metal pressure plate.



Here, three circuit boards are connected one above the other with gold-plated contacts.



Elastic connectors with gold-colored inclusions are located between them.

By tightening the two pressure plate screws differently, I was able to get the T90 working again.

Only during continuous operation did the mirror occasionally stop working. Inserting two elastic connectors one above the other improved performance.

I also cleaned and polished the externally accessible contact surfaces of the release magnet. But as I touched the release magnet with the screwdriver, the camera jammed and EEE was permanently on the display.

This problem now occurred with two T90s after assembly. The cameras worked before.

I suspect that after parting the boards, the connection via the elastic connectors no longer works reliably.

I couldn't figure out how these work. They connect all rows of contacts without short-circuiting them, but must still be conductive because they connect two boards each.


I've now ruled out any mistakes during disassembly and reassembly as it already worked and I strictly followed the instructions by Larry Lyells in the SPT Journal.

It's a shame, but I'll keep at it.

A few defective T90s are still waiting in the box
 
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Andreas Thaler

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This makes it clear why this T90 has not left the EEE error status anymore.

Either because the mirror box was pulled out or because the mirror mechanism came into contact with the screwdriver (and the resulting blockage of the charge motor), the mirror charge lever was bent.

This meant that the mirror could no longer be tensioned, and if a station fails during tensioning and release, all subsequent stations also come to a standstill. The T90 reports this with EEE. Switches built into the mechanism report to the control electronics when a process is completed and the next one can start.

Lesson learned and the problem has been identified. This should make a solution possible.



Does anyone know of these elastic connectors? Is there a replacement or alternative?
 
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koraks

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Andreas Thaler said:
Does anyone know of these elastic connectors?
Click to expand...

No, haven't seen them before, but they look like a kludge that bridged the gap until proper FPC's became available. It also looks like the conductor tracks are visible on there; have you measured conductivity of the strips to see if the pattern of the connectors is perhaps present in conductive material on the strips themselves?
I can see how you might DIY a fix for this. The crudest and easiest way would be to simply solder the connections together, but this of course may make later servicing and repairs more time-consuming and risky. You might be able to clean these strips as the problems you're observing reek of fouling or corrosion.
 
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Andreas Thaler

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In order to thoroughly clean the shutter of the sticky damper (this problem affects every T90), it must be removed and disassembled.

Until we find a solution, there remains a risk that the camera will no longer work after reassembly with EEE.

So I‘ve been looking into whether the mirror box can be lifted out far enough to remove the shutter without opening the front connector. This is possible, but it puts stress on the circuit board, which should be able to withstand it.


+++

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

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As a test, I soldered thin enameled copper wire between the contacts to see if that would solve the problem. But it quickly became confusing.

Direct contact without elastic connectors didn't change anything.

Then I tinned the contacts and soldered them with hot air. That killed the T90
 
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Andreas Thaler

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koraks said:
have you measured conductivity of the strips to see if the pattern of the connectors is perhaps present in conductive material on the strips themselves?
Click to expand...
They are high-impedance according to measurements.

koraks said:
I can see how you might DIY a fix for this. The crudest You might be able to clean these strips as the problems you're observing reek of fouling or corrosion.
Click to expand...

I tried using electronics cleaner.
 
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Andreas Thaler

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Summary of my previous knowledge regarding problems and repairs
  • The shutter is easily accessible for removing/cleaning the sticky damper.
  • Removing the mirror box, which is necessary for this (for a thorough cleaning) is quick and straightforward.
  • The Copal S shutter is secured with just one screw, and booster springs between the slats are easy to attach.
  • The curtain travel times can be adjusted after minimal partial disassembly of the T90. However, the associated adjusting screws and gears in the shutter are made of plastic, which can break. Therefore, the shutter should be removed and adjusted visually.
  • Errors EEE and bc can have various causes. Information on this can be found in the SPT Journal and the C & C Associated Troubleshooting Guide for the T90. They indicate that a process in the camera is blocked (see links below).
  • Problems can be caused by dirty magnets (four in the T90), contact problems in the front connector (connects three circuit board components by pressure), defects in the DC/DC converter (a separate circuit board on the bottom of the T90, which can be easily replaced), and also by dirty switch contacts (especially the command wheel).
  • Electronic components such as the ten integrated circuits or SMD diodes and transistors appear to be robust; at least, they shouldn't be the usual cause of failures.
  • The 3-volt backup battery seems to last forever; I haven't seen any damage due to leakage.
  • The two documents below provide assistance and instructions for troubleshooting, as well as for the structure and operation of the T90. They are comprehensive, understandable, and thus offer a realistic chance of troubleshooting.

Canon T90 Repair Guide

From the Society of Photo-Technologists Journal
learncamerarepair.com



See also on PHOTRIO;

Canon T90: The infamous HELP/EEE error fixed through guided and systematic troubleshooting

We've discussed the Canon T90, arguably the most complex 35mm camera with MF, several times here. It's encouraging that most of the common problems can be solved DIY, see: https://www.photrio.com/forum/threads/canon-t90-recommendations-and-notes.202884/ However, one problem that means the...
www.photrio.com

Canon T90: Address unresolved problems; broken plastic parts in Copal S shutter, epoxy, EEE/HELP/bc errors, troubleshooting partially successful

The Canon T90 is particularly fascinating because back in 1986, every available technology was used to create the most complex 35mm SLR without autofocus. Electronically and mechanically, packaged in a stylish and robust housing. Even today, the T90 is satisfactory, but knowing and mastering...
www.photrio.com


+++

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

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I still have a few defective T90s and will stay on the topic.

I'm fascinated by the T90 and would like to increase its chances of being repaired by DIY. However, this will require a few more attempts and trials. Spare parts are readily available in discontinued units.

My DIY workshop is taking a break for the time being; I need my desk for other work and will continue to work on digital electronics. Also to better understand digital circuits like those of the T90.

And after my sessions with the T90, I need a rest

Thank you for your interest and all contributions!
 
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