In fact, this problem could happen to any camera.
I may case i had to repair a 7d which has been dropped down with a flash on it – so the hot shoe contacts were damaged (pulled out from the flex cable, with the half of the flex cable. I tried to repair the flex, but only a 50% result, some flash functions were ok, others not.
We decided with the owner to buy a replacement part for the camera. This is an expensive part, but he has found one on the ebay at acceptable price (a disassembled one, without LCD and contacts – but we needed only the flex).
As You can see it has been sold in “good condition and works perfectly”. Super. I have go it in my hands, I transferred all the missing parts from the old one to the new one. I pushed the battery in the camera, checked the focusing, exposure, flash – it was ok.
I finished the work, mounted the other parts. At the final check I observed, that the camera works – but I can’t switch it off….
Ok, I tried to find out, if the switch (a slider) is shorted. Not. I mounted back the old one. Swith on/off was ok. I disassembled the contacts. Nothing. The two flex cables were from this point of view pefectly identical.
After a lot of measuring I verified the flex optically under the lupe. And I find something. Then I checked under the microscope. Here is the picture:
Measuring again with the ohmmeter – but this time instead of continuity normal resistance.
After wiping off the small oxide dot, the resistance was infinite, as normal. The problem with the continuity test is, that multimeters have a preset threshold for open circuit, which is 20-30 ohm….
OK, camera repaired. the problem is, that I have spent 2 hour of labour only because an ebay seller has been lying about the part.
IMPORTANT: in an electronic circuit always check resistance instead of continuity. Camera on/off switches are NOT power switches, they just give a signal to the PCB, so they work on higher resistance too, as the electronic needs only a voltage level to get logic 1 or 0.