Motor transistors keep dying on me

[EmilFrid]

Ok, I will bring out the scope and poke around a bit then

 

[inkster]

I serviced an 80-8 years ago with a similar issue and it turned out to be a regulator, but I don't recall where in the circuit.
I maintain one of those machines for a local studio...next time I'm there I'll take a look at the manual and if I have time measure some values at the motors.

 

[EmilFrid]

@inkster thanks a lot! That would be very helpful. This is the single most confusing issue I've met, and I have worked on both my machines extensively.

 

[EmilFrid]

There are no regulators directly affecting the motor though, as they are supplied AC directly from the transformer. And my measurements show the correct DC voltages on the reel control card

 

[JohnRoberts]

And also I replaced the caps in the RC network as one of them exploded

It's not unusual to replace the driver transistors (the small ones driving the bigger ones) after power transistors fail.

JR

 

[EmilFrid]

I think I will do that then. It's not like I'm losing anything by putting new ones in even if it turns out something else was the culprit.

 

[sahib]

Yes, C111 or C112 (as they're just connected point to point under the chassis it's difficult to say which one of those two exactly), so I changed both of them for two that can handle s little higher voltage

Have you replaced CR1 and CR2? As I mentioned before these are capacitor resistor networks (0.01uF + 120R) and will probably look like this type.


I replaced a lot of these in photo-flash units (from early '80s) for a friend of mine who was a photographer. If there is space replace them with much higher voltage rating.

 

[radardoug]

There is still no explanation for you getting a reading of 300V. This should be impossible. The motors should have separate supply windings on the transformer. Have you checked between those windings for leakage?

 

[sahib]

I have explained that. It is due to the capacitors being charged through two separate windings on t=0 and summing. They then discharge and settle at 150V. See below