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

 

[tk@halmi]

How old is this contraption? If the diodes have checked out fine then capacitors would be next on my list to replace.

 

[Khron]

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.

View attachment 139989
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.

If those are the same construction as their (Rifa's) metallized paper X2 mains capacitors, those are ticking time-bomb and they WILL (destructively) fail

https://www.google.com/search?sca_esv=04e68c913ef2a454&q=rifa+fail&udm=2

 

[EmilFrid]

@sahib no I haven't changed them. They haven't exploded but may be a good idea to replaced just in case.

@tk@halmi yes I'm thinking of changing motor caps. A little difficult to find perfect replacements, but maybe it's ok to change them with caps that are slightly different value? Like a 1uF off

 

[EmilFrid]

What is interesting is that the CR components are 0,1 uF and not 0,01 like in the schemos

 

[sahib]

If those are the same construction as their (Rifa's) metallized paper X2 mains capacitors, those are ticking time-bomb and they WILL (destructively) fail

https://www.google.com/search?sca_esv=04e68c913ef2a454&q=rifa+fail&udm=2

Indeed. And with a mighty bang. Here is the proof.

What is interesting is that the CR components are 0,1 uF and not 0,01 like in the schemos

That is not a problem. 0.1uF is fine. But I strongly recommend that you replace them all because of their age now. And again use much higher voltage rating.

 

[EmilFrid]

Will do!

 

[marcelvanwijk]

Semiconductors generally fail from overheating or over-voltage. 5W emitter resistors suggest some power in the area. If not overheated (literally melting the silicon) voltage spikes could punch shorts between collector emitter. Can I ASSume the bad transistors measure shorted collector-emitter? Less common failure modes are open bases (from excessive base current). Back in the 60s as a junior technician I would grind the tops off metal can transistors to look for voltage spike punch throughs (visible as a dot where the collector-emitter is shorted, with some local melting around the dot from the heat caused by that short. )

I see a RC spike snubber, and a CR snubber (?), across the transistors. Dried out caps in the snubber circuit could diminish effectiveness. I am not even sure what a CR component is maybe some kind of capacitor?

Good luck

JR

It is possible to clip the base tension with some diodes in series and the collector tension with voltage dependent resistors.