Super 8mm sound projector

[Patrick McGrath]

Anybody here ever tinker with sound film projectors?

I have a very nice Yashica Super 8mm that has suddenly developed a loud hum not effected by the volume control and a couple of frying resistors!

What the?

I already swapped all the electrolytics when I got it and while working on the motor speed control(a completely unrelated, unattached circuit)
the sound suddenly went up in smoke.

Any takers?
If I post a pic that would probably help, eh?

BTW, my first post and a cry for help at that...doesn't bode well does it?

[Iain McNeill]

Originally posted by Patrick McGrath
and a couple of frying resistors!

you mean they're getting hot???


yes, a picture would be worth a thousand words

[Patrick McGrath]

Can you spot the fried resistor?

[Iain McNeill]

my eyesight is failing.
Can you post a bigger pic

[Patrick McGrath]

I uploaded a photo...maybe it will come up later?

Anyway, the resistor in question is getting hot on the verge of smoking. Actually I witnessed wisps of smoke! I tried replacing it, but the new one is also getting hot and the hum remains.

[Cal Weldon]

Moved to video

[Iain McNeill]

somethings shorted or pulling too much current. What's downstream of the resistor? follow the traces if you can or probe with a continuity tester.

edit: I see now. Uh oh - looks like bad transistors. Could the amp output have been shorted?

[Patrick McGrath]

I may have accidentally crossed the speaker terminals on the board. That was my first thought but I wasn't sure what havoc it might reek.

How can you tell the resistors are bad? Is it from the symptoms I'm describing and location of bad resistor?

Are we talking about the larger square transistors mounted to the heat sink? Or the three legged jobs?

I have already cross referenced them with replacements as I had a suspicion.

From what I understand, both have to be replaced as they work as a pair, correct?

[Iain McNeill]

As a rule of thumb, when you let out the smoke, the device stops working. Exceptions are high power resistors which can survive some abuse.

To test a resistor you may need to remove one end from the board to measure it as the rest of the circuit may affect the reading.

If you can follow the traces from the resistor in question it will lead you to one of the transistors which must be regarded as suspicious. However, the failure of one output tranny can take out the other and even the driver trannies as well. The domino effect can be quite devastating.

Others here may have better diagnostic tricks but I usually take several detailed photos of where everything is with a few notes to clarify. Load the pics onto my PC and check that I can see the part numbers etc and then remove whole stage. Then I can test each transistor by looking at Vbe and Vbc - the two diodes should conduct one way only, pnp opposite to npn of course. Check resistors and capacitors, ensure traces haven't vaporized etc.

Could be quite an exercise - are you sure it's worth it?