Fisher Amplifier Question

[bigpun4fubu]

The Left channel works fine but the right channel out put is showing 62v dc on the speaker terminal. I have replaced the output transistors but no luck so far , any ideas?

[bear]

Don't fire it up full power, or your transistors will likely fry again...

Assuming it is a +/- supply which polarity you are pulled to tells you which side might be a problem in the circuit

Is this a discrete transistor (older) Fisher? You might be able to find the schematic online.

I'd start by checking all the transistors on the board for shorts - you can compare to the "good channel", power off, zero voltage residual on the PS rails using an ohmeter, or the "diode check" found on many DVMs. Assuming no shorts, and no weird readings and nothing burnt up (did you look with a bright light at the resistors for darkening bands or brown areas?), then take OUT the outputs, fire it up and measure the base to emitter voltages of the smaller transistors - without shorting out the transistors by slipping with the probes. They should all fall within 0.5vdc - 0.7vdc. Also you should not find the same voltage on the base as the collector.

That should get you most of the way there... look also for shorted and leaky electrolytic caps AFTER you are sure the transistors are ok AND there is still an offset. You will need to check the "new" output devices. The Hfe position on many DVMs will probably be good enough.

No matter what it is a bad idea to flip the power switch on and hope that nothing blows up. A Variac and an ammeter (your DVM will work) and a lot of care in wiring this up are essential when trying to fix a solid state power amp. A scope is not a bad idea either.

I could not repair amps without both of those devices without getting very lucky...

_-_-bear

[bigpun4fubu]

Thank you I will try that , I have an oscilloscope on the way , I may have to replace the outputs again anyway but thats ok , I figured i would after swapping them old ones out and it was doing this .