question on troubleshooting power amps

If I am dealing with a power amp that has a very low output, would I assume that one of the power transistors is bad?

Can a power transistor go bad and not blow out the main fuse?

If a power transistor goes bad, will I always get no sound or can I still get some sound from other transistors that are working?

Thanks.
Joel

1. Assume nothing.

2. Possible, but the usual failure mode is a short, which (ideally!) equals a blown fuse.

3. It depends...

Why not tell us the particulars? That'll get your amp fixed faster than vague Q&As.

[quote author="Mbira"]If I am dealing with a power amp that has a very low output, would I assume that one of the power transistors is bad?[/quote]

Does the PA have any form of volume control on it? That way you can help to pin-point to where the problem is.

Can a power transistor go bad and not blow out the main fuse?

Click to expand...

Yes.

If a power transistor goes bad, will I always get no sound or can I still get some sound from other transistors that are working?

Click to expand...

Sometimes you can get sound from the other transistors - it might depend on the design though.

Is there any crackling or variation in volume associated with the problem?

Thanks guys. Dave, I'll post more details in a while...I'm not by the amp right now.

The main reason for my ambiguous questions is that I was wondering if there is a "first thing" I should be looking at when getting very low output out of an amp. No output is usually easy for me-often look for the smoke...but the low output thing still has me a little baffled as to causes. These are always in solid-state stuff too. Tubes are much easier for me to find the problem.

One of the pieces is this peavey 8-channel powered PA head. It gives very low and very distorted output. I have traced coming out of the preamp section and all looks good up to that point.

The other is a bass head.

Joel

Check the output transistor emitter resistors. Then check the voltage across them to see if the transistors are conducting. Check the base-emitter voltages from the output transistors, back to the drivers, the input stage, etc. Low, distorted output oftens means that the transistor(s) are not biased on. Check for open resistors or diodes in the "bias string", open driver transistors, etc. Troubleshooting a typical direct-coupled power amp can be a pain because a problem anywhere in the chain results in voltages being wrong throughout the circuit.

If you find and replace a bad component, be certain to power up through a current limiter and use a dummy load, not a speaker. There might be more unpleasant surprises lurking. A lamp in series with the AC line makes for a good current limiter; choose the lamp based on the amount of current the amp should be drawing at idle when operating normally.

Thanks Dave...I'll check it out.

Stay warm up there.
Joel