Expert: Cleggsan - 10/4/2008

Question
HEY HOW'S IT GOING?  LONG TIME NO CHAT!!!
so I have a question about an amplifier that has a problem, dont know if you or anyone you know knows how to repair them, I assume its just a bad main diode or transistor but dont know how to locate it.  here's the problem.

Its a small 2 channel class a/b amp.  it was never abused but a while ago it started passing DC out one of the channels.  the amp still works 100% (audio out both channels) but like I said there's a steady flow of DC out one channel, I blocked the DC with a capacitor in series with the speakers and the amp sounds and works fine.  I am wondering if that points to a certain component on the board that could be replaced if you know which one. I know how to solder and where to get replacement parts.  I'm just not too good with how to tell which part has gone bad.

THANKS!


Answer
It is a very simple matter, but you must know the circuit of the audio output stage and how it is biased.  

The difficulty may be a defective component in the biasing scheme or more likely a sick or going sick output transistor.  

So, you might start by replacing the output power transistors; if it is complimentary there will be a matched pair of outputs but one will be an NPN and the other a PNP.  If it is quasi-complimentary out put the two outputs will be the same transistor. They are mounted on a heatsink to keep them cool during operation.  If it is a small power rating, say under 10 watts output, the output transistors may have a heat sink mounted to the transistor somehow.

If you give me make and model I can help further.  The above comments are general in nature which is all I can give you without knowing the specific amplifier you are talking about.

Hope this heads you in the right direction.

C