AMP BH-260 Low Output

[harry]

Hello all,

I am fixing an AMP BH-260 bass head for my friend. The output was acting all kinds of crazy, asymmetrical and fading with increased input signal. I replaced all the electrolytics and it seems to be behaving in a more stable fashion but now the output is really low.

Can anyone tell me the purpose of the 2N5638 switches just after the power amp input? I have a nice healthy signal before R2, and then it just completely disappears at the junction of R2 and R3. There are -13.5V at C2/R5 (where it says -9V). At the gates of the 2N5638s, there are approximately 2mV.

The wire leading to the 2.2M/1N4148 junction is labelled 'MUTE' on the PCB... which is exactly what it seems to do!

(schematic here, but it is massive...)

http://img194.imageshack.us/img194/4...ampschemat.png


Any help would be much appreciated!

Thanks,

Harry

[harry]

Ah - I've figured out it's a power on mute. As I'm just testing it going into a dummy load I've taken out Q1 for the time being (until I can get a replacement), and the signal passes through OK.

Now I have an amp that works nicely upto about 4W output, then intermittently amplifies only the top half of the wave. The more you increase the input, the more erratic the amplification becomes and the peaks actually reduce.

This seems to have a bad impact on the +ve supply rail too, which looks stressed and ripples under increased input. It has a lesser effect on the -ve rail.

Could this be the product of a bias problem? The schematic states for 7mV DC between the power transistors' emitters (no signal, no load - cold); yet with no load the voltage is tiny, and with a speaker attached it won't go below 14mV.

Again, any help would be much appreciated!