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May18-09, 10:05 PM
Sci Advisor
P: 4,047
Need help with a 12 watt Audio amplifier

I'd have to agree with Waht on this one.

There shouldn't be any audio on the heatsinks because there should be insulating washers between the transistor bodies and the heatsinks. They might be little rectangles of mica or some other insulator.

If you have a multimeter, make a chart of voltages (relative to the negative rail), redo the picture of the schematic so it can be read and put the list and the new photo on this Forum.

That schematic is pretty bad. Get some more light on it and focus it and hold the camera steady on a tripod. And don't shrink it so much.

Measure the resistance from the junction of the two output transistors (on the heatsinks) to the large capacitor that leads to the speaker and then from the capacitor to the speaker. There seems to be a fault just there, somewhere.

Check that you have the right transistors in the right positions on the board, particularly the transistors on the heatsinks.