Behringer A500 thumping in one channel


2010-11-22 11:12 am
Hi, A Behringer A500 amp has the following conditions;
The right channel has a loud thump noise and the led indicators pulse full scale about once per second. This happens if there is OR isn't a audio input present, but only happens when the main gain control is set almost full. I have a circuit diagram. Have replaced the electrolytics C31, C28, C39 and the non-polarized cap C34, but still the same. I suspect the amp IC4b - will replace it when spares arrive. This channel also has a lower audio output than the left channel.

Question is; Can anyone explain the bias pots adjustment procedure. I have not touched the pots, but would like to know what voltages at what points, should be measured when the pots are set correctly.

I also noticed that 2 resistors R71 and R72 are shown (voltage divider) in the diagram, but are not fitted on the board.

Any help will be greatly appreciated. Thanks