Thanks for your help Phil- I will have another look at the power amp- will pull the heatsink diodes cr6-8, plus cr10/12 and check the electrolytic caps. If anyone has any ideas about how to track this fault down, I would be glad to hear them. Thanks, Andy
Just ran the preamp out into another amp. The buzz is there but it is quieter now and does not sound like the previous 100hz hum. With the main filter caps replaced, could the next suspects be diodes breaking down under load in the power supply?