My vintage mini amp has developed a significant speaker thump on power up. The two channels read around 30mv DC. There are no pots to adjust or relay to clean. I have a parts unit, so I tried changing the power switch, no change. I don't have a service manual. I could change the power supply caps. Any other ideas? This little unit is really well built, but being in such a small package its alot of work to replace parts. I seem to be having trouble with the search engine.