I need help, please, with where to start looking for faults with my new build Amp15.
Problem: when muted, the output voltages appear normal (2.6V), but unmuted, one channel can be trimmed to 0V but the other remains stubbornly at -29.4V (measured at the + output).
I've successfully built 2 Amp11s, a Trupath and an Amp32-PS, all working, so I think that I am ok on my soldering skills and general build quality. Having said that, some of the spacings on the Amp15 are a challenge.
Here's more background:
1. I first built the PS section and confirmed it working with a 35V 300VA transformer, i.e., VPP=+49V and VNN=-49V and the VN+10 and V5 all were fine.
2. All the jumpers are correct per the drawings. Only the builk caps have been left out at this point.
3. I have rechecked the soldering with a magnifying glass, resoldered all contacts and cleaned thoroughly with a toothbrush and IPA
4. When powered, both FETs are receiving the VPP and VNN voltages at their Hi and Low pins respectively.
5. Unfortunately, I have soldered the 3020 directly to the board (yes, yes, I know).
Any advice on what to test? Is the -29.4V an indication that there is a problem with the FET, or a problem with the 3020 driving it?
Many thanks in advance for your help.