B1 Problem

I've recently finished my third and fourth B1 boards (bought from the official source, with the JFETs). The two boards were built identically (for balanced operation, eventually), with extra-high quality parts (e.g., Z-Foil resistors in the signal path).
One of them is working perfectly, but the other is giving me problems. I'm guessing it is because, when first testing it, I hooked up the power supply backwards. Presumably, that could have damaged (or worse) the filter caps. Before I swap those out, though, I thought I'd ask here, and see if anyone had other ideas.
The symptom is bad distortion in both channels, and what seems like low volume, as well. I used to hear something similar when using batteries with my other B1, when they would start to die. But voltage measures OK at e.g. the gate of Q100.
I'll attach a photo of the board in my test enclosure, though I don't know if that will be much help.

20220802_190900.jpg
 

ranshdow

Member
Paid Member
2020-07-10 6:19 am
SF bay area
Is the diode still okay? Are either C1 or C2 internally shorted?

FWIW I accidentally hooked my B1 up to a dual rail +/- 24V supply while the output of one channel was connected to an amp I was playing with. The output had a path to ground and the output wires off the B1 PCB were getting hot enough to melt the insulation. Yet, when I hooked it up to normal power afterwards, the B1 was fine, except I think C1 or C2 read lower than I was expecting in situ.
 
Thanks, ranshdow, I have some spare diodes and will check C1 and C2. Probably I should just go ahead and replace them.
Before I realized what I'd done, I was measuring the voltage where the supply connects to the board, and it kept cycling between 4 and 9 volts (negative, I eventually realized). So something weird was happening, and I wonder if that fried the JFETs? Fortunately, I also think I have some spare JFETs lying around, so if worse comes to worst....