JLM AUDIO

Hey all--I was going to post this at The Lab but can't seem to get into my account there.

I just built the BA4, and three channels work perfectly. Channel 3 is dead though. Here's the weird thing that happened...when I tested each, I got silence from 3...until I flipped the phase. Then I could hear a weak signal. I took out the board, checked all my solder points, resoldered everything, and rechecked the I/O and power connections. Put it back in, powered up--now it was totally dead, and even the phase trick didn't work.

Any idea what might be wrong, where to look, or how to test it?

OK, I found the source of the problem...but I have no idea how to fix it. It is very weird.

There is a 10K resistor right underneath the JLM99v. On the side where the power comes in, it reads 48v. On the other 3 channels, the opposite side reads 24v. On this channel, the other side reads 0v.

I replaced it TWICE, both times with resistors that successfully tested at 10K on the ohmmeter. But every single time, it reads 0v on the other side. What the hell? To make sure it wasn't bad soldering, I took a reading on the lead itself right before it enters the resistor. 48v on one side, 0v on the other.

Why on earth would this be happening?!!? Are all the other 10K resistors on this board going to be bad, too???

Hmm, I guess the resistor is not the problem. The 48v is getting to the JLM99v, but the values on the other pins are all wrong. Looking at it from the top, as on the wiring overlay, the left column should be 24v, 24v, and the right column 24v, 0v, 24v, 48v. Instead I have: left column, .1v, .8v, and in the right, 4v, 0v, 0v, 48v.

Do I have a bad opamp?

Also, resistance between pins 1 and 2 of the input XLR is not between 3k and 4k as on the other channels, but 7.22k.