XLR preamp output problem

[cdfac]

The symptom: I'm getting less volume from the right XLR output, whereas the RCA outputs are both even. The right XLR seems to have some extra static or crackling, too.

First of all, I've narrowed down the problem to the preamp, and it's not the tubes (there are only 2 12AU7s, and I just switched in some NOS cleartops). The pre, a Belles 21A/Auricap, is a hybrid design, if that helps. I'm still working on getting the schematic from David Belles, in case I need to send it in to someone.

I did some RMS voltage measurements on the pins using a test tone, and here's what I found:

Left Channel:
1->2____64mV
1->3___125mV
2->3____64mV

Right Channel:
1->2____68mV
1->3____78mV (the least trustworthy, it seemed to fluctuate a bit)
2->3____24mV

I always had a gut feeling it was the tube socket, since the right channel has always been a little more prone to the occasional wave of hissiness. But if that's true there must be some pins that are used only for the XLR, right? My other guess would be a bad solder joint. Does anyone have any ideas of what's going on, or what I should be looking at next? Appreciate the help!

Chris

 

[cdfac]

Well, to tie up this thread, I was asked to ship it in to Belles himself after speaking to him a few more times. The blessing is that he got to it within several days and was able to finish the repair in an afternoon - I guess a few parts were bad. The curse was that it was not cheap! Ouch, although this is mitigated somewhat by having the designer do the work. And that cost included a new cover (the original was beat-up, at least by A-gon standards), which I now regret not asking the cost of beforehand. All in all, I'm not going to sweat it, as this preamp is too good to complain :thmbsp: