My Pro-39 seems to have a problem that I think is in the squelch circuit. I
don't have a schematic but here is what happens. When it is first turned on,
it works normally. After a few seconds, it acts like the squelch was just
turned all the way up. It won't receive anything. I have 2 of these so I
swapped boards and found the problem to be on the middle board. Can anyone
help me out with a schematic or does anyone know what the problem could be?

Thanks,
Chris