This is a little off topic, but there seem to be many experts in many areas
that may be able to help me.

I have a guitar amplifier that I'm trying to fix.  It's a Kustom KLA-185
(~1990), and I was able to get the schematics to the pre-amp, but not the
amplifier section.  Although I'm not really sure what kind of misuse caused
one of the ground traces to burn and physically lift off the PCB before
blowing the fuse, something bad happened, and I was able to narrow down most
of the problem to a blown diode in the bridge rectifier of the power supply.
I had the whole thing apart, threw the new part in and it worked!  Ok, so
then I put the whole thing back together, turn it on, and get what sounds
like a square wave (maybe around 20 Hz) for about a second before the fuse
blows.  Take it apart again, replace the fuse, works fine?? What's the
difference?? The front panel of the amp is grounded to earth ground wire
straight from the outlet.  The front panel is metal and conducts through the
metal 1/4" jacks on the front, which are connected to the internal ground
plane on the pre-amp section.  So, I get a little scared when I realized I
just played through this without it grounded, but decide to flick the ground
wire on the edge of the input jack -- big spark (well, I'm sure some of you
have seen bigger, but it was big for me).  So, then I measure the difference
in potential between the two grounds, and get almost nothing (~90 mV).  One
more thing, the amplifier is separated into two pcbs.  One is the pre-amp
section and includes a lot of op-amps / discrete components for treble /
bass knobs as well as some JFETs for the clean / distorted switch.  The
second board is the power amp / power supply section which rectifies the
incoming AC (already through a transformer) and amplifies the signal sent
from the pre-amp board.

I disconnected these two pcbs and have connected the ground wire (from the
outlet) to the internal ground coming from the rectification circuit, and
nothing happened (the reading of ~90 mV between the internal and outlet
ground was still present here), but when I power up the pre-amp part, the
internal ground does "the bad flashy thing," so I assume something in the
pre-amp is causing the trouble.  Is this a correct assumption?

Does anyone have any experience / guesses as to what would cause this sort
of a problem to happen?  What could I do to further isolate the problem?
None of you think it is safe for me to play with this without the ground
wire on the front panel, do you? (Even though the devil tempts me to)

-Joe

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