Some amplifiers had capacitors to ground, or neutral at the AC input. If
this one has them, check the capacitors..

Vern

Joe Schneider wrote:
>
> 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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