isa 428 - one channel with too much gain and overload.

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Cpartipilo

Active member
Joined
Jan 27, 2009
Messages
37
Location
Miami - USA
Hi guys,

I'm back to ask for help!

I purchased a "broken" Isa428 which was blowing the fuse. Checked and the power transformer was overheating and drawing more current than it should which was the main problem.  I ordered an original tranny from ThermalRelief and got the unit going.

So far so good, i'm testing channels until i got to channel 3.  The channel has so much gain that without a mic connected i can still hear environment noise in my headphones.  It's like if the box becomes a microphone, if i tap the case it sounds like if there was a mic inside of it.

If i plug a mic into that channel then it overloads wildly so i'm guessing is something in the gain control circuit (if there is such a thing in a preamp)

Schematics are not available but maybe someone can guide me in some general way as to what i should look for.  I have 3 good channels so i could try to compare test points from the other channels.

Thanks!
 
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