Vintage Synth Repair

Hello to all:

   I am repairing an oberheim OB-X.  Unfortunately, I'm stuck and 
need a little help to get over the hump, hopefully someone will be 
able to help.  I usually do amplifier repair, and this is my first 
keyboard.

   Problem.  The first (physically closest to the x-former) bridge 
rectifier is heating up, and the board underneath it is charring, 
causing smoke.  I replaced the original rectifier and re-capped the 
supply, carefully noting all the polarities.  The x-former 
secondaries check out, and are symetrical.  After replacing all the 
filter caps, I brought it up with a variac, and the same thing 
occurred: abnormal heat from the rectifier, and charring/smoke from 
the pcb.  There was no load was connected to the power supply 
section.  I checked the test points while it was on, and the 
specified 19 volts was absent.

    Now this thing did sit for a while, hence my off-the-bat re-
cap.  My experience says that the pcb has become conductive between 
the bridge's connections, but I've never worked on anything that 
used this type of pcb before.  Is there something I am missing, or 
anyone with experience in this sort of problem?


Thanks very much


Mike

Please?

   A little help?



Mike


--- In vintagesynthrepair@yahoogroups.com, "sabolick" 
<sabolick@y...> wrote:
> Hello to all:
> 
>    I am repairing an oberheim OB-X.  Unfortunately, I'm stuck and 
> need a little help to get over the hump, hopefully someone will be 
> able to help.  I usually do amplifier repair, and this is my first 
> keyboard.
> 
>    Problem.  The first (physically closest to the x-former) bridge 
> rectifier is heating up, and the board underneath it is charring, 
> causing smoke.  I replaced the original rectifier and re-capped 
the 
> supply, carefully noting all the polarities.  The x-former 
> secondaries check out, and are symetrical.  After replacing all 
the 
> filter caps, I brought it up with a variac, and the same thing 
> occurred: abnormal heat from the rectifier, and charring/smoke 
from 
> the pcb.  There was no load was connected to the power supply 
> section.  I checked the test points while it was on, and the 
> specified 19 volts was absent.
> 
>     Now this thing did sit for a while, hence my off-the-bat re-
> cap.  My experience says that the pcb has become conductive 
between 

> the bridge's connections, but I've never worked on anything that 
> used this type of pcb before.  Is there something I am missing, or 
> anyone with experience in this sort of problem?
> 
> 
> Thanks very much
> 
> 
> Mike