[sdiy] Arp Omni PSU question

Roman Sowa modular at go2.pl
Tue May 19 08:45:25 CEST 2015

If there are burn marks under 4.7k resistors, those should be the first 
to check. Usually most if not all resistors in power supply are playing 
major role in setting output voltages.
Next source of troubles are the sockets for the transistors on heatsink. 
better to solder them in, or at least clean.

BTW, I could not find 4.7k resistor in the schematics, but I have found 
4.7ohm in very crucial spots - as current sensors. It's hard to imagine 
how could anyone burn a black hole in PCB with 4.7k over 15V, while it's 
very easy when there's 4.7ohm. So maybe someone have replaced them 
without checking schematics, relying only on burnt colors, which 
obviously were discolored after burn.


W dniu 2015-05-19 o 04:15, Travis Thatcher pisze:
> I've got an Omni MK I with a power supply that has me stumped. I
> replaced the 2 1000uF and 250uF caps, the 2 10uFs as well as the 1uF
> tants and replaced both the lm301 and the 723 as well as all the 4001
> diodes.  Still, I'm getting about 36v on the v+ and -6 on v- with the
> trimmers doing very little. Someone else has already done some surgery
> on this synth before me - you can see the burn marks under the 4.7k
> resistors. I checked and I'm getting a healthy +- 36v out of the
> transformer. The power tansistors aren't heating up - would that be
> the next place to look or does anyone have any suggestions for
> troubleshooting. I have a power one supply that I can stick in but was
> hoping to save the original. If I go the power one route - I'm
> assuming I can just wire v+, gnd and v- from the power one to the
> appropriate inputs on the lower voice boards and ignore the V/gnd
> sense lines?
> Image of the current PSU is here: http://recompas.com/xfer/omnipsu.jpg
> Thanks in advance for any help!

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