[sdiy] Arp Omni PSU question

Travis Thatcher recompas at gmail.com
Tue May 19 16:10:19 CEST 2015

Thanks Roman - You made me realize I was staring the most likely
problem right in the face! Whoever butchered this thing previously
installed 4.7k resistors instead of 4.7ohm which would explain a lot.
Will grab some hopefully tonight and install them.

Does anyone have any good recommendations for substitutions for the
power transistors?

On Tue, May 19, 2015 at 2:45 AM, Roman Sowa <modular at go2.pl> wrote:
> 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.
> Roman
> 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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