[sdiy] Arp Omni PSU question

Michael E Caloroso mec.forumreader at gmail.com
Wed May 20 03:30:50 CEST 2015


There is no "bad batch" of tantalums caps.  Generally ANY tantalum cap
from the 1970s and 1980s should be replaced.  The dielectric does not
age well and their maximum voltage rating degrades.  I had one
literally smoke while I had the hood open on my OBX.  They were in
ARPs, Oberheims, and lots of music gear (not just synths) from that
era.

MC

On 5/19/15, mark verbos <mverbos at earthlink.net> wrote:
> You can assume that all of those 1uF tantalum caps are bad.ARP got a bad
> batch or something.
> You will likely be replacing them in an Omni for many hours. There's one on
> each key.
>
>
> Mark
>
> On May 19, 2015, at 11:52 AM, Travis Thatcher wrote:
>
>> Yeah - I found that one of the tantalum caps had shorted on the
>> negative rail which I figured caused those resistors to blow.
>>
>> On Tue, May 19, 2015 at 11:45 AM, Terje Winther
>> <terje.winther at wintherstormer.no> wrote:
>>> The burn mark under the resistor is not because someone did some surgery:
>>> it
>>> is because the original resistor was blown because of a short circuit.
>>> You need to swap the resistor for the right value, and then you need to
>>> swap
>>> out the power transistors.
>>>
>>>
>>> Den 19. mai. 2015 kl. 04.15 skrev Travis Thatcher:
>>>
>>> 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!
>>>
>>> --
>>> http://www.travisthatcher.com
>>> http://www.soundcloud.com/thevoiceofsaturn
>>> _______________________________________________
>>> Synth-diy mailing list
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>>
>>>
>>> Terje Winther
>>> terje.winther at wintherstormer.no
>>> http://wintherstormer.no/
>>>
>>>
>>>
>>>
>>>
>>
>>
>>
>> --
>> http://www.travisthatcher.com
>> http://www.soundcloud.com/thevoiceofsaturn
>> _______________________________________________
>> Synth-diy mailing list
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>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
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