[sdiy] Arp Omni PSU question

[Travis Thatcher]

Success - replaced the 4.7k with 5.1 ohm (what I had on hand) and
everything trims out nice and stable. Luckily nearly all of the
envelope caps have been replaced on this - though judging from
previous work I need to double check it all. I am beginning to see why
some folks steer clear of repairing Omnis :) Hope to bring this one
back to life though. Thanks all for the help and suggestions.

On Wed, May 20, 2015 at 2:35 AM, Ullrich Peter <Peter.Ullrich at kapsch.net> wrote:
> Similar experience here:
>
> When I had a Korg MS-10 it stopped outputting a signal from one day to te other. No smoke, nothing, simple no output.
> I found out that the ADSR charging/discharging capacitor made a 100% short to ground and so the VCA was set to complete attenuation.
> It was a blue drop tantal type that died.
>
> This was about 20 years ago, but no further tantal died (the MS10 is now at a friend)
>
> Ciao
> Peter
> ________________________________________
> Von: synth-diy-bounces at dropmix.xs4all.nl [synth-diy-bounces at dropmix.xs4all.nl]" im Auftrag von "Michael E Caloroso [mec.forumreader at gmail.com]
> Gesendet: Mittwoch, 20. Mai 2015 03:30
> An: mark verbos
> Cc: Synth DIY
> Betreff: Re: [sdiy] Arp Omni PSU question
>
> 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
>>> Synth-diy at dropmix.xs4all.nl
>>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
>> _______________________________________________
>> Synth-diy mailing list
>> Synth-diy at dropmix.xs4all.nl
>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>
>
>
> The information contained in this e-mail message is privileged and confidential and is for the exclusive use of the addressee. The person who receives this message and who is not the addressee, one of his employees or an agent entitled to hand it over to the addressee, is informed that he may not use, disclose or reproduce the contents thereof, and is kindly asked to notify the sender and delete the e-mail immediately.
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy



-- 
http://www.travisthatcher.com
http://www.soundcloud.com/thevoiceofsaturn