[sdiy] Memorymoog VCO trim

[Barry Klein]

This was it!  Two outputs had a short feedthrough pulse that caused the 
issue.
Replaced it and all is well (well, I haven't tried the MIDI ports yet...)
Thanks,
Barry

----- Original Message ----- 
From: "ChristianH" <chris at chrismusic.de>
To: <Synth-diy at dropmix.xs4all.nl>
Sent: Monday, June 30, 2008 9:52 AM
Subject: Re: [sdiy] Memorymoog VCO trim


> On Mon, 30 Jun 2008 09:06:31 -0700 "Barry Klein" <barryklein at cox.net> 
> wrote:
>
>> I have 2 oscillators that have some audible low frequency modulation. 
>> Not
>> due to the VCO chip
>> as did not follow in a chip swap. The two are on different voice cards. 
>> Not
>> immediately apparent what the cause is.
>> Annoying.
>
> I once had an annoying gargling filter modulation in my MemMoog, and it
> turned out to be a leaky s&h channel on the DMUX board. Apparently, the
> gargling sound came from the s&h refresh rate bleeding through.
>
> Those 4051s and TL072s are prone to failure anyway, the OB-8 also had
> problems there evrey now and then.
> I haven't done any measurements, but I suspect that they don't like
> still having charge in the s&h stages for some time, while being powered
> down from the PSU. In particular, the CMOS 4051s may be damaged by this
> transient state.
> Maybe they should have included a shutdown command, in order to zero the
> cap voltages before supply power goes away...
>
> Christian
>
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