[sdiy] Memorymoog VCO trim

[ChristianH]

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