[sdiy] 4051 spikes

[ChristianH]

Yep, S&H 4051s are dying pretty often, even in non-Oberheim synths.

The other suspicion I have is that on power down, the S&H cap may hold
its voltage longer than the PSU. CMOSes don't like signal voltage
without being powered.

Chris


On Mon, 27 Apr 2015 21:34:13 -0400 Michael E Caloroso
<mec.forumreader at gmail.com> wrote:

> When I started restoring my OBX I was determined to uncover why so
> many of these show up broken.
> 
> Turned out that these things eat 4051s because the Vdd 5.6v rail
> circuit is a poor design.  It was derived from the 15v rail using a
> diode/resistor drop, which isn't a regulator at all.  It will vary
> with load current.  When the CMOS ICs are in transition they are
> drawing more current which pulls down the 5.6v rail, zapping the 4051s
> over time until Poof!
> 
> The OBSX and OBXa have that design error too.
> 
> It didn't help that they used first generation RCA CMOS with ZERO
> overvoltage input protection.
> 
> MC
> 
> On 4/27/15, Florian Anwander <fanwander at mnet-online.de> wrote:
> > Hello
> >
> > Thanks to everyone, who answered.
> >
> > I think old crows hint regarding the usage of the INH line together
> > with  basic explanation with stray capacitance  makes sense. And someone
> > asked about the grouding of the probe: The ground was taken direkt from
> > the supply connector (not in the picture).
> >
> > For me it was not a "problem", I have used 4051 in many circuits without
> > any problems. I was only interested why I saw this in my simple test setup.
> >
> >
> > Ah btw: 38 out of 40 chips are working :-)
> >
> > Once again thank you!
> >
> > Florian
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