[sdiy] 4051 spikes

[Michael E Caloroso]

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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