[sdiy] Moog Memorymoog clone almost completed

[Tom Wiltshire]

Since when did a 4051 have a direction? It can be 1-to-8 or 8-to-1, can’t it? In which case, everything’s an input, everything’s an output.

Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================



> On 19 Oct 2020, at 18:37, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> 
> The trend I have been seeing is RCA 405x have a high failure rate.
> Those 1st generation RCA components did not have a long life.  The
> majority of CMOS failures I have seen were RCA parts.
> 
> The absolute maximum rating for CMOS inputs is VEE-0.5 < Vin < VDD+0.5.
> 
> Some polysynths have a design flaw in that the opamp buffer after the
> DAC lacks clipper diodes on their outputs.  DEMUX circuits using 405x
> often are on VDD/VEE power rails other than +/-15V.  That opamp buffer
> is on +/-15V rails, and on power transitions the state of those opamps
> is indeterminate.  In other words, they could generate voltages that
> can violate the CMOS absolute maximum rating.  While they are brief,
> they can degrade CMOS over time.
> 
> Other polysynths use a flawed design for the supply rail to the 405x.
> I uncovered this in my Oberheim OBX and OBSX, and it also exists in
> the OBXa and Matrix6/1000.
> 
> I am not convinced that S&H caps are a contributor to 405x failures.
> The 405x are sensitive to input voltages relative to supply voltages;
> the S&H caps however are on the OUTPUTS of the 405x.  Even if a 405x
> output were to be active during power transition, the S&H cap would
> simply charge to the output voltage.  That's not a condition for
> damage.
> 
> MC
> 
>> I have seen lots of 4051 problems on other polys as well.
>> 
>> For years I've been suspecting that on power down the S&H caps hold
>> their voltage longer than the PSU. I think CMOS chips don't like signal
>> voltage without a supply voltage.
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