[sdiy] Moog Memorymoog clone almost completed

[Michael E Caloroso]

Good point.

MC

On 10/19/20, Tom Wiltshire <tom at electricdruid.net> wrote:
> 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
>
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>        Electric Druid
> Synth & Stompbox DIY
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>> 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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