[sdiy] Moog Memorymoog clone almost completed

[Michael E Caloroso]

Maybe my website?

http://analoguediehard.com/studio/keyboards/oberheim_obx/index.html#designerror

Never found an Oberheim ECO to address the power transients.  I don't
claim to be the author of the clipping diode solution on the opamp, I
had seen them on other instruments.

MC

On 10/19/20, Roman Sowa <modular at go2.pl> wrote:
> Not about what is output or input, but the voltage at the 8x side of
> 4051 (you see, I didn't say in or out :) ) can never be outside power
> rails of the chip because it comes out there after passing through the
> chip. While at the 1x side it can exceed power rails, because the opamp
> can go anywhere between -13 and +13 roughly during powerup. With opamp's
> short circuit current of 30mA or more, it's already more than maximum
> rating for input protection of CMOS chip.
>
> I believe there was even some engineering change orders in Oberheims to
> prevent that spike. Or was it os some repairman website?
>
> Roman
>
> W dniu 2020-10-19 o 20:26, Tom Wiltshire pisze:
>> 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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