[sdiy] Moog Memorymoog clone almost completed

[Roman Sowa]

Yeah, I remember that website, I've read this lengthy page in full back 
then.
So it was you...
Thank you sir for writing this stuff. It gave me head start during OBX 
repair a couple of years ago.

Roman

W dniu 2020-10-20 o 06:31, Michael E Caloroso pisze:
> 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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