[sdiy] Help with an OB-X issue

[Michael E Caloroso]

(copying to the list)

The output of A9 is DC coupled to the VCF of the voicecards.  The
defective 1458 may have latched up presenting a nasty DC offset to the
VCF.  This DC offset was amplified with increasing resonance, sending
the opamps downstream (in the VCF) into latchup which was likely the
"thump" sound you heard.

Glad it was a simple fix.

MC

On 1/1/16, Tim Privitera <tim.privitera at comcast.net> wrote:
> I checked signal starting at the MM5837, moving forward in the schematic and
> I wasn't seeing signal off the 1458 (IC A9).  Swapped that with an extra I
> had and noise problem solved.
>
> Strange (and really good) thing is the cutting out of the signal with high
> resonance on those two voice cards seems to have TOTALLY disappeared after
> swapping the 1458.  Looking at the schematic, I can't really tell how that
> chip has anything to do with anything other than the noise circuit and I've
> done nothing else.
>
> I've had the OB-X on for over an hour and power cycled it a few times and
> the problem has not returned.
>
> Thanks for the guidance,
> Tim
>
> -----Original Message-----
> From: Michael E Caloroso [mailto:mec.forumreader at gmail.com]
> Sent: Thursday, December 31, 2015 9:01 PM
> To: Tim Privitera
> Subject: Re: [sdiy] Help with an OB-X issue
>
> I've never heard of MM5837 failures.
>
> Did you verify the select lines to the 4051?  The TTL drivers like the 7442s
> and 74174s also have a habit of failing.
>
> No line on original OBX voicecards, sorry.  They are very hard to come by.
> The crowbx cards were tweaked using a real OBX and sound pretty darn close.
>
> MC
>
> On 12/31/15, Tim Privitera <tim.privitera at comcast.net> wrote:
>> MC,
>>
>> Thanks.  Per all the guidance on your OB-X restoration website, I did
>> replace all the 4051's (and all RCA 4xxx ICs) with new TI versions.
>> Also swapped in the new TL082s in the sample and hold section and
>> fully rebuilt the power supply, recalibrating everything.
>>
>> I probably hit the 4051 you are referring to during the upgrade.  How
>> common is it for the MM5837's to fail?  I've got a couple on order, so
>> it will be easy enough to swap.
>>
>> I will test those diodes in a couple of days when I get back into the
>> project.  I figured you might have seen such an issue or know the
>> probable cause.
>>
>> Appreciate it,
>> Tim
>>
>> PS - got a line on a couple of original cards?  I'd like to make my
>> 6-voice and 8 and didn't want to chance the CrowbOX cards having
>> slight variations from the 6 originals
>>
>> -----Original Message-----
>> From: Michael E Caloroso [mailto:mec.forumreader at gmail.com]
>> Sent: Wednesday, December 30, 2015 4:44 PM
>> To: tprivitera at nc.rr.com
>> Cc: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] Help with an OB-X issue
>>
>>> 2 of my OB-X voice cards have an issue with patches that have high
>>> resonance
>>> (ex: Tom Sawyer sweep) . The envelopes abruptly “shut off” in
>>> mid-sweep with a thump.  Almost like the VCA Sustain cuts off.  This
>>> is not something I’ve been able to “calibrate out” and I’ve tried
>>> swapping 3310’s and also swapped the cards around and confirmed it’s
>>> a card issue (problem follows the card, not the slot).  If I lower
>>> the control panel resonance setting enough, I can get it stop cutting
>>> out.
>>> Filter on all other patches sounds normal, just an issue with hi-res
>>> stuff.  Any ideas or experience with a similar issue?
>>
>> Verify the diodes in the feedback path of the filter, they limit the
>> signal during high resonance.  If either/both diodes are defective
>> then the OTA and/or opamps will latchup which may be what you are
>> hearing.
>>
>>> Also, the noise quit working.  The LEDs light with I hit the
>>> Half/Full buttons, but no noise on any of the cards.  Anyone have
>>> experience with the
>>> MM5837 chips from China on eBay?  Can seem to easily locate an NOS
>>> version.
>>
>> Check the 4051 and its select lines first.  First generation CMOS from
>> RCA have a bad history and the OBX is loaded with them.
>>
>> MC