[sdiy] Arp Omni strange filter behavior

Michael E Caloroso mec.forumreader at gmail.com
Tue Apr 12 03:13:20 CEST 2016

Sounds like an opamp processing the filter CV has its common mode
limits being exceeded.  They will reverse polarity when that happens.

ARP liked those LM301s (aka SLxxxxx in house marked) but they weren't
very robust opamps.


On 4/11/16, Travis Thatcher <recompas at gmail.com> wrote:
> I'm nearly finished with an Arp Omni MK I restoration for my friend
> that started with recapping the whole synth with a kit from New
> England Analog. When he turned it over to me it was nearly all
> working. I had to replace one of the CMOS switching chips to get the
> bass voice to turn on properly and now I'm left with 2 problems that
> have me clueless. First, the filter works in that the ADSR does what
> it should however the filter only opens up on the last 15% of the
> sliders for the ADSR and Cutoff. Seems the Res slider may also only
> work at the end of its range. I tested the sliders and they are
> working correctly. If I monitor the voltage on Pin 4 of the 4075
> filter and keep all sliders at zero except the cutoff slider the range
> seems to go from +66mv to 0 for most of its travel and then jumps from
> 0 to -77mv. If I remove the 4075 the voltage at where pin 4 would be
> on the main board goes from -1 to around -2.6. It seems there's
> something in the 4075 clamping the voltage the wrong way? The caps are
> called changed on the filter itself and the 4.7k resistors have been
> swapped out for 2.2k to increase the freq. range.
> The schematic for the 4075 can be found here:
> http://www.till.com/arptech/pdf/4075.pdf
> R29 and R28 check out with the correct values. On the main board the
> resistor values in the cv mixer seem to check out as well, those the
> resistances change with different slider settings which I assume is
> correct. I also verified that one end of the slider is getting a
> proper -V with the other at ground.
> Service manual is here:
> http://www.synthfool.com/docs/Arp/ArpOmni/ARP_Omni_2480_service_manual.pdf
> The other issue is weirdly similar though may be a red herring. The
> bass volume also only seems to work on the last 15% of its travel.
> Seems really odd that its the same behavior as the filter controls but
> not really part of that circuit.
> I'd appreciate any ideas as I'm stuck!
> --
> http://www.travisthatcher.com
> http://www.soundcloud.com/thevoiceofsaturn
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