[sdiy] Arp Omni strange filter behavior

Travis Thatcher recompas at gmail.com
Tue Apr 12 02:54:47 CEST 2016

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:

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:

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!


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