[sdiy] Arp Omni strange filter behavior

Travis Thatcher recompas at gmail.com
Tue Apr 12 03:35:03 CEST 2016

I also forgot to mention that I replaced the the 1458 opamp that is connected to the cv input for the filter. I didn't replace the 301  on the board. I can try that tomorrow as well.

> On Apr 11, 2016, at 21:13, Michael E Caloroso <mec.forumreader at gmail.com> wrote:
> 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.
> MC
>> 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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