[sdiy] Arp Omni strange filter behavior

[Adam Inglis]

What if you restore the original 4k7 value resistors - does it scale normally then?

AI
> On 12 Apr 2016, at 10:54 am, 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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