[sdiy] Arp Omni strange filter behavior

Tom Wiltshire tom at electricdruid.net
Fri Apr 15 02:04:55 CEST 2016

Hi Travis,

I've had a look at this this evening. Various things strike me:

If the CV at pin 4 of the VCF module is ok with the module out, that suggests the problem has to lie on the module. But we don't know for sure what that CV is supposed to be, so we don't know where the problem is for sure. There are two possibilities:

Assuming the problem is with the CV, there are a lot of sources feeding CV to that module. You say that the ADSR slider doesn't respond correctly. But you're sure that all the others are ok: LFO, VCF Freq control, and Pedal and External CV input if you can test them? An offset from one of those other sources might throw things out at Pin4. There's also a "VCF Freq Cal" trimmer, which could be way out, or could have given up the ghost.

If we instead assume the CV is ok (e.g. assume that the measured -1V to -2.6V is what's expected) then that would mean the problem is with the 4075. That CV leads straight to the exponential convertor, so that would have to be prime suspect. But if there was something wrong with that, *none* of the CV sources would work accurately. Which then makes me think that the first assumption might be better - if the problem really *only* affects the ADSR slider, then it has to be on the synth board and not the module. We'd then be looking for an offset generated by the ADSR module that would hold the filter closed until the last bit of the travel.

That suggests the ADSR generator. Does the ADSR affect the VCA correctly? Is there anything there which would suggest a problem with the ADSR? I notice that whereas the VCA takes the ADSR output only from the final amp (the 0 to -10V env indicated) the Filter also has a connection further back into the circuit via R41. It looks to me (Red flag! guesswork!) like it's a feedback path for a comparator to trip the circuit from Attack to Decay or something similar, but I can't work out all of the ADSR at the moment.

I hope seeing the process I went through thinking about it helps jog your memory or triggers some thought that had eluded you.


On 15 Apr 2016, at 00:09, Travis Thatcher <recompas at gmail.com> wrote:

> I put the 4k7s back in and no change. I also replaced the LM301 on the
> filter slider pcb and no dice. I replaced Q13 and Q14 on the 4075 and
> no change. With the module removed the control range seems okay but
> perhaps that is misleading somehow?
> On Mon, Apr 11, 2016 at 9:16 PM, Adam Inglis <21pointy at tpg.com.au> wrote:
>> 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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