ARP Quadra (ARP Omni section) voicing issue on strings

[Blue Jinn]

Has anyone worked on the Omni? (which is likely similar to other top octave divide synths?) This is a Quadra, but from what I understand it is basically an Omni and a Solus put into one package with some simple digital controls added.

The string synth in the lower register "warbles" on some, but not all of the notes. I have a schematic, but I am at a loss as to even know where to begin troubleshooting. The Omni uses "top octave dividers" which is all I know. The notes may be the same note in different scales I need to check that out. But any suggestions/links/words of encouragement apprecieated. 

There is also a note that doesn't work, but I think that may have to do with a CMOS chip...

 

[etheory]

Could you record a sound sample please so I can hear what the warbling exactly sounds like?
I have a pretty good rapport with analog synthesizers so I'm sure I could help out.

 

[Blue Jinn]

Will do tonight, THANKS!

 

[Blue Jinn]

I just did this real quick on a laptop mic so let me know if it is too lousy. 4' and 8' separately. You can tell where the split occurs and the sound evens out.

https://sites.google.com/site/bluejinnaudio/mp3/quadra/quadra_strings.mp3

I did look a  bit at the schematic, which is somewhat hard to follow, there are about 7 sheets, nicely hand drawn though, a lost art. I don't know if you are familiar with the Quadra/Omni, but there is a master tone generator, and there is a string section and a "poly" section, the latter you can make, e.g. an organ sound of sort etc.

I haven't corresponded the lower voicing notes, but because the fault doesn't present itself on any of the higher notes, I would guess the fault lay in the lower voicing board. Also, in addition to the distortion, they are out of tune.



I love the Quadra, but damn it is a beast to keep running. The 4' bass string voice doesn't work either, the led doesn't come on, but it does lock out the lower 2 octaves, (which is what it is supposed to do so you get an exclusive bass register....So it has now **three** faults....

 

[etheory]

That sample sounds to me like some of the divide down voices are being mixed in with the current note, but not all the time.

I'd be checking the following schematic:
http://www.synthfool.com/docs/Arp/ArpQuadra/Arp_Quadra_service_schematics.pdf

And be looking at the voicing boards to ensure that there isn't note leakage happening for certain note combinations. It's possible that one of the switching transistors is leaking, or something similar.

 

[Blue Jinn]

Thanks. and sorry for the late reply!

I have the Quadra and Omni schematic. The voicing circuits look very very similar (although the boards themselves have different part numbers.) The Omni SM goes into a bit more detail. I am trying to actually learn how it is supposed to work. But I am assuming you mean the transistor arrays are leaking causing the bleeding. I think what you say makes sense as I listen to the fault that notes are being mixed causing,; if I am reading correctly, the notes are always  all on and the transistor switches allow the particular note to pass through? That part I don't understand as to how the bleed over could occur? I'm devling more into the service manual though to get my head around that and where to actually put the test probe.