[sdiy] Ideas on resonance problem on single OB-Xa voice

[tprivitera at nc.rr.com]

The overall resonance isn't as strong in 4-pole mode, but it does seems like the problem is only happening in 2-pole (12db) filter mode.

I did just get in there with a DMM and noticed something interesting.  Pin 9 on one of the offending card's 3320's (assuming it's the 2-pole filter chip) is measuring 8.2v and every other chip on the other voice cards are measuring 7v at that pin.  Pin 9 is the pin for resonance.

I'm not that good at reading schematics.  Is this 7v coming individually to each card from the control board or is there a common voltage that a resistor on each card drops to 7v (indicating a bad resistor on my card)?

Thanks,
Tim

-----Original Message-----
From: Tom Wiltshire [mailto:tom at electricdruid.net] 
Sent: Saturday, February 21, 2015 1:44 PM
To: tprivitera at nc.rr.com
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Ideas on resonance problem on single OB-Xa voice

Hi Tim,

Does the voice do this in both filter modes? - e.g. with the 12dB filter and the 24dB filter?

If it does, that might suggest something going funny with the resonance control voltage for that voice, since the resonance is unlikely to have failed simultaneously on both filters. If it the resonance is fine on one filter but not the other, I'd expect the problem is in the actual filter circuit itself.

HTH,
Tom

On 21 Feb 2015, at 19:22, tprivitera at nc.rr.com wrote:

> Hello everyone,
> 
> I wanted to see if anyone had any ideas on an issue I am having with the resonance on one of my OB-Xa's voice cards.  With patches that have more extreme resonance settings (ie: the resonant sweep patch 'C4' aka Van Halen 1984 patch), instead of sounding "whistling" at the top of the sweep, this voice sounds "buzzy", with no whistle, if that makes sense.  That voice sounds fine in other patches without resonance or with resonance at a lower setting.  Here is what I've done so far:
> 
> - Recapped all 7 electrolytics on the card
> - Cleaned all the connections on the card and pins on the voice card motherboard that seat the card
> - Pulled every chip on the card, cleaned pins and reseated a few times
> - Swapped multiple CEM 3320 filter chips in to confirm it wasn't a bad chip
> - calibrated the card correctly.
> 
> I've calibrated this whole synth several times in the years I owned it and since it's not a calibration issue and I've hit all the usually fixes, I'm kinda stuck.  Anyone have this specific problem before or can think of what might be causing this?
> 
> Thanks in advance,
> Tim
> 
> 
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