Arp Odyssey mkIII VCO2 tracking issue

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Arp Odyssey mkIII VCO2 tracking issue

Postby 4075alive » Mon Apr 24, 2017 3:22 am

Hi there. First post. I have been restoring an Arp Odyssey mk. III. I've solved a few problems with some help, but now I'm at an interesting standstill: I can't seem to figure out how to get Osc2 to track properly at 1V/octave. I can match, say, the bottom note to Osc1, and then the top note won't quite get there using the calibration procedure.

Here's what I've done so far. As per the service manual, I calibrated both oscillators to be 20Hz on their square wave outputs with coarse slider down and fine slider in the middle. Then I calibrated OSC1 v/oct so low C is 100Hz and high C is 800Hz. Then you're supposed to do the same with Osc2, but no go - I can get the 100Hz of the low C, but run out of trimpot at around 780 Hz trying to calibrate the high C. As you can imagine, this makes OSC2 more and more out of tune as you play up from low C...

I've tested the sliders (new) and they seem to be putting out the requisite 0-15V coarse/-15 to +15V (fine slider). I initially thought maybe it was a problem with the keyboard CV going to Osc2, but I notice even if I put S&H on, the higher the note the S&H plays, the more out of tune it is (assuming, again, I've tuned both OSCs to match at the low C of 100Hz). This makes me wonder if it's something in the summing circuit for all the CV inputs.

Any ideas? Thanks much.
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Re: Arp Odyssey mkIII VCO2 tracking issue

Postby madtheory » Mon Apr 24, 2017 12:41 pm

I don't know. There are a lot of tantalum caps in this thing though, replace them all they're well past their sell by date. The failure mode as you probably know is to short, this may be what's happening somewhere in the summing ciruit.
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Re: Arp Odyssey mkIII VCO2 tracking issue

Postby 4075alive » Mon Apr 24, 2017 2:26 pm

madtheory wrote:I don't know. There are a lot of tantalum caps in this thing though, replace them all they're well past their sell by date. The failure mode as you probably know is to short, this may be what's happening somewhere in the summing ciruit.


Sorry, I should have said all tantalum and electrolytic caps are replaced including PSU and filter is upgraded using Synthchaser's upgrade kit, which I recommend. Sliders are Bourns with adapters.
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Re: Arp Odyssey mkIII VCO2 tracking issue

Postby WhinyLittleRunt » Wed Apr 26, 2017 7:45 pm

You think this would also contribute to not staying in tune as well? Mine has never been recapped and I'm considering it...
roland jupiter 6 | oberheim ob-8 | moog minimoog | moog prodigy | moog opus 3 | yamaha s90es | yamaha cp30 | arp omni-2 | arp odyssey mkIII | linndrum | simmons sdsv | acetone fr-3 | 82 rhodes mark II stage | wurlitzer 200a | hammond m100
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Re: Arp Odyssey mkIII VCO2 tracking issue

Postby 4075alive » Wed Apr 26, 2017 11:10 pm

A recap of PSU and replacing old tantalums seems like a good place to start before troubleshooting. I'm a relative beginner but it could be the tempco resistors need testing and/or replacing. Mine is a V/oct issue, not a drifting issue.
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