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MK1 Odyssey drifting/unstable VCO2! Help please!
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devinw



Joined: Jul 18, 2018
Posts: 10
Location: Portland, OR

PostPosted: Mon Aug 06, 2018 8:46 pm    Post subject: MK1 Odyssey drifting/unstable VCO2! Help please! Reply with quote  Mark this post and the followings unread

Hi all! So, I have a MK1 white face Arp Odyssey which I fixed up about a year ago and it has been working great...out to gigs etc.. always been rock solid. Well, there was one little issue with a bit of high frequency bleed from the VCOs when she was gained up. I read on some other threads that the Odyssey's really like the original RCA 4011 chips on the VCOs and can help kill that bleed, so I bought some and popped them onto my board (already have a DIP socket on VCO1 and now added one to VCO2).... Go to re-calibrate and VCO1 is fine but VCO2 will not sit still! I tried switching my original non-RCA 4011 into VCO2 and it still has the issue! So I don't know if I did something while swapping chips or if this just by coincidence cropped up right before I took the unit apart. Craziness!

I saw a video online by Synthchaser which had a similar unstable VCO and he had a bad JFET, so I basically changed every single transistor on the VCO2 block and I still have the exact same problem. I also though maybe the VCO2 tune trimmer pot might be bad and changed it...nope still there. In addition I replaced the 680pf mica cap by the 4011 chip, no dice. The VCO sloooooowly drifts up and down by 10Hz or more and really doesn't want to settle at 20Hz. The oscillation is not tied to the LFO, and I can change the LFO freq and this thing just continues to wander on its own.

As a last ditch I went to change the transistor pair and noticed that the schematic for my B1 board says the pair should be a 2N581 and a 2N81 but both my VCOs have a 2N81 bonded to a 6067! Well, I have all of those parts so I glued some together with thermal epoxy and the 2N81/581 pair didn't work right at all! The brand new 2N81/6067 did however work, but still have the exact same problem of drifting.

So, any help or insight here would be HUGELY appreciated with regard to:
1) What do you think might cause this instability
2) Why does my synth not have the same tranny pairs as the schematic shows? They are painted red and white which leads me to believe they are original.

Thanks!
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devinw



Joined: Jul 18, 2018
Posts: 10
Location: Portland, OR

PostPosted: Fri Aug 10, 2018 8:13 am    Post subject: Reply with quote  Mark this post and the followings unread

Well, i fixed it, for those who may read this thread later. It was the DIP socket...well not exactly. It was the solder on 1 or 3 of the legs. You see, the Oddy boards are double sided, but don't have thru vias, so if there is a trace on the top side (in this case pin 1, and pins 13 and 14), you need to be doubly sure you have a good connection on there. But the problem, with the DIP socket is you can't SEE at all...so I was counting on good heat and flux and getting the solder to wick up there. Apparently this created a spotty connection on top that had some varying capacitance and made the osc drift like crazy as the components flow current and microscopically move, etc...

Took the socket off, and soldered the RCA 4011, making sure to actually flow solder on the top pad/traces on 1, 13, and 14 and voila, rock solid oscillator.

BTW, RE the exponential converter pair: I ended up using an unmatched pair of 6076 and TZ81 glued together with thermal epoxy and it works great. Ironically I couldn't ever get a TZ81/TZ581 pair to work, even though that''s totally what ARP's MK1 B-1 board schematic shows! Very weird.
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