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Memorymoog modulation offset issue

Posted: Wed Oct 26, 2016 8:03 pm
by rjd2
Hi folks, I'm wondering if anyone has any ideas on an issue I have.

Working on a non-plus MM. The problem: activating the "osc _ freq" button in the modulation section creates a pitch offset on the respective VCO(the mod wheel is completely off). so, for example, if the machine is tuned to exactly zero, and all the VCO's are tuning up perfectly, pushing the "osc 1 freq" button in the modulation section shifts the pitch of osc 1 up by about 40 cents. it does this with each osc, respectively, so "osc 2 freq" button pushes the pitch of osc 2 up by about the same amount. same w/ osc 3. so there is obviously an offset that needs calibrated, somehow.

i've followed the factory calibration procedure from psu thru the vco's. the modulation section calibration mentions nothing about a modulation offset. i'm kinda stumped. any ideas? am i overlooking some other section of the service manual? thanks for any possible help! -RJ

Re: Memorymoog modulation offset issue

Posted: Thu Oct 27, 2016 1:13 am
by zaiusz
Hello RJ!

I had a similar problem which only affected one voice, where pressing the modulation button would offset the oscillator pitch. I traced the problem to the countour/glide board after finding that there was no calibration to fix this. By memory the problem was due to a bad op amp (probably a 353) connected to one of the 3310's. Are you are reporting that all voices suffer this or all oscillators on one voice?

Re: Memorymoog modulation offset issue

Posted: Thu Oct 27, 2016 10:23 am
by rjd2
zaiusz-thanks! whew! that's actually super valuable information. thanks!

all 3 vco's exhibit this, but respectively. so activating "osc 1 freq" ONLY causes an offset on osc 1, not osc 2 or 3. same with 2 and 3-depressing "osc 2 freq" in the modulation section offsets osc2 freq, not 1 or 3. ditto for osc 3.

hmmmm. ok-opamp bad possibility. i am tracing a voltage bump on the common analog board right after the 4016, so for osc 1, R105 sees a bump of about .6v when the "osc 1 freq" button is depressed.

i had a similar issue on the contour/glide board, with the "hold" button causing a similar offset, and i was able to calibrate that out(don't remember which trim pot from memory). maybe correlated? i'll sniff around.

Re: Memorymoog modulation offset issue

Posted: Thu Oct 27, 2016 5:43 pm
by rjd2
problem solved! U12 on the common analog board, a 741 opamp, was causing the offset. swapped it out and it solved the issue. thanks!!!

Re: Memorymoog modulation offset issue

Posted: Thu Oct 27, 2016 5:53 pm
by zaiusz
good job. so being on the common board I suspect this problem must have affected multiple voices at once?

Re: Memorymoog modulation offset issue

Posted: Tue Nov 01, 2016 11:40 pm
by synthguy1
Hello zaiusz
synthguy1 here. You seem to know a bit about the MM. I know a bit myself having purchased mine in 1983. This particular keyboard has quite a history be it
performing, recording, or just playing. I'd have to say, it's greatest history lies within repairing. I see that your location is 'Torontoish'. Well, me, too. You can see and hear the MM in question at about or just after 7 minutes, 10 seconds on this video here:
This is/was/still is my favorite keyboard ever but right now it's so 'eff'ed' up I just want to smash it with a hammer. I've rebuilt this thing too many times and I have had it working so perfectly so many times that I don't think I can bear to even open it up again - it's such a pain in the 'a##' to do and I ain't gittin' any younger. Still, I'm naked as a musician without it, or so I feel. Do you fix this jive? 'cuz I got $. I was going to send it to Rudi in Germany (I bought a LAMM from him but had to sell it - needed the $ at the time) but he wants almost 8000 euro, way more than I paid for a whole one in 1997, and I don't have that kind of $$$$. Any ideas?
Cheers! synthguy1

Re: Memorymoog modulation offset issue

Posted: Fri Nov 04, 2016 10:57 am
by zaiusz
synthguy1 wrote:Hello zaiusz
synthguy1 here.
Hi synthguy1, I sent you a pm