Hi, I am new here. Just bought a Minimoog Model D at a yard sale for a very good price. Its serial number is in the 5000 range, but has a replacement oscillator board (the newer more stable version), I’ve always wanted one so I am very excited to be able to finally own one. Just to give you some back ground on it, it is in very good cosmetic condition, needs the key bushings replaced (which are on order), and at first appeared to work perfectly. After playing it for a little while I noticed the oscillators were not in tune with each other, so I figured it just needed calibration, which I though would be no big deal, I had done it on other synths before with no problem. When calibrating it I could get a single note in tune, but could not get the rest of the keys in tune. I have cleaned the buss bars because as I was also having jumpy notes, and though that might have something to do with it, but that didn’t help with the tuning (the jumpy notes are better). I haven’t been able to find any one with a similar problem. I am suspecting something is wrong with the control voltage coming from the keyboard or the contour generator board as this problem exist on all 3 oscillators. I thought it may also be the op-amps that are responsible for scaling the CV from the keyboard, but from what I understand there is a exponential converter for each oscillator. I have a good amount of experience with electronics (EE), but before I started getting into it too much I wanted to see if there was an expert on the Minimoog that could chime in and if they have ever encountered this problem.
I had the same problem with my arp 2600 when i swiched from mono to duofone keyboard here you have a cv scale pot next to each osc. so it was a easy fix
Dont now about mini’s tough.
The most important part of the procedure is to adjust the scale trimmer for each VCO such that all octaves are in tune. Written calibration procedures are designed to be confusing in my opinion so I rarely read them. My approach is to tune a low note to 100 hz (using a frequency counter) or using a tuner, set the note to pitch. Play a note 1 octave higher to start. A frequency meter should read 200 hz. If the count is high, or the tuner shows sharp, the VCO is scaled WIDE. Scale trimmers never work the same way - even on the same synth, one VCO might reduce the width with clockwise motion and another VCO might increase the interval with the same clockwise motion. Additionally, reducing the width might LOWER the low note leading you to believe the width increased, but the top note would be even more flat if in fact the interval was reduced. Therefore, don’t pay any attention to anything except did the interval get wider or narrower with a clockwise, or counter clockwise tweak. After making an adjustment, re-tune the low note, then check the high note (or vice versa - it doesn’t much matter as long as you correctly observe the WIDTH of the interval and correct accordingly). As you zero in on the scale setting, play 2, 3 or 4 octave intervals for more precise observations. The newer oscillator board has separate octave range trimmers for each VCO. Check to make sure the octave range buffer mod was installed. Without this mod the octave range controls can have some slight interaction between the VCOs. Calibrate the octave range trimmers after the keyboard scale is set. Also check the pitch bender function. If the dead band mod has not been installed, it is very easy for the pitch bender to cloud the tuning issue as it does not return to zero reliably. A sideways glance at the wheel can change the VCO pitch by 5 or 10 cents. Hi-track is least important. Even if this trim is way out you can still get lower octaves to scale up OK. Lastly, set the front panel tuning controls to 12 o-clock and adjust the offset trims to concert pitch.
If your Mini VCOs are not tracking together, this procedure should correct that condition, or you should learn exactly what issue is preventing them tracking properly. Let us know what you discover. Congratulations on your yard sale purchase!
Larry, I don’t follow the Moog instructions either, but with all due respect, your instructions seem even more confusing to me.
Nathan (the original poster) wrote me privately a few days ago and we discussed his problem.
I’m not sure what’s up with his Mini as he appears to have a good grasp of tuning.
Regardless, I’m posting now because, as I said, I’m even confused by how you described the procedure.
So not to usurp the thread or your answer, but I find this below is easier to follow.
No slam to you, but I think it should be pointed out that newer and older VCO boards tune up in opposite ways.
(using a known tuning reference)
If an old VCO board, hit a high note and adjust the RANGE trimmer.
Then hit a low note and adjust the SCALE trimmer.
Go back and forth doing these adjustments until correct.
If a newer VCO board (with uA726 chips), the tuning is the opposite.
Here, one hits a low note and adjusts the RANGE trimmer.
Then hit a high note and adjust the SCALE trimmer.
Go back and forth doing these adjustments until correct.
After all VCOs are tuned, then adjust the OCTAVE trim or trimmers.
Old boards have one trim for all VCOs.
New boards have one trim for each VCO.
Hey Kevin: Back in the early 80s when I had several Minis in the shop on any given day I probably remembered which way the CV offset affected the scale trim and whether a clockwise turn made the interval wider or narrower. Now days I don’t get to see more than one every once in a while and that is too much info to remember - especially when I also see an occasional Arp, Oberheim, SCI, etc. Then there is the Momorymoog where some of the precision scale trimmers are inserted one way and others are reversed, so you have to remember which end of the device the screw is on to cause a clockwise trim to widen (or was that reduce?) the interval. Some of these folks seeking advice may never calibrate more than one Minimoog and as an instructor I would never require they memorize those steps in any procedure. So, I reduce it to the simplest terms. If the interval is too wide, make it narrower. It it is too narrow, make it wider.
It’s kind of like Ohm’s Law. I can hardly remember where the E, I and R go in the formula, but since I understand Ohm’s Law, I “see” the relationship and understand where the letters go and why. I don’t have to worry about retaining a visual image of how the formula looks. Checking the interval for wide or narrow and making the appropriate correction works for any VCO on any synth without any calibration procedure. No slam to you either and with all due respect, I’m not sure why that would be confusing to you. Maybe I could have said it in simpler terms.
I’m curious about this Mini. You said you had private communication with Nathan and did not know what the problem is (?). That part of your post seems to be missing this morning or did I dream that I read that last night? Perhaps we could all learn something useful by following this through. What is going on with your VCOs Nathan?
I completely understand what you’re talking about in regards to one synth doing things one way, while another is different.
It does get confusing sometimes. We’ve both worked on a lot of synths.
But when I read your post, I didn’t see where it specifically said to adjust the scaling pot.
That was the confusing part.
We both work on all sorts of varied synths often, but most owners don’t.
They usually want specific instructions, especially after trying what they’ve read and not finding success.
It also doesn’t help that the first printings of Moog’s own Mini service manual stated it backwards either.
(They later produced an addendum correcting their error.)
Since the original poster wrote me saying he tried tuning his Mini several times and it just wasn’t working, I was just being more specific and absolute.
I wasn’t sure quite what directions he was following and it’s a little known fact outside of service-tech-land that the two basic Mini VCO designs are tuned up in opposite ways.
If he was a synth tech, I might have said “The later Mini boards are tuned much like 2600s.”
But he did try several times and it just wasn’t happening.
As to what you’re seeing or not in the forum, I’m not sure. I never edited my post above.
I even just scrolled back up to check and it still says “Nathan (the original poster) wrote me privately a few days ago and we discussed his problem.
I’m not sure what’s up with his Mini as he appears to have a good grasp of tuning.”
As to why he’s having a problem, I did suggest a few things.
But he’s also an EE (or studying to become one) and I need not be pedantic and explain that the synth is 1v/oct, power supplies are checked 1st, to measure voltages and all that.
He seemed pretty bright to me.
So I just left it at that and verified he was doing the correct way.
For all we know, he could have one bad resistor on the env board that sets the keyboard current.
But first things are first and that’s where he was.
No hard feelings or criticism towards you or your tech chops. I’ve recommended you to many people.
I just didn’t think your explanation here was specific enough for his needs. That’s all.
All, thanks for the replys, the problem ended up being as simple as cleaning the contacts on the oscillator and contour generator boards. Now the Model D is tuning up well. Now that I have it working next step is to go through and reclean all contacts, replace electrolyic caps as needed, and get the keyboard all leveled out. Evenually id work like to refinnish the case as it has a spot wear the finnish has worn off. Thanks for all the replys I appreciate it!
