Model D calibration problem

The Moog Forum Moog Music General Topics Forum
1

I am finally getting around to tuning the model D. The problem I am seeing is that the instructions I am reading say to tune the high A at 16’ to 440, and the low A at 16’ to 55hz, and my low A will not go down that far even when tuning down all trim pots on all 3 oscillators. It seems that 110 hz at low A is where the trimpots seem to gravitate to. I thought it might be the pitch wheel tuned up an octave, but I checked the resistance at the detent and it it 15.6k. Any suggestions?

edit: I can get a 55z at low A/16’ only if I turn the tune knob all the way down.

2

Old OSC board or new OSC board ?

3

new, I think this may have to do with the pitch wheel. Will the mini function without the wheel assembly attached?

4

okay, but can you play a low A at 32’, and tell me the frequency? Is it 55 or 22.5?

5

I’ve got an old board…

Your problem is for OSC 1 ?

6

all oscillators. I am going by:

http://balduin.wordpress.com/category/my-stuff/minimoog/

says at 16" high A is 440, low A is 55hz

7

Okay. I’ll get my facts straight. I’ve brought out my iPad with Strobosoft and iRig, plugged in the headphones out of my Mini.

On 8’ range, I get A5 (880) on highest A, and A2(110) at lowest A on keyboard.
On 16’ range, I get A4(440) at highest A, and A1 (55) at lowest A on keyboard.

8

It could possibly be an intermittent contact on either the keyboard connector, or most likely the mod wheel connector. Easily checked by simply removing the two front screws on the top of the mod wheel assembly. You can then reach the cinch-jones connector and “jiggle” it while playing a key and see if the pitch changes.

9

Yeah that’s what i thought, thanks. Something is screwed up. I don’t think it really matters, I can just tune an octave up, which is where it was at since I’ve had it I guess. I’m pretty sure the pitch wheel is what is causing it, even though it is reading the correct resistance. Do you think its safe to remove the wheel assembly connector and turn the mini on? Sorry for all the questions.

10

I checked this, its fine. Whenever my pitch wheel came loose, and I fixed it, I think I tuned it an octave up. The problem is, when I fixed it, I think I tightened the allen screw too much, so the plastic cracked a little and the pot arm lodged into place in the wheel, and now the wheel will not come loose.

11

Yes, but the pitch will be completely offset…


…Sorry for all the questions.

That’s perfectly okay. That’s what forums are for !


If, as you say in an earlier post, your pitch bend pot is showing 15.6K at notch, then it’s in the right position. You should then be able to get A1 (55Hz) playing the lowest A on the keyboard with range at 16’.
If not, then something is wrong…

12

So the pitch wheel actually adds a voltage to the pitch? It would seen that the pitch wheel at detent would be 0 volts, but I have no idea. I removed the wheels and was able to get A 55hz at 16’ on all oscillators. This tells me something is wrong with the pitch wheel. You say 15kohm to 15.6 kohm is the proper resistance on another thread, maybe if I got it down to 15k it would work.

13

Yes the pitch bend pot sends an additional voltage and act as a second “tune” knob.

At the notch detent, it should measure 15K (according to service manual). I don’t think a variation from 15k to 15.6K would result in a complete octave shift however. Of course when it says 15k ohms, that is relative to ground. In other words, measured from the ground side of the pot to the wiper in the middle. Not the other way around, that is to say, from 10V side to wiper… Especially since the pot only makes a 90 degrees turn…

14

I’ve taken out my mod wheel assembly. At detent position, the pitch bend pot measures 15.2K. Fully down it measures 10.2K. Fully up it measures 20K. Measurements taken with mod wheels assembly disconnected from Minimoog, and DMM probes connected to ground (white/black wire) and wiper (orange wire) on pitch bend pot.

15

Thanks, you didn’t have to do that. My ground wire is white, while the wiper is orange, hot is red. Reading 15.65 at detent, 9.99 down, 21.76 up. So I guess the question is whether I should tune the pot down, or the oscillators up an octave. If the pot is tuned to a different resistance, it doesn’t seem like it could do any harm, as long as the keyboard tracks well.

16

If your Mini’s oscillators are scaled correctly and track accurately, it would be much easier just to offset the position of the plastic wheel on the pitch bend pot shaft, instead of retuning all three oscillators.

That pot is supposed to be a 25K linear so, in theory, it should provide the same impedance variations (range) over 90 degrees anywhere along its travel. In theory.

And the other question is, can you loosen the setscrew enough to be able to offset the plastic wheel position on the shaft ?

17

Do you think its safe to remove the wheel assembly connector and turn the mini on?

It’s safe to do so, but it probably won’t play at all.
The env gates run through those connectors.
And yes, the pitch will be way off anyway.

1st thing to always check is the power supply.
The voltages, measured either at the rectifier board or first three pins of any board, should be +10, gnd, -10 respectively.
Make sure the main tuning knob and VCO 2 & 3 tuning knobs are centered.

If it’s the old board (7 trimmers), the basic routine is to tune the oscillators together or to a reference when hitting a high note (adjust range), then hit the lowest note and adjust the scale trim(s.)
Repeat back and forth until perfect.

If all the oscillators track with each other fine, but are still off compared to A-440, it could be the keyboard interface.
Specifically, the 590 ohm 1% resistor at the top of the env board (board #2.)

I also wouldn’t mess with the octave selectors at all during this.
Set them all to 8’ or 16’, tune and scale the oscs, THEN adjust the octave trimmer on the bottom of the osc board (if an older board.)
Later boards have an octave adjust trimmer separately for each osc.

Sometimes they won’t tune because of something overlooked (or we haven’t been told) like a recently installed buffer board, recent repairs, etc.
It may be a good idea to start with all pertinent trims centered too.

By the way, the tuning instructions in the service manual are wrong.
Moog later printed a correction sheet.
But it’s not all that important if you grasp the concept of tuning the range trims for high notes and the scale trims for low notes (and repeating until perfect.)
If it’s a newer osc board, the order is reversed: low notes are first adjusted via range trims, high notes are adjusted via scale trims.

And yes, if everything is scaling fine and the entire instrument is still too high or low in tuning to a reference, it may be the pitch wheel isn’t on its shaft correctly.

But again.. make sure the power supply is perfect first.
It’s a voltage controlled synth and if the power supply is off, everything will be off.

Good luck.

18

Very pertinent info from Kevin.
Especially since it’s a V/oct synth, a deviation of only one volt from the PSU could mean a difference of up to one octave at the oscillators…

Anytime one wants to perform a recalibration of Any analog synth, the power supply Must be the very first thing to adjust precisely, indeed. I had assumed that unfiltered37 did this prior to starting his endeavour. Then again, maybe not…

Good reminder, thanks Kevin.

19

I had similar issues but found once I got a frequency counter and set the high range of Ocs 1. To 3250?? if I remember correctly, If this isnt correct it’ll throw it off.