Model D calibration problem

[thealien666]

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 ?

[Kevin Lightner]

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.

[thealien666]

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.

[Wilby]

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.