The Hi end pot

[Palle]

Can someone, please, explain what the Hi end pot does on the later Model D:s? What is affected when turning it?
As I understand it, Octave tunes a key, and Scale determines the width of the pitch relative to the keyboard. Calibration seems to work well with those two pots, which makes Hi end a question mark to me.

[till]

This is adjusting the keytracking in higher octaves. This is needed, for even more accurate tuning over several octaves, as the exponential function of the transistors is not a perfect exponential one. You might get close to a usable tuning without hi end adjustments on some units, but you need it on others. Not all transistors are 100% identical in their curves. And it depends also on how perfect you would like to have the keyboard tracking.

I made some A/B with a Prophet-5 Rev.4 against two vintage Rev.3.x ones two years ago. The only easy to difference was, that the top octave of the vintage ones were both a bit flat in tuning. The hi end was most likely not calibrated perfectly here. But the vintage instruments were not mine.

[Palle]

Thanks a lot for answering. So, it's a kind of Scale for higher octaves. Any specific ranges or frequencies?

[till]

VCO on 2' footage, top A (should be at 3520 Hz if unit is tuned to A=440 Hz).
So we are talking about close to the highest normal playable note (2' top C = 4186. 009 Hz) of a Minimoog.

From the Service Manual of later (above SN #10175) Minimoogs:

NOTE 1
To tune OSCILLATOR 1, turn A-440 ON, OSCILLATOR 1 ON, octave RANGE 1 at 8' and center TUNE control.
To tune OSCILLATOR 2, turn A-440 OFF. OSCILLATOR 1 and 2 ON, octave RANGE 1 and 2 at 8' and center OSCILLATOR 2 control.
To tune OSCILLATOR 3, turn A-440 OFF, OSCILLATOR 1 and 3 ON, octave RANGE 1 and 3 at 8' and center OSCILLATOR 3 control.

NOTE 2
To obtain a zero beat, it may be necessary to make a slight adjustment on the front panel as follows:
TUNE control when tuning OSCILLATOR 1
OSCILLATOR 2 control when tuning OSCILLATOR 2.
OSCILLATOR 3 control when tuning OSCILLATOR 3.

5.3.1 SCALE TRIMPOT ADJUSTMENTS (Figures 5-2 and 5-4)
Set octave RANGE at 8.' Refer to Note 1.
b. Press low A (55Hz) and zero beat with shift trimpot. Refer to Note 2
c. Press high A (440Hz) and zero beat with scale trimpot.
d. Repeat steps b and c until low A and high A zero beat

5.3.2 HIGH END COMPENSATION
a. Octave range is 2' Refer to Note 1 and substitute 2' for 8' -> actually substitute 8' for 2'!
b. Press low A (440Hz) and zero beat with shift trimpot. Refer to Note .2
c. Press high A (3520Hz) and zero beat with high end trimpot.
d. Repeat steps b and c until low A and high A zero beat.
e. Recheck paragraph 5.3.1 and repeat paragraphs 5.3.1 and 5.3.2 if necessary

The 12 trim holes at the later Minimoogs backside of the panel are:
The rows are OSC 1 to OSC 3 from top to bottom.
The columns are (left to right):
HI END
SCALE
OCT
SHIFT

[Palle]

Exactly what I need, thanks!