I’m trying to adjust the scale trimmer but it maxes out at only 4.6 volts instead of 5 volts.
Do I need to replace the trim pot with a modern multi-turn trimmer?
No. The scale circuitry is designed as voltage divider. If you replace the trim just by another (e. g. multiturn), the resistance is kept. So you can adjust more precise, but don’t increase the end voltage and you won’t reach 5V.
To ensure proper operation of scale do the following:
1.) Set SCALE potentiometer (front panel) to center position.
2.) Increase R10 (5k1, see service manual) to let’s say 10k.
3.) Try to adjust the SCALE trimmer to obtain 5V at highest note.
Reason: By increasing the resistor R10 the voltage divider line of the SCALE circuitry is divided in new voltage portions per component. The upper part (see service manual) with R4, SCALE trimmer and SCALE potentiometer gets a bigger voltage part, so you can provide more voltage with the scale process.
Thank you for this clear and detailed explanation. I will follow it and report the result here.
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I’d like to point out a printing error in the Moog Modular service manual which can be seen in the schematic excerpt that you posted. Q5 which is a 2N3702 is shown as an NPN transistor when in fact it is a PNP transistor. The emitter should be shown connected to R6 and the collector should be connected to the resistor string on the keyboard. The circuit will not work correctly with an NPN transistor installed for Q5
I see. Thanks for clarification. I cloned the 950 which has a similar circuitry, so I did not notice the error in the 951 circuit drawing.
But this does not affect the tests with R10 I recommended above.