Tuning

Moog Prodigy Tuning

Step #00: Physical setup

Set the keyboard on it’s left side panel (side with wheels).

Remove 4 screws on bottom (sides), 1 on lower center of back panel.

Remove the bottom panel, to which the keyboard is attached, and place it down on the bench.

Remove all knobs and switch caps.

Carefully open and remove nuts holding circuit board with 5/16” driver.

Monitor the main output with an electronic tuner, a small audio amplifier, and an oscilloscope.

Step #0: Initial Settings

Set pitch wheel to 0 (center). Use DMM to confirm wiper is near 0.000V. (Reset pitch wheel position with 1/16" allen key if needed.) SYNC control should not change OSC 1 pitch if pitch wheel is properly centered.

Set TUNE control to 0 (center). Use DMM to confirm wiper is near 0.000V.

Set INTERVAL (OSC 2) control to UNISON (center).

(Optional step: Reset all trim pots. Only if tuning seems impossible and you want to "start over.")

Center all these trim pots:

R14 Chip Temp Osc 1

R35 Chip Temp Osc 2

R1 Osc 1 Freq

R7 Osc 1 Scale

R20 Osc 2 Freq

R28 Osc 2 Scale

Fully turn these trim pots: (Whichever way makes the high notes go FLAT.)

R42 Osc 1 Hi Freq

R60 Osc 2 Hi Freq

Chip Temp Adjustment

CHECKING THAT THE HEATER CIRCUIT IS WORKING:

Vce of Q6 and Q12 should be closer to 6V than 1V or 12V if the circuit is working properly. Near 12V indicates the heater is cutoff/cold, and near 1V indicates the heater is saturated/hot. Near 6V, the heater has room to swing in either direction to compensate. NOTE THIS IS A Vce READING, WHICH MEANS ONE LEAD ON THE COLLECTOR, ONE LEAD ON THE EMITTER (NOT BLACK LEAD TO GROUND).

Check pin 7 on both U2 and U4. This should be around -9V. If it is +10V, then the heater circuit is saturated, will not track temperature, and you will get drift problems.

Trims R14 and R35 adjust the voltage at pin 6 of both U2 and U4. Turning the trim, you should be able to get pin 7 to a negative output. If the output is stuck at +10V, then check and/or replace Q6/Q12 (2N3904) and/or U1/U3 (CA3046 transistor array). The heater circuit will not work unless pin 7 on both U2 and U4 is negative.

Q6/Q12 operates as a current sink for the Q5/Q10 inside the transistor array. The Vce of Q6/Q12 (voltage from collector to emitter) should indicate that the transistor is somewhere in its forward active range. If you're seeing 12V across it, it is probably cutoff and the heater circuit is off/cold. If you're seeing less than 1V across it, it is saturated, and the heater circuit is maximally hot, probably not able to track temperature (pin 7 on U2/U4 is probably at +10V). Q6/Q12 is forward active when there is about a 600mV Vbe and Vce is somewhere between 1V and 12V (6V would be ideal for maximum swing in either direction).

Since Q5/Q10 has a grounded base, the emitter should always be around -0.6V, and the collector of Q6/Q12 will therefore be somewhere between -0.6V (closer to ground than that indicates cutoff condition) or near -11V (indicating saturated condition).

What about Q3/Q11? These are the temperature "sensors" of the system. The Vbe (voltage from base to emitter) should increase slightly with temperature. This slight change of voltage will be "read" at comparator U2B/U4B's non-inverting input, and the output should change slightly to adjust the current sink, which adjusts the current through "heater" transistor Q5/Q10. Since our "sensor" and "heater" are on the same IC chip, this should result in a temperature compensation system.

ONCE VERIFIED THAT HEATER IS WORKING, THIS IS THE SERVICE MANUAL CALIBRATION FOR HEATER CIRCUIT:

(This is an optional step. Adjusting these trims means you will probably have to retune! This is probably only needed if the heater circuit was found to be cutoff or saturated, or if transistor array chips U1 or U3 were replaced.)

Short R17 and R38 with alligator clips. (This simply turns OFF both heater circuits.)

Wait 5 minutes. (Allowing the "heater" transistor to cool off and not contribute to the "sensor" temp.)

Record voltage of U2 pin 5. (Non-compensated "temperature sensor" voltage.)

Remove short on R17 and record voltage of U2 pin 5 again. Watch and wait for 30 seconds, or until voltage seems stable. (Compensated "temperature sensor" voltage.)

Adjust R14 so that the normal reading is 60mV less than the shorted reading. (Assumes 72ºF/22ºC room temp; higher drop for colder temps, and lower drop for warmer temps - see Service Manual "Table 1")

(Repeat for OSC 2:)

Record voltage of U4 pin 5.

Remove short on R38 and record voltage of U4 pin 5 again. Watch and wait for 30 seconds, or until voltage seems stable.

Adjust R35 so that the normal reading is 60mV less than the shorted reading. (Assumes 72ºF room temp)

NOTE: R14 and R35 will also set the range of tuning. If tuning seems impossible, use R14 and R35 to adjust the oscillator’s range, and try calibration again. Be careful not to saturate the heater circuit when doing this. See previous section to test if heater is active, saturated, or cutoff.

External CV Input ("OSC IN" Rear Jack)

If you have an "OSC IN" jack, connect this to a something that will give you accurate 1V steps and use this to scale the synth to 1V/Oct. I've found that doing the regular keyboard tuning first may not result in 1V/Oct tuning.

If you have an external CV input, leave the keyboard either disconnected, or hit the low F so that the keyboard's contribution is near 0.000V. Tune up using your external 1V/Oct CV, then adjust the "keyboard scale" trim (R92) to match. You probably have the "KYBD IN/OUT" jack as well, and when you are done this should give you a 1V/Oct output.

The following procedure assumes there is no external CV input.

Keyboard Scale Adjustment

(Service manual procedure)

Play F1, read voltage at U12 pin 1. (Should be near 0V)

Play F2, adjust R92 so voltage = [first reading] + 1V. (1V/oct)

(Note: I've found this can lead to inaccurate 1V/Oct tuning. It is better to monitor the "KYBD IN/OUT" node for 1V/Oct.)

Tuning Oscillator 1

Only listen to Osc 1. Set filter open and envelope to just full sustain. Mod switches off.

Set Osc 1 to 32’

Play F1 and tune to F 43.6 Hz using "Range" R1 (R3 on early units)

Play F3 and compare observed note to F 174.6 Hz. If flat, use "Scale" R7 to make more flat, if sharp use "Scale" R7 to make more sharp.

Play F1 again, retune using R1 (R3 on early units), and repeat.

Adjust R14, chip temp, if you reach the end of the trim pot (see notes in that section, above).

Hi Freq Compensation

Set Osc 1 to 8’

Play F3 and compared observed note to F 698.4 Hz.

Note should be somewhat flat. Check that R42 is set completely FLAT. If note is SHARP, redo scaling until octaves from low note up get progessively flatter.

When ready, use R42 to stretch the high F up to pitch, and recheck all octaves.

Tuning Oscillator 2

Only listen to Osc 2. Set filter open and envelope to just full sustain. Mod switches off.

Set Osc 2 to 16’

Play F1 and tune to F 88.8 Hz using "Range" R20 (R23 on early units).

Play F3 and compare observed note to F 349.2 Hz. If flat, use "Scale" R28 to make more flat, if sharp use "Scale" R28 to make more sharp.

Go back and forth until both are correct.

Adjust R35, chip temp, if you reach the end of the trim pot (see notes in that section, above).

Hi Freq Compensation

Set Osc 2 to 4’

Play F3 and compared observed note to F 698.4 Hz.

Note should be somewhat flat. Check that R60 (R63 on early units) is set completely FLAT. If note is SHARP, redo scaling until octaves from low note up get progessively flatter.

When ready, use R60 to stretch the high F up to pitch, and recheck all octaves.

Tuning VCF Sine Wave Osc

Turn OSC 1 and 2 mixer levels to OFF.

Set CUTOFF FREQ control to 0 (at center).

Set EMPHASIS control to full CW (self oscillation).

Check that FILTER MOD switch is OFF.

Turn CONTOUR AMOUNT fully CCW (off).

FILTER KBD TRACKING to OFF.

Play F1 and verify that there is oscillation.

Adjust R140 until oscillation is 660Hz +/- 50Hz.

Monitor U16B pin 7 for an output of “-23dB +/-3dB” (I think this is 50mVrms, or just whatever sounds like a good level in the output mixer).

SWITCH FILTER KBD TRACKING TO FULL.

Play F1 and set CUTOFF FREQ control to F 349.2Hz.

Play F3 and compare observed note to F 1396.5Hz. If flat, use R152 to make more flat. If sharp, use R152 to make more sharp. Retune F1 using CUTOFF FREQ, and repeat until tuned.

(These designators may be different on early units.)

VCA trim

Set LFO to full CW (30Hz), and set for square wave.

Turn off all inputs to the mixer.

Take a 20k resistor, and use it to connect the raw LFO square wave (U13A, pin 1) to the VCA’s control input at the emitter of Q39.

Monitor U16A (pin 1) output and adjust R120 for minimum output of square wave.

(These designators may be different on early units.)

This concludes the Prodigy tune up.

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