Vintage Synth Repair

Hi All,

A Micromoog with a good steady power supply (-15.00V, 14.53V), and all other circuits working, has an incorrectly scaled oscillator, that can't be tuned by R331 Osc scale trim pot adjustments (trimpots are original) back into spec. The symptom is tuning the 2' range (with the keyboard control set to Drone) to exactly 698.5Hz (F5), and then switching down successively to 4', 8', 16' and 32' ranges detunes the oscillator by nearly a semitone per octave (i.e E, Eb, D, Db). The R330 Osc octave trim also isn't sufficient to compensate for the pitch drop. With the keyboard control switched on, and the Lo F key tuned to F5, mid F is roughly 1 semitone sharp, Hi F nearly 3 semitones. With the instrument correctly warmed up, the oscillator is steady, no scale or range drift, just a constant mis-scaling.

I checked the oscillator range is ok with R337 Osc. range trimpot.

Checking the keyboard CV at R331 Osc. Scale Trim, the keyboard circuit is generating a perfect 1V/Octave scale: 1.00V at mid F, 2.00V at Hi F. Even with R331 (determined the trimpot is fine) at lowest value (0 Ohms), which divides down to 1.91V at the centre tap of the trimpot (pressing Hi F), with 2.0V feeding from the Kbd CV, we still get a tone high (G2, not F2).

Checking the IC305 resistor pack and octave switch SW301 for the octave divider chain matches the resistance of a Multimoog circuit, - so it does not seem to be the divider chain.

Following the service manual Table 5-1 symptom G: Osc Summing Network Fault, I checked all oscillator CV inputs finding no unusual offsets, i.e. 0V for all inputs except the correct Kbd CV and Octave selector values. Acting on the manual's advice, I tried shorting each of the resistors to ground to try to diagnose the source of the pitch offset, without any change detected.

IC306 pins 3,8	R302	Octave select		short = no diff
IC306 pins 4,8	R330	Oct osc. trim		-3.62V (@ R329)
		R331	Kbd CV			Looks ok 0-2V correctly scaled.
IC306 pin 5	R339	Ribbon			0V ok
IC306 pin 7,8	R343	Extern Osc In		short = no diff
IC309 pin 1,2	R371	Mod routing		0V ok
IC309 pin 2, 3	R372	Mod routing		0V ok

CV values at octave divider SW301 to R329 (wrt Gnd of course):

32' 		-14.22V
16'		-10.66V
8'		-7.11V
4'		-3.55V
2'		0V
Wide		-7.11V (of course)

There is a minor pitch difference between SW302 Oscillator drone setting and when kbd selected of about 17.6 cents sharp.

I checked the oscillator control summing op-amp IC301A pin 1 output with the SW302 set to drone and switching SW301 octave range:

32'		4.88V
16'		3.90V
8'		2.92V
4'		1.94V (Manual p5-2 expects  ~ -2V for 4' wave length)
2'		0.96V
Wide		(a range of course)

I assume (can anyone confirm?) I should be seeing almost exactly 1V increments per octave switch.

Checking pin 7 of IC301b produces:

32'		-5.41V
16'		-4.43V
8'		-3.43V
4'		-2.44V
2'		-1.42V

All nicely offset by 0.42V. Switching the SW302 between drone and kbd changes the voltage by 10mV. Switching SW302 to oscillator off changes the readings by -10mV, i.e very little difference.

I replaced IC301 with another MC1458CP, with no variation in behaviour or voltage readings, but that was expected since I would assume a failing op-amp would have produced an unstable oscillator, but now the IC is socketed so I can (if need be) check current supply at pin 2. Pin 2 is reading 0V as it is acting as virtual ground (see service manual page 2-6) for the oscillator control current Io.

The next possible causes/questions to investigate are:

1. The paired transistors Q307 & Q308 functioning as zener diodes are leaky. How would I check? Reading current?
2. CR303 has failed (I doubt it's noisy). I need to check the bias voltage, I assume?
2. There is loading of the dual exponential current source due to impedance mismatch? This is suggested by page 2-6 in describing the octave divider chain. Hmm, wonder how I check that?
3. The filter control summing circuit IC302 is saturating causing the control inputs to couple into the oscillator control producing "a mysterious shift in filter [i.e oscillator in the reverse case] frequency". However checking IC302 pin 1 shows sane voltages for octave shifts and keyboard presses.
4. Any thing else anyone can suggest? Any kind soul with a Micromoog able to take some voltage/resistance readings for me? The Multimoog is also in need of repair (possible circuit board short), so it's a little limited in it's reference value at the moment.
5. Any other circuit readings anyone care to suggest?
6. Give up and play guitar instead? :^)

I noted a track from IC304 pin 1 to SW302 has been cut, but comparing against the Multimoog circuit, I suspect this is a factory modification as it isn't present on the Multimoog board, which is clearly a newer version of the main board than the Micromoog.

I have the service schematics, DVM, scope and determination. Any advice would be greatly appreciated!

Thanks!

Leigh
--
Leigh M. Smith
mailto:leigh@leighsmith.com
http://www.leighsmith.com