Moog MG-1 fall time sustain problem

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1

Picked up two of these recently and other than the black “goo” they are in good shape other than a few dirty sliders which have been desoldered and cleaned and resoldered again. The one remaining issue is that when one of the MG-1’s is switched to “contour” the “fall time” slider acts as a volume for a sustain, which is not on. The “rise time” works as it should. As mentioned already I have cleaned the sliders and that particular switch. I have checked it with the other one I have and that is not how it should work…obviously.

Any ideas?

2

Possibly defective U14 (CD4007) maybe ?

3

Would that be the case if the “rise time” still functions correctly?

4

The 4007 consists of three complementary pairs of MOS transistors. It’s possible that only one pair is damaged or malfunctioning inside the chip.

It could also be the Sustain IN - OUT switch that isn’t making proper contact. Check for proper operation on all pins with a multimeter.

If the switch is working correctly, then it could also be a defective (U13) 4558 (prone to failure).

When in doubt, replace. Very inexpensive.