[sdiy] Hot-rodding the Moog 921B [long]

[Paul Schreiber]

In conjunction with previous email on the 921A........

DISCLAIMER

Moog 'purists' may barf up hairballs. Certain list members will equate this
procedure
with body part removal using a dull butter knife.

+++++++++++++++++++++++++++++++++++++++++

In order of preference

1) Replace drifty summing node resistors

The following resistor is the #1 source of drift error in the 921B: 150K 5%
resistor R28. I *think* it's
R28, but without a SILKSCREEN, who can tell? It is connected to the CV
summing node (pin 2 of IC2)
to the wiper of the RANGE trim pot. Now, for some unknown reason, this
resistor is a common 5%
carbon film (+-300 ppm) resistor. This needs to be at least a 1%, 50ppm
part. Or better, a HOLCO
0.1% 25ppm part. It is located in the upper left corner, near the 2 trimpots
(brown, green, yellow bands).

Next, replace the following RN60D 1% parts all with HOLCO 0.1% 25ppm parts:

R103 (?), the 4.99K resistor to the right of the big honkin' polystyrene cap
R4, the 24.3K resistor (mine are a sky blue color) directly above the LM741
op amp
in the upper left corner.
R2, the 5% 56K (green, blue, orange) resistor right below the trimmers in
the upper left corner.
This is the #2 source of drift. Another 5% resistor to the summing mode
<gack>.
R10, the 18.7K reddish-brown resistor in-between the 2 diodes, under the
dual FET thingy.

2) Replace LM741 op amps.

Replace the 4 LM741 op amps with LT1006CN8. DO NOT replace the LM748 op amp:
it forms the
PULSE out waveshape. Want to preserve that :)

3) Replace the 0.01 poly cap

Replace the 0.01 28yr old cap with a factory fresh one from Mouser, part #
23PS310.

4) Replace rusted lockwashers

Might as well replace all the Switchcraft jacks, while you're at it.

5) Add new heat sink compound

With a small flat-bladed screwdriver, carefully pry the chip out of the
socket by the poly cap.
SURPRISE! It has a 1K tempco resistor lurking underneath. The chip's bottom
side is in
thermal contact with the tempco. Wipe off the caked white goop and apply new
white goop.
Replace chip.

THAT"S ALL! Recal and enjoy Moog VCO sound without wandering all over the
map.

Paul S.