[sdiy] [AH] Improving Moog 921A [long]

[Paul Schreiber]

I thought I'd share the procedure to *greatly* improve the performance of
the Moog 921A
Oscillator driver module. Yes, in my "spare" time I restore Moog modulars.

DISCLAIMER!

Moog 'purists' may croak over dead. Kevin Lightner may call me names.

This eliminates 95% of the temperature drift in Moog
VCOs. If you think Klaus Shultz is correct, in that tempreature drift of
VCOs is what
makes a Moog, well, a MOOG, then don't do this. However, if you really
prefer your VCOs not
to drift 3 semitones when the air temperature changes 10 degrees, read on :)

The cost of this mod is about $25. I did it in about 1 hour.

The following mods are listed in order of preference.

Step 1: Replace the rusted lockwashers

All Moog modulars I have seen (especially the RA Moog ones) suffer from
having non-plated
lockwashers. These rust horribly. Now, normally this is not much of an issue
because they
are behind the panel. BUT...Moog did something 'naughty': he uses the panels
as a common
ground connection. Since the front of the panels are anodized (ie
*insulated*) then he is dependeing
on the lockwasher to 'bite' into the rear panel for grounding.

Well, he *didn't* count on the washers rusting. And they rust magnificantly.
I measured 6 off of the
VCA, and all 6 read over 10M of resistance. You may get lucky, and have the
bushing threads
contact the inside of the mounting hole.

So, simply remove the jack, toss the lockwasher, and replace with a new one.
These are standard 0.375 ID
internal-locktooth washers. MOTM uses nickel-plated ones.

Step 2: Replace the 3 100K input summing resistors + feedback resistor

Here is the *second most* source of error (temp drift, mainly) in the module
(remember, this thing is driving
the control voltage to the VCO). The resistors are located in the lower left
corner. Notice they are marked RN60D.

RN60D is a standard military resistor designation. The RN60 means 1/2W. Now,
you certainly don't *need*
1/2W but in 1972 these were the easiest to buy. We want to use RN55 instead
(1/4W). But the bad news
here is the 'D'. This is the *lowest* grade in the line: in fact, the specs
are -500ppm to +200ppm drift. This is
actually *worse* than the standard carbon film resistors today at +-200ppm.
These 3 resistors, which sum
the main VCO CV, MUST BE REPLACED unless you really like the Moog's VCO
drift.

They are easily de-soldered (Moog used single-sided, non-plated pc board).
Replace with Mouser (www.mouser.com)
part number 279-100K at a staggering $1.08ea. These are HOLCO 0.1%, +-25ppm
drift parts. An
improvement of 10 TIMES lower drift.

Next, replace the 48.7K RN60D resistor with Mouser 279-47.5K. This reduces
drift gain error and the
slightly smaller sizer makes the trim for 1V/OCT must easier.

Step 3: Replace the 3 LM741s with LT1006CN8

Back in '72, the LM741 op amp was king. Well, amost by default :) Not many
op amps were out there. Moog
even sprung for the more $$ '393' grade (see the data seet at
www.synthtech.com, CEM section). But, in light
of today's op amps, the LM741 look pretty shabby.

So, order 4ea LT1006CN8 from DigiKey (www.digikey.com). These unsolder
easily, and the LT1006 is an exact
pin-for-pin replacement with 100 TIMES lower dc drift over temperature.
Wonder why the MOTM-300 VCO
is "Ultra"? Here is a clue :)

The extra part is used next.

Step 4: Replace the LM308D op amp with LT1006

This is the only "trick" part. But, it is THE #1 drifting component!!

a) remove and sell on eBay the following no-longer-need parts: both 100p
disc caps and the 330 ohm resistor.
These are right next to the op amp. SURE would be nice in Moog and paid $2
more per pcb and had them
SILKSCREENED!! (cough cough).

b) Remove the pretty gold-plated ceramic LM308D. Sell on eBay for $100.
Claim Emmo peed on it on '76.

c) Now is the TRICKY part. The LM308D has 14 pins, the LT1006 has 8. So,
being clever we note
all the pins are tranlated EXCEPT the -15V pin. So, insert Pin 1 of the
LT1006 where Pin 3 was for
the LM308D.

d). Solder a small wire from Pin 4 of the LT1006 (now really where pin 6
was) to the empty hole where
pin 7 was (and where -15V is).

THAT'S IT!

Recal the 3 trimmers, and if you are going all-out, replace the tarnished
Switchcraft jacks (www.newark.com).

Paul Schreiber
Synthesis Technology