[sdiy] Improving Moog 921

[Terry Michaels]

Message text written by riccardo
>Hi, 
I do not have the Minimoog schematics right here, but if  I remember well,
its summing section derives from the 921A.
So, as the Mini also drifts in tune, what about resistors and 741 op amps
on Minimoog's summing section?

ciao
riccardo
<

Hi Riccardo:

I did an upgrade on my Minimoog, it has the earlier oscillator board that
uses the SG3821 (CA3046) expo transistors, not the later oscillator board
with the UA726.  I made two changes to this Minimoog:

#1.  Replaced the 741 CV summing amps with OP07's.

#2.  Moog originally put the tempco's several inches away from the
SG3821's.  This is fixed as follows:  Remove the Q81 type tempco resistors
from the circuit board, add several inches of insulated wire to the leads,
and reinstall the tempco's right on top of the SG3821's.  The added wires
go to where the tempco leads originally were soldered to the board.  A few
dabs of silicon glue hold them in place.  Oscillators 1 and 2 share a
SG3821, so you will have two tempco's on that one chip.

I didn't make careful before and after tests to determine how much of an
improvement these changes made, but this Minimoog seems to be much more
stable than before, I find it requires much less front panel adjustment and
tweaking to keep it in tune.

The really early (R.A. Moog) Minimoog with the all discrete VCO board could
be upgraded by moving the tempco resistor as above, but it used a discrete
opamp CV summer which would be more difficult to replace.

Terry Michaels