[sdiy] Moog 904A filter questions

[Michael Buchstaller]

Hello List friends, gurus etc,

inspired from the thread on the 904A, i decided to build a clone, too.

In general, my module works, but i have some questions:

1. CL-1 Diode at the base of Q6:
What type of Diode is that ? I used an 1N4148 and a 1N4004, but with both
i cannot get close to the +0.64V indicated in the schematic.
Also tried a germanium diode, but no luck.

3. R11 (5 K pot) for fixed CV:
Shall this be be a linear pot ? How is it connected to the panel ? (-6V CCW and 0V CW,
or the other way round)

2. CV input amplifier:
The Input summing node ("control node") seems to be extremely muted.
When i apply a CV to one of the inputs, i can hardly detect any changes at this
point. However, wt the collector of Q5 i can see changes. (should be 0...0.12V, i measured
0.006 ... 0.126 V)
But i am having problems calibrating the scale with the R8 trimmer. When the fixed CV pot is
set to "0" (assuming that this is the center position), the R8 has hardly any effect. it works only
over the area of 0...-3 V.

3. Loudness problem
When having the filter calibrated as far as possible (all differential amps nulled out, 2V over R23, etc.
and i do a filter sweep from fully open to fully closed, the output signal becomes louder (appros. 3 db) at one
point of the sweep, regardless of range used or input signal waveform (Resonance off or set to 50K)
What could that be ?

4. CV bleedthrough
I can null out the CV pretty good, but at some point (dpending on the CV level) it leaks through with an
amplitude almost the same than the audio signal.
This could be the same position as in #3.

5. Output signal is inverted
Is this an error, or is it intended to work this way ?

6. The service manual says in step 1 of the calibration procedure: "set R17 and R25 to mid position" and
then never again mentions those pots. How is one supposed to set them ?
I think R17 shall null out the diff amplifier Q22/Q23, yes ?
But what shall i set the 2.5 Ohm R25 to ? I can see no reaction when i change its setting.

7. Is R45 really intended to be 0.47 Ohms ?


I have used the original 2N3392/2N4058 transistors, matched to 2 mV with the recommended circuit.
Power supply is stable at 12.06/-6.04 Volts.

Many thanks to all yof you for any hints !!!


-Michael Buchstaller