MOTM-110 Tune-up – Additions 2/27/02

IN September 1999, Paul sent out some "factory approved" modifications
for the MOTM-110 VCA. He clearly stated that "your results might vary" since
the 100 K pot is +/- 15% and the CEM is +/- 10%. And, mine did. The values
he suggested did not work out for me. However, he did get me started. So, I
spent a lot of time determining exactly what did work for me.

First, I wanted to set the zero point on the gain knob. This is done by adjusting the
value of R26. I adjusted my knob so that at full clockwise rotation, it points at the last
tick. Call me anal, but I don't like a knob that turns past either the first or last tick.

Paul's suggested change for R26 was to go from 56K to 27K. This offset my zero
point too much the opposite way. To best match MY pot VR2, I found that 39K was
the correct number for R26. If you want to dial yours in exactly, here's how:

- Make sure nothing is connected to CV IN.
- Stick a 100K pot in the place of R26.
- Set the GAIN right on the zero tick.
- Put your voltmeter on pin 2 of VR2 (or where it connects to R5)
- Adjust your temporary pot until VR2 pin 2 reads exactly zero.
- Read the value of your 100K pot.
- That is the size of your R26.

Paul suggested changing R5 from 100K to 39K. On MY 110, this produced too much
gain when the pot was turned all the way up. On MY 110, the value that caused unity
gain at the "1" dial position was 47K. When complete, my 110 gain is 0 at exactly 0, and
1 at exactly 1. However, the reduction in R5 causes the gain at full clockwise to be
greater than 2.0. It is closer to 3.0. In fact with a full 10 peak to peak input, clipping can
start near the last tick. However, I left it that way because for lower level signals, the
extra gain is nice. Since the 1.2 silk screen number was an "mess-up" (those were
Paul's words), I just removed it. If you want no clipping possible for your 110 with a full
10 volt peak-to-peak signal and the gain adjusted to full, maybe 56K might be a better
choice for you. I like having the gain = exactly 1 when the gain knob pointer points to 1.

The other thing I found on MY 110 was that with the gain set to zero, using CV control
and the CV mod set to 10, I could not get even up to unity gain from the approximate 5
volts from a MOTM-800 EG. So, I decided to adjust R1. The smaller the value of R1,
the more effect the same voltage will have on gain. I wanted unity gain with the CV MOD
knob set at 10. For me, that meant changing R1 from 56K to 47K. I arrived at that value
by connecting one of my 800 outs to the CV in and setting the CV Mod knob to 10.
I carefully measured (with scope) the input peak-to-peak voltage. I adjusted R1 until
I measured the same voltage OUT as I had IN.

The one other modification that Paul suggested was putting a 22 or 33 pF cap directly
across R27. At higher gains, there tends to be some high frequency oscillation. I did
add a 22 pF cap and it put an end to the ringing or oscillation. Thanks Paul.

Summary - So, here are the changes I made to my MOTM-110:
- Change R26 from 56K to 39K – sets exact zero point for gain knob
- Change R5 from 100K to 47K – sets gain at 1 when gain knob points to 1
- Change R1 from 56K to 47K – sets CV mod effect to exactly 1.0 when the knob
is turned to 10 (full scale). This was determined using a MOTM-800 for control.
Performance with other EGs might vary if their CV out is significantly different than
the MOTM-800.
- Add 22 pF cap across R27

Disclaimer: I am not an engineer. I am not suggesting that you change your 110 or
take any of my advice. However, I am reporting MY RESULTS on variations of some
modifications originally suggested by Paul. In the end, I am glad that I gave my 110 this
"tune up." Special thanks to Paul for publishing the suggested modifications in the first
place. Remember, your results may vary.

New! MOTM User submission – I recently received a nice note from Paul Haneberg
with photos showing how he completed his 110 tune-up with trimmers since results do
clearly vary from module to module. To see Paul’s modifications and photos, click here.
Thanks for sharing Paul.

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