Archive of the former Yahoo!Groups mailing list: MOTM

Last September, 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 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) anyhow, I just removed it.

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 knob 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 go 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 22pF cap and it put an end to the
ringing or oscillation. Thanks Paul.

800 modification:
I also added the one resistor and diode required to change the 800 so it
will work with gate only. These changes are documented in Paul's newest
schematic for the 800. And, Roy Tate gave a very detailed explanation of
how to add the parts a while back. I found his suggestions to be just
about the best way to do it for me too. I did get some 1/16" heat shrink
and use it where I soldered the resistor to the wire so I could lay it down
securely to the board without fear or shorting against anything. This is a
very useful modification to the 800.

On a side note, when I was doing my testing, I found out something useful
and cool about the 800. I don't know if this is true of NON-modified 800s
since I didn't discover until I had modified all of mine. As you know, the
800 will work with a trigger only as an AR EG using the Attack and Decay
knobs (with sustain set to zero). Well, I inadvertently plugged a gate
into my 800 trigger with nothing in the gate jack. To my surprise, I found
that the EG still acts like an AR EG with one HUGE difference -- the EG
starts on key release !!

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
some modifications suggested by Paul late last year. 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.

Larry Hendry