Archive of the former Yahoo!Groups mailing list: MOTM

I went to work on Friday, but nobody else showed up, so I figured it
was a good day to hack my synth. What follows is a summary of what I
changed.

1. MOTM-101 Modifications

Changed R11 from 100K to 27K to get 10 Vpp on RANDOM output. Random
level is now similar to PINK level.

2. MOTM-120 Modifications

Changed R10 from 33K to 200K to get 10 Vpp output from any one of the
sub octave outputs. As designed, the only way to achieve 10 Vpp
output is to have the two IN level knobs and all of the SUB level
knobs set to 10. With this mod, the two inputs and each of the sub
octaves will output 10 Vpp when their associated output level knob is
set to 10.

Note that this mod multiples each of the 5 Vpp outputs from the logic
circuitry by two to get 10 Vpp levels. This doubling of amplitude
also applies to the A IN and B IN signals. If the MIX A switch is set
to NORM and a 10 Vpp signal is applied to the A input, the output
level will be 20 Vpp when the A IN knob is set to 10. The same holds
true for the B input. A voltage divider can be constructed directly
across the input jacks but I haven't bothered to do this; I can just
turn down the A/B IN levels.

3. MOTM-410 Modifications

Changed R53, R56, and R59 to get the filters to track each other more
closely. I have two 410s and when all six filters were tuned to
generate maximum output from a 1 KHz sine wave, the settings for the
FREQ knobs varied between 2.0 and 4.5 (a knob variance of 2.5). After
making this mod, the knob variance at 1 KHz is zero and is less than
0.5 over a 10 KHz range.

Procedure
---------
1. Replace R53, R56, and R59 with 500K pots. Preset the pots to a 180K value.
2. Set SWEEP, DEPTH, and all FREQ knobs to zero.
3. Power up module. Apply 1 KHz sine wave to IN jack. Put one channel
of scope on IN jack. Let circuit warm up for at least 10 minutes.
4. Put second channel of scope on OUT 1 jack. Set FREQ 1 to 4.5.
Adjust R53 until OUT 1 is at maximum (output will be 180 degrees out
of phase with input).
5. Put second channel of scope on OUT 2 jack. Set FREQ 1 to zero, set
FREQ 2 to 4.5. Adjust R56 until OUT 2 is at maximum (output will be
180 degrees out of phase with input).
6. Put second channel of scope on OUT 3 jack. Set FREQ 2 to zero, set
FREQ 3 to 4.5. Adjust R59 until OUT 3 is at maximum (output will be
180 degrees out of phase with input).
7. Power down module. Disconnect 500K pots and measure pot values.
Replace R53, R56, and R59 with measured values.

Make sure that only one of the FREQ pots is set to non zero at any
time. Otherwise it will be difficult to get maximum output at
resonance (the three filters interact with each other causing a dip
in the output when more than one is set to the same frequency).

Give the circuit plenty of time to warm up before doing this mod (at
least 10 minutes), the filters tend to drift quite a bit when power
is applied.

My Results
----------

410 #1
------
R53 = 180K (did not change, used this as the reference)
R56 = 200K
R59 = 375K

410 #2
------
R53 = 91K
R56 = 191K
R59 = 220K