Archive of the former Yahoo!Groups mailing list: MOTM

While building up a couple of MOTM-110 VCA/Ring Mods, I found the GAIN dial
did not accurately reflect the actual output levels.

Now, don't panic! in 99% of the VCA uses, all you set is the 0 gain point,
and the
CV mod voltage does the rest.

Also, the 100K pot is +-15% or so, the CEM3330 is +-10% or so, which means
without trims (not doing that!) you will be off no matter what.

Here is what I found worked best:

a) change R26 from a 56K to a 27K
b) change R5 from 100K to 39K

what I found was in "0 point knob cal" mode (which as you recall you turn
the pot, without the knob on,
to the point where the VCA starts outputting signal [ear or by 'scope] and
∗then∗ attach the knob, with
the pointer at 0), the pointer was very close to both the '0' GAIN and the
'1' gain points being correct.

Now, there is a mess-up on the panel label: the "1.2" is really "2"! I just
never caught it until now.

With the current values, setting the '0' point makes the unity gain point
more like half-way between
'1' and '1.2' on the scale. The new resistor values "shove" the range CCW.

Also, and this is CEM3330 dependent, at +5V CV IN, mod=10, the gain on 3
units was not unity
but like 0.87. You can fix this by adjusting R27 up.

In conclusion: there is NO REQUIREMENT to do this mod unless you are anal
about the knob
scale (why do I care?....∗magazine reviews coming soon!∗) Don't want to have
this become an issue
with SOS (they actually check this sort of stuff!)

MOD #2

I found that adding a small cap, of 22pf to 33pf directly across R27 (on top
of the part, solder to the
pads) reduces noise and excess ringing on square waves when driving longer
(say 2 feet+)
patch cords. When the current pcbs run out, I will add this to the pc board.
This is a quick, easy
fix that costs less than 20 cents from Mouser. This mod is not required, but
"highly suggested".

SIDEBAR

These mods bring up a point: all DIY'ers/MOTM builders ∗need to have a
supply of parts∗. By this
I mean:

Some 5% resistors (Mouser sells a great assortment pak) + get some 100K 1%
Some axial ceramic caps (22pf, 33pf, 100pf, 470pf, 1000pf)
Couple of electrolytics
Couple of diodes (1N4148)
Couple of TL072ACN op amps (can be used for most any dual op amp in MOTM as
a back-up).

Certainly, everyone has a nice supply of plastic bags to arrange these! :)
This will prevent the late night discovery that I messed up the resistors in
your kit and
your are short a 1K! (the upcoming MOTM-320 has 80 resistors. I'm sure I'll
mess
somebody up as we are talking close to 5000 resistors sorted, counted, and
bagged).

Of course I'll send out any missing parts, but also for mods and the
occasional "drop&roll
across the tile floor" useage it's nice to have a little stockroom.

Paul S.