MOTM

I just finished testing of an MOTM-444 OTA filter kit.  The past two  
months
have been the first time in two years that I've had any time to work on
synths or music -- our house is nearly finished, yeah!  (We moved in
back in October, but a lot of the interior finishing still wasn't  
done at
that time.)  I'm still looking for things that I haven't seen since  
two moves
ago -- my scope probes, the conical tip for my soldering iron, my MTA
connector tools, etc.  Oh, well.  At least I've got the soldering iron.

Scope wouldn't work at first -- it's been in storage for two years  
like a lot
of the rest of my gear.  Turned out that the channel 1 input mode switch
(the AC/DC/GND switch) is dodgy.  Will work on that later, but by  
jiggling
it around, I got it to work.

At first, the filter sounded kind of tame, and the resonance control  
didn't
seem to do much.  (Among other things, it wouldn't self-oscillate.)  But
at least it basically worked.  I went about testing all the inputs and
controls.  Discovered that I had the bass enhance switch in upside down.
(When in doubt, read the instructions to see what you did wrong.)

Then I set about calibrating the 1V/oct input.  I was in some doubt
about how I was going to do that without being able to hear the
filter self-oscillate.  Turned the resonance all the way up, and fed
it a nasty AM'ed noisy signal from the function generator.  Connected
a Dotcom standards module to the 1V/oct input.  Turned the octave
control.  It seemed to be hitting the right places.  However, at one
point, when I tapped my screwdriver on the trimpot, the sound
changed drastically for a moment.  Hmm.  Tap on it a couple of
more times.  Suddenly the displayed waveform on the scope
changes, and I'm hearing a definite tone that wasn't there before.

Turned down the input.  The filter is self-oscillating!  Tap on the
trimpot.  The self-oscillation comes and goes.  Tap on the edge
of the board.  Same thing.  Wiggle the trimpot.  No particular effect.
Turn it back and forth.  The oscillating continues.  Hmm.  Try to
wiggle the resonance trimpot.  No effect, and it's absolutely secure
on the board and flush up against the panel.

I started running my fingers across the components.  When I hit
a spot near the trimpot, the resonance acted up again.  Touching
around, I found that C8, one of the polystyrene caps, was a bit
loose.  Looked at it carefully -- cold joint on one leg, the solder
didn't flow through the hole.  Very carefully, I touched it up from
the component side with the iron.  Problem solved.  I was lucky
that the component with the cold joint happened to be near the
trimpot, and my fumbling around with it had pointed up the problem.

Anyway, it's working great now.  The calibration turned out be
very close already -- once I figured out to trust my ears rather
than trying to measure it on the scope.  In fact, doing this
uncovered the fact that the time base on the scope is pretty
far off.  What I did instead was pump in a triangle wave from the
function generator and tune the filter to the fundamental.  Then,
I advanced the standards module one octave, and tweaked
the trimpot until I got a nice fundemental-plus-second-harmonic
display on the scope.  Listened to it some more, went through
several octave jumps, tweaked a bit more by ear.  It's now
nicely calibrated over five octaves.  And, setting up a beat
pattern using the function generator and the self-oscillation,
I let it run for 20 minutes and I heard no drift.

Fun evening, and the first chance I've had to do anything with
the modular in a while.  The OTA filter will be a nice complement
to my Cynthia Steiner filter.  Now to put it to use -- I've got a
song in mind that needs a really cool solo sound!