MOTM

I've been soldering for more years than I care to admit.  I generally 
now wear a magnifying head visor and when I am done soldering, I also 
use a magnifying glass to carefully look at every solder joint. That 
will show up poor soldering connections that I can't normally see.  I 
usually find a couple that I touch up.

Glad you found it and it is working.  It's one of my favorite filters, 
the MOTM-440, that is.

Dave

--- In motm@yahoogroups.com, David Cornutt <cornutt@...> wrote:
>
> 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!
>