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!
>