Archive of the former Yahoo!Groups mailing list: MOTM

Well, I won't say I'm the last to get my VCOs going, but I didn't get in
any hurry. I got them both powered up tonight and they seem to be doing
what they are supposed to do. I set the V/Oct trimmer at 11.66K ohms as
Paul indicated that was the "calculated" amount for correct V/Oct
performance (yes, that was close to 12 turns from max). It is very close,
and the tracking is already great before I have it completely dialed in.
I'll get to that tomorrow.

I am also very pleased with the tracking of the Kenton Pro2000 that I am
using for my control of my MOTM. looking at about 7 octaves I found that
the V/Oct actual voltage according to my Fluke 77 is within .02 volts
across the range.

I have not had an opportunity to try all the modulation stuff, but I have
noticed a couple of things I will toss up for discussion or consideration.

1 - My FM1 switch position has some effect on pitch with the FM1 pot turned
all the way down and nothing plugged into FM1 input. In the DC position,
flipping the switch from EXP to LIN causes a shift in pitch. The same is
true in the AC position, BUT to a much lesser degree. Actually, unless you
have two oscillators fine tuned to the same frequency, you might not be
able to hear the difference in the AC position. I certainly do not see
this as a problem, since these switches will be left in one particular
position if nothing is used in the FM1 input. However, I wondered if
other's VCOs did the same (ie: am I normal?).

2 - I admit my ignorance to VCO sync. However, My hard/soft sync switch
seems to be operating backward to my thinking (yes, I will check the wires
again following sleep). In the soft position, my VCOs are dead locked. In
the hard position, I get a VERY loose sync. In fact, if one oscillator is
slightly de-tuned while I have a sync like this, the sync breaks. Unless I
am missing the obvious, the only connection required is one cable from sync
i/o of one oscillator to the other. I hope to learn more tomorrow when I
hook up the scope to set the sine symmetry.

3 - There is significant value in checking everything again before actual
power up. I try and take my time during construction so I will not make
stupid mistakes. I assembled both my VCOs at the same time. However,
after I was 100% complete, just before first power up, I took 15 minutes to
scan each circuit board for the correct orientation of parts. Sure
enough, I found that I had inverted U4. Since I found it prior to power
up, nothing was damaged, and a quick desolder and re-install corrected the
trouble. THANKS Paul for laying out the circuit board so all chips,
transistors, and diodes have the same physical orientation. It makes it
easy to spot such errors.

BTW, two VCOs tuned a 5th apart, both fed into the MOTM-120 on "cross" mode
is one awesome fat nasty waveform.

Larry Hendry