[sdiy] Debugging MOTM-300: oscillating at bat frequencies

[Aaron Lanterman]

Sometime last semester, I posted about having trouble getting the MOTM-300 
my students and I put together (I had them start, and then I took over 
once we got to the expensive transistors so I wouldn't have to yell at 
them if they fried a MAT-02).

I thought nothing was coming out at all, so I erroneously focused on the 
LS3954A (I thought I might have had a solder or organic goo bridge). Paul 
steered me away from that direction.

I finally got around to getting back to it, and this time took it to the 
lab to hook up to a scope. I should have done that immediately (I had 
previously been just playing with it at home, where I am scopeless), since 
I found that it was quite happily oscillating, and changing frequency as 
you twist the Coarse control knob, making happy sawtooth waves and squares 
and sines of the appropriate amplitudes.

However, it's making them between 96 kHz (Course knob all the way left) 
and 236 kHz (Course knob all the way right).

Here's some measurements, for those of you who know your way around a 
MOTM-300:

VX is 4.8 (right according to the instructions) - this is the top voltage 
of the reference.

Pin 3 of U4A (A 1013 op amp), is at 0.317 V (set by a simple voltage 
divider)

Pin 2 of U4A is around that, so golden op ajmp rules apply - this is the
voltage the other end of the reference resistor is held at.

The main output of the CV summing section happily runs between 7 and -7 V
as you twist the Coarse knob (the manual says 8.3 to -8.3 - but note I 
haven't set the 1 V/oct trim yet)

As you twist the Course knob, the voltage on the base of the MAT-02 (i.e.
at the top of the 1K Tempco resistor, which runs to ground) runs between
around 1.68 to -1.63) - the voltage divider has a 44.2K and 1K resistor,
so that seems reasonable.

The one thing that doesn't quite match what it says in the manual is pin
one of U4, which is the output of the op amp that goes to the emitter
of the MAT-02 through a 4.7L resistor. The instructions say it should
vary between approx -0.3 V to -14.2 V as you twist the Course knob - mine
goes between -1.45 V and -12.5 V.

Oh, another measurement... The base of the 2N4403 resistor seems to hang 
out of around 5.6 Volts.

I'm at a loss at where to go from here. Rather than just e-mail Paul 
directly (Paul, you just posted so I'm guessing you're checking e-mail), I 
figured there are lots of people on this list who have built the MOTM-300 
and I thought someone else may have encountered a similar problem.

- Aaron

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Dr. Aaron Lanterman, Asst. Prof.
and Demetrius T. Paris Junior Prof.    Voice:  404-385-2548
College of Electrical and Comp. Eng.   Fax:    404-894-8363
Georgia Institute of Technology        E-mail: lanterma at ece.gatech.edu
Mail Code 0250                         Web:    users.ece.gatech.edu/~lanterma
Atlanta, GA 30332                      Office: Centergy 5212