[sdiy] Debugging MOTM-310: was working, now not working

[Aaron Lanterman]

I just posted about trying to get the GaTech MOTM-300 working.

I had a working MOTM-310, and since 300 and 310 core circuits are so 
similar, I figured I'd bring out the 310 and take some measurements there 
to get a feeling about how things were supposed to look.

At some point during my explorations into the 300 and 310, the 310 stopped 
working. Unlike the 310, which does oscillate in a controllable 
exponential fashion (just not in a range of human hearing), the 310 just 
stopped oscillating. I'm not sure when/how it happened, and of course my 
big concern is that I somehow fried a part while I was poking around 
taking measurements (maybe accidentally shorted something to something 
else).

The closest you can get to the original oscillator is one end of the 
capacitor - nothing happening there. Also, nothing happening at the output 
of the dual FET buffer (which we expect, since nothing is happening at the 
cap).

However... I do get a somewhat crazy sawtooth out if I apply my finger to 
the part of the underside of the board containing C9, R24, and R27. The
resulting audio does look like a sawtooth on the scope (although more of a 
droopy sawtooth, like a decaying exponential), and by applying more 
pressure I can increase the frequency.

I can also make it oscillate by touching the top of the FET buffer.

If I touch the underside of the board to make it make noise, I can then 
make a European sireon sound by alternately touching/not touching the FET 
buffer.

The sound created by touching the board does not seem effected by the 
Course knob.

At that point, I figured it was time to go home for the day.

I did take some measurements first:

The output of the CV summer section runs between -6 and 6 V, as the 
instruction say it should (noticed that I had previously set the 1V/oct 
trim, unlike with the MOTM-300)

VX main voltage ref is at 4.4 V (as manual says it should be for the 310)

other end of voltage ref is at 0.319, as expected, held at a virtual 0.319 
by an op amp.

As you turn the Course knob, the base of the MAT-02 runs between -0.132 
and 0.132 (seems right looking at the 44.2K and 1K tempco voltage divider)

Base of 2N4403 seems to hang out at 5.94 V

The one thing that does seem quite off is pin 7 of U2B, the 1013. This
connects to the MAT-02 emitters via a 4.7K resistor. According to
the manual, this should very between -0.3 V and -14.2 V as you twist the
course knob. It most emphatically doesn't - it varies between
-0.29 and -0.55 volts!

This must be a clue - it seems if the base goes between -0.132 volts
and 0.132 volts, and that pin 7 goes between -0.29 V and -0.55 volts (I 
forget which direction, i.e. what is associated with what location of the 
knob), then the left side of the MAT-02 is never quite turning "on." (I 
had measured that before, and I think it was operating something much 
closer to what it says in the instructions).

But now I'm not sure what to do with that clue...

- Aaron