[sdiy] asm1 vco problems continued

[Gene Stopp]

Hi Josh,

I take it you're using a scope... where are you seeing this waveform? Is it
at the sawtooth output pad? Take a look at pin 7 of U5 (the unused op-amp
that's set up as a sawtooth buffer). You should see this

   /   /   /   /  --- +5v
  /   /   /   /
 /   /   /   /
/   /   /   /     --- 0v

Avoid using the scope probe tip on the output of the 3140. If the tip slips
and shorts anything it may zorch that part. I love 3140's for their giant
input impedance but they are such wimpy chips.

If you see the above, then you know the VCO core is making the right
waveform. Then I would follow the signal to the next opamp. I wanted a -5 to
+5 bipolar sawtooth, so the following opamp does the 2x gain plus a sum with
V- thru a 560K resistor to shift the sawtooth up. Funny thing is, your
description goes against having a wrong resistor in the 560K location, since
a lower value would be a more positive offset!

Anyway remember that the two most significant building errors are:

1. Wrong part in the wrong holes
2. A short somewhere

Best Regards,

- Gene


-----Original Message-----
From: Josh Randall [mailto:jorandal at vt.edu]
Sent: Tuesday, April 09, 2002 3:20 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] asm1 vco problems continued


I wrote the list a while ago because my saw outputs were shaped as
follows:
\     \     \
 \     \     \
  \     \     \
   ---   ---   ---
Looking at it further I realized that the top of the teeth are around 0V
and the bottom are around -12 so I wandered if there could be some reason
that the wave could be twice that amplitude that they are suposed to be
and the reason the shape is funny is because it is hitting the power
limmits.  If this seems reasonable, how would I go about fixing it?

Thanks,  Josh Randall