VCO4d lives !

Harry Bissell harrybissell at
Mon Nov 15 07:56:00 CET 1999

Just got 2 VCO 4d up and running,

1) The pulse output could use a "pull-up" resistor to ground. Without it
the pulse is mushy on the positive side. I used 1.5 Kohm

2) I made R11 1.1 Meg.  When U1 pin 7 hits the negative rail the
accuracy of the expo converter goes down (also wave distortion starts).
With this I get from 1 cycle in 15 seconds to ultrasonic.

3) I added a 220pF cap from U5 pin 1 to pin3 for AC hysteresis. Keeps
the pulse from going ratty at the high end.

4) I made R39  smaller than the pot (ie.. 8.2Kohm with 10K pot.  I used
a 25K pot and a 22K resistor) The 82K doesn't let you reach a square
wave (3V level needed...)

I want pulse that goes both sides of square... to extinction. I think
your intent (Tom) was to go from 0-50%. I go from 0-100%. When I'm
finished I'll bias the pot a little negative to get below zero.

I used the MPF-102 fet. No problems.

Triangle convergence is right on. I think the sine might be a tad hot at
the bias input. I'm going to try some different values to lower sine
third harmonic distortion (audible).

I used the MAT-01 and tempco in place of the CA3046...

Hats off to TomG for a great board.  Nice sound and easy to get running.
Looking forward to more EFM boards...

(Hey TOM. How about adding a couple of extra pads on the positive and
negative rails and esp. ground.  It would be convevient for hookup on
the bench. I hate scraping off the nice solder mask (but I will do it

:^) Harry

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