[sdiy] Weird MFOS VCO max CV problem

[Neil Johnson]

Hi,

On 10/04/2012 04:47, Travis Thatcher wrote:
 > Right. I figured I'd swap it just in case, but I was thinking the same
 > thing.  I tried the circuit with both an LM394 and SSM2210 for U3 and
 > no change.

Have you probed around the circuit with a DMM when this happens? 
Swapping opamps to try to fix the problem is like changing tyres on a 
car when it won't start - it only works if you know that the tyres are 
the fault.  From reading your emails it looks like you haven't.

I suggest you:
1) print out the schematic
2) force the circuit into the fault mode
3) go round with a DMM and measure the inputs and outputs of U1 and U2 
and note the readings on the schematic.  Also worth checking with a 
scope as well to see if any of the op-amps have gone into 
self-oscillation (could be several hundred kHz).

Of particular interest is what is happening to pin 7 of U1.

Once you have those numbers then come back to the list and we can start 
to make some headway.

Cheers,
Neil
-- 
Modules and more :: www.cesyg.com
Homepage :: www.njohnson.co.uk ::