Unwanted fm through power supply

[Tony Allgood]

Kevin said:
Any ides what is going wrong here?  It seems to me that the LFOs are at
fault.
Is this design prone to exessive current drain?  Is there any way to better
isolate this module  (or my VCOs)  from the common supply?  This problem
occures even if everything else except the VCO and LFO are taken off the
supply.

This sounds like a high source impedance in your power lines... what power
supply are you using? Good grounding is also one to look out for. But here's
a good one, I had this the other day on my monosynth. When square was
selected the pitch went up by about 5cents. I traced it to the control line,
+15V, driving the 4016 used to switch the outputs. This was a good 2mm away
from the virtual earth that controlled the 3340, but grease and dirt (flux
has the same affect) had allowed some current to flow across the tracks.
Clean your board, perhaps???

Hope this helps,

Tony Allgood, Cumbria, UK

e-mail: oakley at enterprise.net

Rack mounted Moog VCF module. Details to be found at...

http://aupe.phys.andrews.edu/diy_archive/schematics/effects/filter.html