303 VCO and VCA question.

[Tony Allgood]

Hi,

I noticed today that my clone has 50Hz (mains frequency) FM on its VCO.
Nice on some notes, but not wanted at all. I traced it down to the SCR.
The trigger point of the SCR is being modulated by stray hum field, so
the saw resets at different levels, producing a mild FM. Good layout can
certainly reduce this, my original stripboard design had to be changed,
and now it is considerably better, but not gone if you know what you are
listening out for. Has anyone else noticed this problem? I reckon the
base of Q27 with the collector of Q25 is high impedance enough to be of
a problem. This may make my board layout VERY VERY critical.

Can anyone confirm that R126 on the VCA input biasing is 2K2?

Regards,

Tony Allgood, Cumbria, UK

e-mail: oakley at techrepairs.freeserve.co.uk

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

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