[sdiy] ARP FIlter Problem

[Rob Keeble]

Hi DIYers,
I have built an ARP 4012 filter replica (Moog ladder LPF from ARP 2600), but
I am having problems getting it working. Any help/guidance is very much
appreciated !!

The filter outputs the audio signal (inverted) okay, and Offset and Gain
work fine. But turning up Resonance creates phasing and then total signal
cancellation (nice square wave on the way...), and the Frequency control
doesn't do anything.

The schematic is here (144k).

www.emulatorarchive.com/assets/4012.jpg

I have replaced the 2N5459 FET's with a Dual FET 2N3958, this works more
reliably. The 2N3904/6's in the ski have been replaced with 2N5089/87's.
5172's and 6076's have ben used. No transistor matching has been done, as
this is a prototype..
R15 input sees +43 to -135mV with varying Frequency. T14 collector output to
the ladder is constant at 8mA. I have swapped in replacement transistors,
but no joy. I have checked back to the original ARP blueprint and PCB layout
and found no errors.

What should I check next?

Many thanks (wish I'd finishd my degree in Electronics...)

Rob