Monday, June 21, 2010

It Lives!

Really thought that I must get back onto the Moog 921B VCO. I was having a LOT of trouble getting a dual JFET... any dual JFET not necessarily the right one. Getting a through hole one from Mouser came in at a staggering $118 US! I then found a surface mount part that is similar in nature and managed to completely stuff up the footprint... I then did a fair bit of simulation with LTSpice and discovered that the offending section of the circuit is little more than a very high impedance buffer. Since this thing was designed long before FET input op-amps this was effectively what it was trying to achieve. I ended up removing a bunch of diodes and resistors and swapped the OTA for a CA3140 and added two jumpers --Hazaar! the result a sawtooth! I will tidy up the layout a bit so that it can be done either which way but I seriously doubt that it is going to make much sonic difference....

1 comment:

Florian Stämmele said...

Hi, I'm rebuilding a 921b, too, and I wonder which value Resistor No. 29 has (The parallel one to the Tempco).. The Moog service manual doesn't show it... Do you have any idea?