[sdiy] Old unique VCO - Tune drifting problem - adapting a servo conversion‏

[Idris El Kasri]

  Hi everybody!

I got an old modular synth from the 70's that was previously owned by Brendon Pollard, and previously (the legend says) by Jethro Tull, here's a pic :
http://www.muffwiggler.com/forum/files/elvis_big_351.jpg

  The VCOs are totally incredible, I'd say comparable to Jurgen Haible's Living VCOs and maybe even fatter (without any kind of disrespect of the giant J.Haible's memory).. Some UA726 unfortunately passed away, Y.Usson kindely advised me to replace them by a tempco and a lm394/or ssm2210 (mat02 etc...)...

  The problem is that the tune drifts a lot!
For example, I tune it on a C2, 20s later it becomes à B2 then a A#2 etc etc during maybe 1h or more then it would becomes higher and higher... big problem!

  Yves advised me to check conversion's capacitor (to put a low t° drift capacitor) and power supply's stability, which I did and the problem remains; he said that considering that there was no servo conversion on this old VCOs from the beginning of the 70s (having a look at the dates on the AOPs) I shoudn't hope very good stability...

  Here is the schematic of the VCO :
http://img826.imageshack.us/img826/646/vco2shematic.jpg

   I didn't found much documentation on the "servo conversion"...

Somebody could advise me some things to try to adapt a servo conversion on this VCOs? I m really lost...

I'm sure that if we can fix that pbm we could really have here some really wild VCOs...



  Cheers.