Tempco measurements

[jorgen.bergfors at idg.se]

Happy new year DIYers!

During the christmas break I made some measurements on my ASM-1 clone VCO (I made my own board, otherwise it is identical to the ASM-1). It has a SMD tempco resistor ($0.60) and a BC847BS dual SMD transistor ($0.20) mounted on a hybride module and covered with Araldite. The module fits in an ordinary 8 pin DIP IC-socket.The timing capacitor is a Wima FKP2 (Polypropylene/metal film).
I put the VCO board (size 50 x 75 mm, 2 x 3") in a shoe box and put on the lid. 20 cm (8") from the box I placed a 300 watt infrared heater. After 15 minutes the temperature inside the box had rised from 24 to 34 degrees C.
With the tempco the frequency of the VCO increased from 518 to 523 Hz. This corresponds to 17 cents or 1/6 of a semitone. With the tempco replaced by a normal 1% metal film resistor, the frequency increased from 424 to 470 Hz. That's 190 cents or almost two semitones.

I don't know how this compares with other VCOs but at least the tempco resistor makes a big difference for sure! Anyone else who have made similar measurements?

/Jorgen