[sdiy] Berfotron/Buchla now triangular, but with 4KHz offset!

[René Schmitz]

Hi Aaron and all,

Aaron Lanterman schrieb:
  > 4) If I turn around the freq = current/(2*Vpp*C) formula, and write 
it as
> 
> current = freq*2*vpp*c, I can plug in 4K and see what current I would 
> imagine would be needed to get that 4K, and it comes out to 170 
> microamps. So it's like all these magic microamps appear out of nowhere.
> 
> 5) Oh, here's the wild part: we can COMPLETELY DISCONNECT THE EXPO 
> CONVERTER altogether - i.e. disconnect the emitters of the diff pair in 
> the OTA from the collector of the right transistor of the expo converter 
> - and it still happily oscillates in the kHz range! (We discovered this 
> when putting in the 910 ohm resistor mentioned above).

Disconnecting the expo is a good "divide and conquer" debugging 
technique, you can test both pieces individually. I often do this as 
well when there is some problem.

> Any ideas? Anyone run into this mystery offset before?

It will be some leakage current again, but its difficult to say where 
exactly this might occur. I'd try swapping the transistors in the "OTA" 
first, and maybe the integrator opamps, since you already encountered 
some faulty components, it might be that there was more damage done to 
other components.

Cheers,
  René