Electronics > Beginners

transistor replacement for VCO?

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FrankBuss:
Looks like it was the soft sync input. I desoldered both trimmers, but still the same problem. Then disconnected each non-essential pin, one by one, starting at pin 1:



When I reached pin 9, the high frequency problem was gone.

I guess the problem was that the triangle output from pin 10 coupled into the sync input of pin 9. A 1 nF capacitor was very close to pin 10:



When I connected the pin again, but desoldered C2, the high frequency problem was also gone. The datasheet says, if soft sync is unused, pin 9 should be connected with a 100 nF capacitor to ground. Probably a good idea to follow this advice.

FrankBuss:
All good now. Looks like it is not a problem with long traces or the trimmers. I guess the main problem was that I didn't ground the soft sync input:

https://hackaday.io/project/160538-cem3340-module/log/153565-next-revision

I need the trimmers, but might be better to use vertical versions and then place it closer to the pins, and the same with the 1000 pF Mica capacitor.

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