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VCO not oscillating
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VCO not oscillating

by a1k0n on Mon Oct 24, 2016 2:16 am

I am building a kit from x0xsh0p.de, and I cannot get the VCO to reliably work.

Instead of 2SC536Fs, the kit has all 2SC946Ps, so in the VCO Q24 and Q25 are C946Ps. Q27 is a 2SA733P as originally specified.

What appears to be happening is that the "SCR" action of Q27/Q25 is getting stuck "on", charging up C33 and not "letting go" once it reaches ~10.6V so it can oscillate (should it be charging to a higher voltage?). What's weird is that I got it to work a couple times by not soldering in the transistors and just sort of sticking them into the plated through holes on the PCB, but after soldering them down it no longer starts up. I put a SIP socket in for Q24 and Q25 so I could try alternate transistors and with them socketed, it no longer starts up with any combination of parts on hand. I guess some sort of resistance in the pins was making it "work" before.

If I short D25, then I get a very small sawtooth oscillation between ~11 and ~12V.

I did all the standard VCO voltage checks, and they all check out, except the voltage at the Q24 emitter is 10.6 volts or so, ~3 diode drops under the 12V supply, instead of the expected 5.4V. Q26 and the whole section connected to it is definitely working fine, in fact all the transistors individually work, but the circuit as a whole does not.

Current voltages:

- 12V supply: 11.89V
- Q25 emitter: 10.70
- Q24 emitter: 10.57
- Q26 pins 1..5: 5.331, 5.332, 4.778, 10.70, 5.267
- IC11 pins 1..8: 4.608, 5.331, 5.331, 0, 2.453 (my injected CV), 2.452, 2.452, 11.89

I'm completely at a loss -- these transistors should be equivalent, no? Has anyone successfully built up the VCO with 2SC946Ps? I've got some 536Fs on order but ... shouldn't this work?

a1k0n
 
Posts: 3
Joined: Mon Oct 24, 2016 1:55 am

Re: VCO not oscillating

by a1k0n on Mon Oct 24, 2016 11:25 am

*sigh* I had a weird whisker of solder bridge between Q27 emitter and collector which explains what I was seeing. Works now.

a1k0n
 
Posts: 3
Joined: Mon Oct 24, 2016 1:55 am

Please be positive and constructive with your questions and comments.