Living VCO dies...sort of

[numbernone]

Have had the VCOs humming nicely since last December. About 2 weeks back the center OSC begins to experience what I would call voltage burps here and there, fluctuations sometimes small sometimes large in the pitch. Yesterday it seems to have stabilised. After about 3 hours the center OSC has dropped out and is producing a profound hissy noise only, most powerful from the square output with the PWM CCW.

I am prepared to dig in for the fix, however my wiring is tight and I want to target potential problem zones first, and this is the strangest VCO problem I have ever had.

All help appreciated in advance. Important gig next week and I want a full trio of power filling the night skies of West Virginia!

[numbernone]

Redacted update...user error

Checking it now, there is some pitch present in all the noise. However even at its maximum CCW setting, the pitch is very high and and increases in pitch and fades the volume out in about 3 full turns CW.

[numbernone]

Having returned after a day off. I have observed its behaviour (at the ear only) from a cold start.

The noise is now very gritty with a digital arcade timbre, given 5 minutes or so this smoothes into whitish noise. There is hi frequency oscillation present, which responds to all controls but to a MUCH greater magnitude. Even the linear detune knob would easily cause more than an "octave" in variation, while the tune pot goes well beyond audible to silence in about 3 full turns. The manual PW sounds like a hi pass filter, cancelling the signal at approx 50% rotation CW.

I have swapped the easy chips to no effect, voltages at pots are right. Will try the 3046 next just to rule out ICs.

Funny part is this is most definitely the most lively noise generator I have ever dealt with. I almost wish I didnt have to fix it.

[jhaible]

Brian Eno probably wouldn't fix it.

Seriously: This sounds like a cold solder joint or some such. Something making contact sometimes, and loosing contact sometimes, and then picking up something on the disconnected, high impedance node ...

JH.

[numbernone]

I have decided to hold off til after the big gig weekend. It has a flavor I might be able to use.

Eno lover here.