The VCO-555

[gimble88]

Well, it's pretty well fixed. The simulation you did was brilliant and pointed to the lack of gain at IC3d as the problem. Thanks so much for that.

We had to expect that with a set of symptoms as goofy as the ones I had that there was a pretty high chance of bone-headed-ness involved. Unfortunately, it was pretty extreme. In addition, I was focused on the 555 and it's interaction with the LM13700 so that's where I was most carefully checking components. Again the simulation made it clear where the problem actually was.

I remembered that as I was trying to get the oscillator to work after the rewire that I replaced IC3. Due to my not-so-great PCB design I found it easier to remove R22 as it was a little bit in the way of drag soldering the IC. Apparently, I replaced it with the wrong value. I believe there were a couple of the little 1206 buggers sitting on the bench and I must have grabbed the wrong one at the last minute as I installed it. I keep a meter with tweezer probes on the bench and check every part as I install. Well.... I guess almost every part.

Thanks again for the patient replies and great ideas.

The only issue I'm chasing down now is that the sine amplitude is only about 6.5V PP. I know this design sometimes has lower output on the sine and mine was about 8V PP before the disaster.

I can't ask for any more help with this oscillator. For one thing it sounds good and if the sine output is low - hit the VCA harder.

I'll continue to sniff around but I have more modules to build including more 555 oscillators.

I gonna get to know that simulator, what a powerful tool.

Again, I couldn't ask for better or more generous help. Hope the vaccine booster blahs go away soon.

spence

[elmegil]

Excellent!

You're quite welcome. I enjoy troubleshooting.

Falstad's simulator is a go-to for me whenever I'm looking at something and scratching my head. Doesn't always work because occasionally you'll have things (like Buchla) that push the components beyond their normal states, but it has helped me SO many times.

[octavebrand]

Hello there! I have a very annoying issue with my vcos 555.
I have two of of them and the issue came almost simultaneously (a week after for the second one) for both. I built them almost 4 years ago and they have always worked perfectly.
The problem is that they get out of tune, drastically. They hold the tuning for a minute, then pwooof: around +5 octave, I tune them back, hold it for 20 sec then pwooof: +3 octave, etc...
I am not expert enough to understand with the schematic what component could be guilty.
This is not a power problem as I tried them in my test rack and the problem remains.
Could it be a capacitor? 4 years is not that old right?
Thank uuu

[elmegil]

4 years is not very long, no.

Was it a kit or did you source your own parts?

What *type* of cap is C4 (the main timing cap)? Is it a film or ceramic or ??

It would be useful to start from cold, just set it to a fixed, reasonably low pitch, measure the voltage at IC2 pin 1, and then wait for it to flip into high frequency mode. Then measure the voltage there again.

You're right a bad cap is one possibility, but I also think that IC2 or Q3 or even maybe the tempco could be at fault.

It is odd that you had it happen on both. So a different sideways question is: how clean is the board? Flux around? If so, what type (e.g. water soluble, rosin, etc).

[octavebrand]

Thank you for that great answer. I did source my own parts, and the capacitor C4 is a film one. I will try the test on IC2 pin1 in a week when I get back from vacations. I'll tell you what voltages I see.