4046 Oscillators progressively getting worse over time

LektroiD

Two of my oscillators are stable (at least as stable as an analog oscillator is going to get), but the other two are sounding rather ill, I have no idea why. They started off in good tune, but one went unstable after about 6 months, and now the second one... Is it only a matter of time before the others go? Maybe someone can help me out. I attached an audio file of a direct square output, although all waveforms are doing the same wonky output....

Osc.mp3

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fonik · Posted: Wed Oct 17, 2012 11:03 pm Post subject:

this sounds...errr... funny?

no, not funny.
just a wild guess: maybe something got loose? it is embarrassing that this would happen on two modules, though!?

are the x-4046 from the same batch? and/or the matched pair?

LektroiD · Posted: Wed Oct 17, 2012 11:11 pm Post subject:

The PCBs were from the same batch...
The 4046 ICs were from the same batch...
The transistors were from the same batch, hand matched...

Everything was working fine but they have gotten to an unusable state now. I tried wiggling all the components by hand one by one, as well as all the pots & sockets, yet failed to stabilise the pitch.

I can't think of anything else that would cause this (the audio is a mild example, it gets a lot worse).

fonik · Posted: Thu Oct 18, 2012 3:24 am Post subject:

for me it still sounds like a loose contact.
but why is it that you experience these problems on two modules, but not on two others?
i would try to think about what was different when building these. solder, potentiometers, other parts...
what is different right now when using them? other cabinet/powering method?

since nobody else experienced such an issue so far we can only try to analyse it.

LektroiD · Posted: Thu Oct 18, 2012 4:46 am Post subject:

I built the 4 oscillators as a batch. I was just wondering what components regulate the pitch (other than the matched pair), if the 4046 might be the culprit, I could swap that out... I'll retouch all the joints and post my results...

elmegil · Posted: Thu Oct 18, 2012 5:12 am Post subject:

If your chips are socketed, can you switch one from a working module to a non working module?

fonik · Posted: Thu Oct 18, 2012 11:41 am Post subject:

and maybe try different pots.

anyways, two modules at the same time???

LektroiD · Posted: Fri Oct 19, 2012 5:37 pm Post subject:

elmegil · Posted: Fri Oct 19, 2012 8:07 pm Post subject:

It looks to me like you've got some kind of extra capacitance on the line, dragging you back to ground. All the waveforms are exhibiting it.

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

If you used film capacitors in the oscillator core, you may have overheated them while soldering. They're very easy to damage, and I've had them do exactly what you're talking about - start out fine, and then get 'sick' over time. Just a thought...
Good luck

Gary

LektroiD · Posted: Fri Oct 19, 2012 9:40 pm Post subject:

LektroiD · Posted: Fri Oct 19, 2012 11:36 pm Post subject:

elmegil · Posted: Sat Oct 20, 2012 6:19 am Post subject:

I meant the audio path, not the power supply.

I would start with caps closer to the output and pull back. I'm assuming those are the output waveforms, do you have anything any better if you probe closer in to the oscillator core or within the oscillator core?

Mongo1 · Joined: Aug 11, 2011 Posts: 411 Location: Raleigh NC

elmegil · Posted: Sat Oct 20, 2012 7:31 am Post subject:

So thinking about the "over time" aspect ... What kind of solder did you use? Did you clean the board when you were done? Is it possible that flux has caused bridges to areas that will cause trouble?

LektroiD · Posted: Sun Oct 21, 2012 2:58 am Post subject:

LektroiD · Posted: Sun Oct 21, 2012 3:06 am Post subject:

fonik · Posted: Sun Oct 21, 2012 5:38 am Post subject:

the effects you experience are too heavy to be realted to just the capacitor type.

BTW did you replace the 4046 allready? or replace the pots? did you re-heat all solder joints related to the expo and the 4046?

fonik · Posted: Sun Oct 21, 2012 5:39 am Post subject:

elmegil · Posted: Sun Oct 21, 2012 6:23 am Post subject:

Sorry if my questions about flux are things you've known how to deal with for ages, just trying to brainstorm for ideas....

LektroiD · Posted: Sun Oct 21, 2012 10:08 am Post subject:

diablojoy · Posted: Sun Oct 21, 2012 1:36 pm Post subject:

check the value of C3 and how it is soldered in
should be .01uf
less likely but also possibly relevent did you use LF442 for IC2 ? or just a TL072
LF442 has substantially better offset specs

Argitoth · Posted: Sun Oct 21, 2012 5:17 pm Post subject:

Did you try removing the coarse tune pot from the circuit? also try removing the fine tune after that. Use DC offset to get it to an audible pitch.. or try 100k trimmers.

sounds like bad pots.

did you try PUSHING on the knob of the pot? So push down as if you were trying to force the knob further on the shaft. See if that affects the pitch deviation. Give it a push pull push pull to see if you hear the pattern.

LektroiD · Posted: Mon Nov 05, 2012 9:56 am Post subject:

That's another one just gone down (same symptoms), I only have one oscillator left now. At this rate it won't be long before my synth is kaputt.

I'll try swapping pots, I just used cheap Alphas. Wiggling them in or out makes no difference, but I'll swap them out and report back...

Benjamin AM · Joined: Nov 04, 2010 Posts: 83 Location: Boise

LektroiD: Just out of curiosity, what are you using for a power supply?