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MOTM 300 Ultra VCO Pitch gone all wobbly
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author MOTM 300 Ultra VCO Pitch gone all wobbly
Hi All,
One of my MOTM 300 Ultra VCOs has suddenly gone all wobbly in the pitch department. I built two of these probably 4 years ago and they've been rock solid until yesterday when one of them suddenly started to drift all over the place. It sounds like patching in a Wogglebug to the pitch mod input as it's a smooth, random change over time.

This wobbling happens even with nothing plugged into the CV inputs so can't be coming from external source, and the other 300 is stable whilst running on the same power supply so it can't be that.

I whipped it out and couldn't see any obvious visual solder problems so it's time to take it to the bench and do some measuring - just not sure where to start..?

My feeling is to look at the temperature compensation resistor/transistor combo, but not sure what measurements would constitute 'normal'. Would anyone mind pointing me in the right direction here?
First test: remove the center wire on the COARSE and FINE pots.

I've seen a bad pot do this.
[edit] Thanks Paul, I didn't see your post at first but that seems to have sorted it applause [edit]

Actually I think I found the potential problem - I disconnected the wiper from the course tune pot and it settled down - I measured the wiper voltage and it's drifting all over the place so I reckon it must have worn out/got dirty. It's not an expensive Spectrol cermet one as is in the original spec so I'll swap it out for one of these to see...
Considering that the quality of the coarse and fine tune pots is an integral part of the stability of these VCOs, you should sub in one of the correct pots. It'll probably work better than ever if you do.

However, beware of bad spectrol pots - despite the huge price, they can be pretty bad. I managed to get a batch of 5 once for a multi vco project, and every one of those 5 were bad - wouldn't hold stable pitches. The next batch I got were fine. Go figure.
Thanks SDuck, yeah I was being a bit cheap when I originally built it and used these cheaper Bi Technologies pots that I already had - k/panel-pot-100k-lin/dp/1760795

Now I can totally see how important it is to use the high spec pots on the course and fine tune. Thanks for the heads up on the potential of the Spectrols to be dodgy too - 5 in a row is pretty bad luck!

Paul; this is an incredible oscillator - I've gigged mine heavily for the 4 years of their life spans and they've been through cold rainy Glastonbury festivals to sweaty hot clubs and are always totally rock solid and dependable!

The PWM and Sync can really tear the air in two over a large PA system - more so than any other oscillator.
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