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[solved]: MOTM-300 VCO gone mad!
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Author [solved]: MOTM-300 VCO gone mad!
werock
One of my MOTM-300 vco's has gone a bit mad, after many years sterling service. It's a bit hard to describe what's happening, but I'll try.

Just listening to the VCO on it's own, with nothing in the 1v/Oct in or any other input, it kind of warbles - it sounds as if there is some bad FM being applied! Then when the Coarse freq is adjusted, it goes all over the place.

I've recorded a short sample - it starts off with me not touching anything, then when it really goes mad is when I am turning the Coarse freq.

If anyone has any ideas where to start looking to fix it, I'd be grateful - if I have to I'll send it back to Paul for repair, but as I'm in the UK I could do without the bother.

Thanks,
Andy.

EDIT: I replaced the coarse tuning pot and now all is well. See last post for details.
sduck
Since your avatar is similar to mine, thought I'd give an answer, but it'll be pretty useless.

Sounds like some electrical connection has gone bad. Not much help, is that? However, I had something similar happen to a 310 - seems I had forgotten to solder one row of pins on one of the ic's, and was getting intermittent connections. So the obvious thing to do is go over everything behind the panel, and see if you can spot any loose connections. Also check the pots closely - the weak spot in the design is the pot/panel connection - too much stress on the bracket and the pots can pull apart, which is hard to spot. You'll probably need to take the whole thing apart to check the back of the pcb too. Also, look around for any stray tiny pieces of wire or such that may have fallen into the circuit somewhere.

Did you build this or get it pre-assembled?
paults
Check soldering on the 3 pins of the Coarse pot. Even if they look good, reflow the 3 joints.
stiebz
I'll have to leave my leeching mode to be able to answer this one ...
But since I suffered exactly the same problem, maybe I can help by telling how I fixed mine :
My 300 was also working fine for a few years until it suddenly started drifting.I traced the error to the part of the circuit that is generating the VX and VREF voltage.(MOTM circuit diagram 1 of 3)
I looked and looked hoping I could find a bad soldering or bad contact, but could not find anything.
The voltage supply to the TL074 was OK, but I discovered that when I removed the feedback resistor R18, the fluctuation on pin 1 of the TL074 was removed (along with the tuning of my 300).
I removed the TL074 and replaced it with a new one and it fixed my VCO !

So, not sure that the problem will be exactly the same in your case, but check all the voltages involved in the VX and VREF voltage. If you can not find any bad contact, the problem could also be in a failed TL074...

Hope you find the problem soon, as it is an awesome VCO !

OK, back to leeching mode with me ;-)
werock
Thanks everyone for the suggestions - I'll probably have to wait until next week before I start looking at it, as we have the painters coming in for a few days d'oh!

I built this myself 7 or 8 years ago, and it has always been fine up until now.
paults
Another tip:

I have seen this issue when the original solder flux was not 100% cleaned off the pc board on both the top and bottom sides.

This is true for people who used their own rosin-core solder (shudder) or the supplied organic-core, but the washing was inadequate.
werock
I finally found the time to take a look at this. When I took it apart I checked the solder connection between the wires and the off board coarse pot - as I was doing that the middle pin of the pot broke off! I put on a new pot (one of the BTI pots from Rapid) and it is now as good as new.

Maybe I was bit over zealous when putting the small J bend on the pot legs when I built it or something, but it lasted a good number of years before it went bad.

Anyway, thanks to everyone for the suggestions.
tobias
Yay!!!! Happy ending!!! applause applause applause
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