Help for FIXING a Dunlop Univibe Stereo Chorus UV1SC - schemo & suggest. needed

[Fuzz]

Hi there!

A friend of mine gave me this 9V Vibe/Chorus because the rate pot has a "dead spot" from 9.00 to 15.00; before and after it seems to work, but in between it simply doesn't oscillate. The effect still works, but there is no phasing sound unless I move the knob by myself thus hearing changes in modulation. So I believe is a LFO matter, but the design of the circuit is quite complex and has no much in common with "usual" vibes.

Oh...before someone asks me...I checked the pot and it's ok (too simple, but I had to try!). I also changed +/- all the caps in what I believe is the circuit "LFO area" with no success. Hard to find appropriate ICs and change them "just for trying" because is a dual layer pcb...

Many thanx!
Fabio

[Fuzz]

 

[Mark Hammer]

Well, you started out right, both in coming here and in checking out the pot.

The pot's behaviour is unusual.  When you say you "checked" it, what did you check?  The reason why I ask is that the range where it seems not to function is the range where you can expect the speed control to have been adjusted the most since it was bought (people use the extremes less often).  Greater use means greater risk of wearing away the resistive element and having poor contact with the wiper in that spot.  I don't want to underestimate your capabilities, but if "checking" simply meant testing continuity from one outside lug to the other, then you might have missed detecting continuity problems with the wiper in the middle of its rotation range.

If this IS the problem, then there are two courses of action, depending on the pot itself, and how it is installed.  You can either just get another pot of the same value and install that, or you can get the original out, pop the back off and attempt to rejuvenate it.  There is a substance I am quite fond of called Stabilant, which acts as a contact enhancer.  I buy it in little 0.5ml vials for 50 cents.  A little drop here and there on a pot's resistive strip does wonders for those 5 micron gaps that develop over a couple of years.  The liquid acts like a solder joint, providing continuity to the wiper.

The problem with the pedal might be something else, but the fact that the pot works so poorly in the "popular zone" suggests that wear and tear is the source of the difficulty.

[Fuzz]

Thanks Mark! But I must say, you DO underestimate my capabilities! 

The pot is ok...but in order to prevent those "fantaelectronic phenomena" that sometimes happen, I even substituted it with a new one, with no success. Thanks the same! Anything else?

[Mark Hammer]

Um....magical fairy dust?

This one has me stumped.  Are there any other symptoms or problems beyond the "dead spot" in the speed pot's range?

[Fuzz]

Are there any other symptoms or problems beyond the "dead spot" in the speed pot's range?

Maybe yes...something intersting (I hope): starting by the "dead spot", when I turn the pot to the "living range" (clockwise), the circuit takes a while before starting oscillating, just like it needed to "charge" itself; this happens every time I turn the pot back to the dead spot, and then rotate il to the max again. Not much time, I think less than 1 second...but that's why I put my interest on the LFO caps.