Faulty Pots?

[steveyraff]

Hey guys,

I built a distortion pedal that seems to work perfectly, but if I move the circuit at all (ie to box it up), the distortion effects cuts out to dry signal only.

Its very frustrating and hard to pin point. To me, it seemed to behave like a poor internal wiring solder joint. I checked them all and they seem fine.

I seem to now have it narrowed down to the Tone pot. If I try to keep the circuit dead steady with no movement, but I apply pressure on the Tone pot lugs with a screwdriver head, the effect will drop in and out - stuttering from the desired distortion effect to a clean dry signal. Would this behaviour generally be relevant to a faulty pot? I have the same value of potentiometer arriving in the post tomorrow so before I start measuring everything and going through every component, I was thinking of waiting and changing it first.

Its a 100k linear, but I do have a 100k log on hand right now. Maybe I should try it to see?

[GibsonGM]

No harm to try it out. It could be a bad solder joint, tho - so I would re-flow those first and see if that helps.  Usually it is something we did in assembly

Pots are pretty tough, and take some abuse...never had one come from the factory broken in box...it it COULD happen, tho wiring is much much more common; I'd start there. 

[steveyraff]

No harm to try it out. It could be a bad solder joint, tho - so I would re-flow those first and see if that helps.  Usually it is something we did in assembly

Pots are pretty tough, and take some abuse...never had one come from the factory broken in box...it it COULD happen, tho wiring is much much more common; I'd start there.

Yea, I thought the same. I think I've pretty much replaced all wiring at this stage. I've always re-flowed all the lugs going to that pot. Its so frustrating trying to track down why movement causes it to stop working!

[steveyraff]

This is odd. The tone pot is a 100k linear. It seemed that when I touched some lugs of it with the screwdriver and applied pressure, it would start working again. So I replaced it with the closest thing I had to hand - a 100k Log pot. It seemed to sound better already and worked. Then I did the same to all pots and they seemed fine... EXCEPT the blend pot, which also seemed to behave the same - ie if I touched the lugs, it'd cut in and out. So then I realised that both the Tone pot and the Blend pot are both 100k linear's that I bought as a pair on ebay, as my usual trusted shop were out of stock. On close inspection, they seem way less ruggedly built that alpha pots. I don't have a replacement on hand to try swapping out the Blend pot, but I'm expecting on in the post tomorrow. However, I looked closed and seen that the lugs kind of move around. They seem slightly loose. I tried to flow some solder all the way up the loosest looking lug, until it reached the main Pot chassis - now it seems to work way better. It'll still cut out if I apply quite firm pressure to that lug, but moving the circuit around doesn't seem to cause any issues now.

I better replace that pot tomorrow too. It seems if they were bought together, and what I've done so far is helping, maybe both these Pots were duds?

Never really thought that would commonly be an issue!

[Mark Hammer]

I don't know that this is your problem, but sometimes the rivets that secure the solder lugs to the ends of the resistive strip can be loose enough that electrical continuity is intermittent.  What I do in these instances is to pry the tabs of the back of the pot up...gently...and just enough to permit removal of the back cover.  You will see something like what is shown here:  http://www.geofex.com/article_folders/potsecrets/potscret.htm

Once the back of the pot is off, you will have enough space to take your best needlenose pliers and gently squeeze the rivets on the pot to tighten them.  Now you can put the back of the pot back on and gently bend the tabs back into place to secure the back of the pot.

While the guts of the pot are exposed you might take the opportunity to clean the resistive strip.

I've had a number of pots over the years that were misbehaving and rehabilitated by this procedure.  It doesn't happen a lot, but it happens.

[steveyraff]

I don't know that this is your problem, but sometimes the rivets that secure the solder lugs to the ends of the resistive strip can be loose enough that electrical continuity is intermittent.  What I do in these instances is to pry the tabs of the back of the pot up...gently...and just enough to permit removal of the back cover.  You will see something like what is shown here:  http://www.geofex.com/article_folders/potsecrets/potscret.htm

Once the back of the pot is off, you will have enough space to take your best needlenose pliers and gently squeeze the rivets on the pot to tighten them.  Now you can put the back of the pot back on and gently bend the tabs back into place to secure the back of the pot.

While the guts of the pot are exposed you might take the opportunity to clean the resistive strip.

I've had a number of pots over the years that were misbehaving and rehabilitated by this procedure.  It doesn't happen a lot, but it happens.

Sounds like this could be the problem. I'll try to repair them as you suggested - but I'll use them for a future build. I have new pots arriving tomorrow so for this build I'll just use them for now to see if that is definitely what the problem was.

Thanks.

[GibsonGM]

Nice.  So now I have seen "broken in box"!  Wow. What happened to quality?  Alpha rules....   

You may 'fix' them and put them aside, use them with the breadboard only (that is what I do with pots from old amps and stuff I have taken apart).   I don't trust the used stuff to go into a new build, but I solder leads on them to BB circuits.  Then new parts for the final build.

[duck_arse]

.... that I bought as a pair on ebay, ...... On close inspection, they seem ...... 

the magic words. loose lugs can mean the death of a pot. one way around the problem (if they have pcb pins) is to mount the pot on a bit of vero or perf, 2 or three rows deep. this holds the pins as they were intended to be, and allows fly-wires to terminate as they like to be - solidly, and perpendicular to the copper.

I've had more than enough wiggle lugs, and as Mark says, there is a "fixx". mine is to use something small and hard and anvil/dolly shaped to support, and something else small and hard and punch shaped (a dud jewellers screwdriver, or a sharpened to a point screwdriver), then I give a few whacks on the rivet while it is supported on the anvil.

then mount the pot on a bit of board so it doesn't happen again. you could maybe practice a method on those two you have.

[antonis]

And don't go wild with soldering heat & duration...

(better use a bigger/flatter tip with lower temperature..)