Trimmer pot lighting up and smoking!

[steveyraff]

Hey guys,

Just caused an accidental pyrotechnics show tuning a Human Gear Animto.

I was tuning the little 10k internal trimmer pot and it lit up like a light bulb and started smoking. Quickly pulled the screw driver out and it stopped. Now its sounding pretty good though. Is it safe to box this up or has it been damaged? It actually done it twice in a row before I stopped but now the pedal seems to function fine. I'm confused!

Steve.

[GibsonGM]

I have done the same, and still used the pot, with no terrible results.  If you are selling it, you'd replace it now, ha ha.  For your own use, I would just use the pedal for a long time to be sure it is ok.   Your call....'lit up like a light bulb' doesn't give me a lot of confidence, LOL!  You might feel better if you just change it out...

Now, I would be curious WHY enough current can flow thru that trimmer...good design would prevent that from happening.  I don't have the schematic, but usually they'd put a limiting resistor or something with the trim to keep the power dissipation within its range...you SURE there is no error around there???

[electrosonic]

In the schematic I have seen, here is a blocking cap on the wiper of the trimmer. If it is smoking then it is probably mis-wired.

Andrew.

[steveyraff]

In the schematic I have seen, here is a blocking cap on the wiper of the trimmer. If it is smoking then it is probably mis-wired.

Andrew.

Checked all that - nothing is miswired, all resistor and cap values are correct and in correct places. Surely if it was 'mis-wired' it'd either not work or sound awful. It sounds good, it just did that once while tuning it. I've tuned the trimmer a few times since and its never done it again. Weird.

[GibsonGM]

VERY weird...because there was a reason it did that.  An intermittent short to your power supply or a potential high enough to cause an excessive current to flow as you decreased the trim's resistance.

Things can be miswired and 'work'...probably not as designed, tho.  You might never find out why that happened, or you might spend lots of time examining the board, your work, and eventually be like "A HA!!!" when you find a solder bridge or something...bit of wire sticking over, or that there was something metallic underneath the board while turning the trimmer.   There is always a reason; that's physics!  Hopefully it did not damage the trimmer.  Sometimes you get lucky 

[anotherjim]

Skeleton pre-sets, the screwdriver slot is the wiper contact. I'd suspect the screwdriver also touched 0v somehow.
Obtain a trimmer tool. A  plastic stick with a metal tang in the end which is not big enough to short anything easily.

[Kipper4]

Sometime the screw driver used can short the trimmer.
Try with plastic /wooden screwdriver.
Wooden you ask?
Yep a 3mm round wooden kebab skewer sharpened to a point.
Ingnore this if you all think it sounds stupid.
I also have a plastic driver I can't remember what it came with.

[steveyraff]

Sometime the screw driver used can short the trimmer.
Try with plastic /wooden screwdriver.
Wooden you ask?
Yep a 3mm round wooden kebab skewer sharpened to a point.
Ingnore this if you all think it sounds stupid.
I also have a plastic driver I can't remember what it came with.

This sounds very like what happened! Thanks Rich!

[GibsonGM]

Or use a fatter Phillips head screwdriver that doesn't sit way in there to make contact.  Or straight 'regular' blade that goes across and stays on plastic.  I've NEVER had that happen - amazing!     

Popsicle stick, kebab as suggested - great ideas.  Mostly, metal will afffect capacitance, so ppl keep a non-conductive thing around like that for adjusting trimmer caps, a good item for your toolbox and cheap.  *free*

[antonis]

Or a metallic screwdriver with flat head swiveled at 90^o at 2-3 mm end..

[MrStab]

well the magic smoke has to come from somewhere!

[antonis]

well the magic smoke has to come from somewhere!

Very good title for the "Manifest of Determinism"... 

[EBK]

I was so hoping this thread would contain a video.   

So, I found one (not a trimpot, but oh well):

Tip: Always remember to use one of your grandmother's handkerchiefs to protect her couch from fire if you attempt this.   

Another tip: If you are going to search the Internet for a lighting up or smoking potentiometer, make sure you do not abbreviate "potentiometer".

[amptramp]

If a trimpot is lighting up and smoking as you say, just put a NO SMOKING sign on the inside of the box.  A good trimpot will respect your wishes.

[antonis]

Just curiosity, Ron...

Should the sign be the same for both horizontal & vertical mounted pots..??

[amptramp]

^^^

You just have to make sure the pots can see them.

[steveyraff]

The section of the 1590BB that the trim pot accommodates was actually allocated as a smoking section in 1971. After the smoking ban came in within this country, the 1590BB remained an independent Sovereign state, and was granted immunity from my countries judicial law system. I can't put up no smoking signs in there now. I think its probably just easier to not use a conductive screwdriver anymore.

[duck_arse]

awwww, there was a video posted here of someone playing their "sounds great" pedal/circuit a few years back, the pot burst into flames as he turned it, but I can't find it now.