Funky Pot Problem

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psycho
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Funky Pot Problem

Post by psycho » Sat Mar 13, 2010 4:19 pm

I went to Goodwill a couple weeks ago and picked up an old racing wheel with a joystick (15 pin dsub) connector. My intention was to use a uC (PIC18F2550) to convert it to USB. It worked out great except for one thing...

For $6, it came with the two floor pedals. It has a total of 8 buttons (on the wheel), 4 pots, and one LED. There is one pot for the wheel, one for a button/switch thing one the wheel, and two in the floor pedal unit.

It seems that one or both of the pots in the floor unit are dirty/dead because when you measure the resistance from one end to the wiper with the pot rotated all the way to one side, you get the full resistance (20k, IIRC). But, move it 1/4 turn or so and it goes from 20K to 0 instantly - no gradual reduction. The pot turns 3/4 or so total but the pedal doesn't turn that much - maybe 25-30 degrees.

Have you ever seen a pot go like this?

btw - the pot looks pretty much like this:
http://search.digikey.com/scripts/DkSea ... 03&x=0&y=0

Kevin

Robert Reed
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Re: Funky Pot Problem

Post by Robert Reed » Sat Mar 13, 2010 5:05 pm

Looks like a standard ultra cheap PCB mount pot. Did you measure the wiper to the other end of pot when it suddenly dies. It may have been abused and skewed the wiper contact so that it is lifting off the carbon element midway thru rotation. With luck, you may be able to open the metal case and get things back into alignment.

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Bob Scott
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Re: Funky Pot Problem

Post by Bob Scott » Sun Mar 14, 2010 9:45 am

psycho wrote:Have you ever seen a pot go like this?
I've seen this. If you connect an ohm meter between the wiper and one end, and see a sudden drastic change in the resistance reading to infinite ohms as you rotate the wiper, the pot is cracked. There is a break in the resistance material. There is no repair, only replace.

Another way to check: Test the resistance between both ends. You should read the value of the pot if it is good. If the resistance trace is broken, it will read infinite ohms.
-=VA7KOR=- My solar system includes Pluto.

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CeaSaR
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Re: Funky Pot Problem

Post by CeaSaR » Sun Mar 14, 2010 9:56 am

Kevin,

I've seen pots go like that and the only reliable way to fix them was to replace them. Looking at your Digikey link,
you'll spend just over a buck apiece for them and if they stay under 8 oz., first class USPS will be about $2.50.
Even if you replace ALL of the pots, you'll still have spent less than $14.00 total for a $80 or more driving set.

CeaSaR

P.S. I did a doubletake at the name of this thread - it sounded like an "organic" question! :lol:
Hey, what do I know?

psycho
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Re: Funky Pot Problem

Post by psycho » Sun Mar 14, 2010 11:18 am

You have all pretty much confirmed my suspicions... I will just replace them and be done with it. I couldn't understand why BOTH of the pots would be dead but then I thought... Well, there is a reason someone gave it to goodwill...

The other two pots seem to be OK but I will get a half dozen of them - I never order one thing from mouser/digikey. If I need one, chances are I will need more later so I will get a few extra. Obviously I can't do that with expensive parts.

Thanks!
Kevin

Oh, and no - I am not into "organics"... Too old to waste my time with that garbage and I never liked it anyway.

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dacflyer
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Re: Funky Pot Problem

Post by dacflyer » Mon Mar 15, 2010 3:13 pm

you could clean the controls - i use a cleaner made by MG Chemicals
it is called NU-TROL control cleaner it is safe on plastics also, it will not eat them up like contact cleaner..this cleaner will restore it to like new condition..
i have used this for many years and always had great luck with it.
the local electronics shop here carries it.. bout $12.00 for a 5 oz can.

good luck.

psycho
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Re: Funky Pot Problem

Post by psycho » Mon Mar 15, 2010 3:15 pm

I soaked it in rubbing alcohol for about an hour taking it out a couple times to rotate it a bunch. It didn't do any good :(

Kevin

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Bob Scott
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Re: Funky Pot Problem

Post by Bob Scott » Mon Mar 15, 2010 3:38 pm

psycho wrote:I soaked it in rubbing alcohol for about an hour taking it out a couple times to rotate it a bunch. It didn't do any good :(
That's too bad. Rubbing alcohol contains lanolin:

"Lanolin (German, from Latin lāna, "wool", and oleum, "oil") also called Adeps Lanae, wool wax, wool fat, anhydrous wool fat or wool grease, is a greasy yellow substance secreted by the sebaceous glands of wool-bearing animals, with the vast majority of it used by humans coming from domestic sheep."

It's no good for tape heads either.
-=VA7KOR=- My solar system includes Pluto.

psycho
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Re: Funky Pot Problem

Post by psycho » Mon Mar 15, 2010 3:57 pm

Hmmm. The bottle says that it contains 50% Isopropal Alcohol and the only other ingredient it lists is water.

Kevin

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haklesup
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Re: Funky Pot Problem

Post by haklesup » Mon Mar 15, 2010 4:50 pm

you reported an intermittant shorted condition (goes to 0 ohms). Dirty wipers and broken resistive elements will generally lead to intermittant opens or higher R.

To get a short, you must either have metallic debris stuck in the pot or something is bent or broken.

Truly, your best option is replacement.

For cleaning avoid rubbing alcohol although it often works if low levels of leakage aren't a problem. Many contact cleaners have lubricants but those would be non conducting. Best to buy either denatured alcohol (ethanol) or methanol or 100% Isopropyl from the hardware store. Never use Acetone unless you are darn sure it has no plastic components (acetone is OK on ceramic, metal and glass but dissolves plastic). If it had been dirty, even rubbing alcohol would have had a positive effect.

If the shorting debris were mobile, try a more agressive wash. Water and soap is OK as long as you rince with clean water thoroughly to remove any soap and rinse again with isopropyl (IPA) to fulsh the water away and dry it quickly.

Important: If that Funky Pot starts to smoke when you hook it up, don't inhale or the cops will come get ya :razz: LOL

psycho
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Re: Funky Pot Problem

Post by psycho » Mon Mar 15, 2010 5:08 pm

Yeah - I am just going to replace them. I will wait until another project comes up to where I need to order something and then order a bunch of stuff for multiple projects. I am a cheapskate and I hate paying shipping (even though mouser and digikey are fair). Until I had the problem with the pots, I had everything I needed on-hand for the project. I used through-hole parts because I am running low on 18F2550's in SMT but I normally do all surface mount nowadays.

I did not know that iso. alcohol could leave residue. I have been using it to clean everything. I use it to clean my future PCBs before I iron on the laser transfer (which I found that using the backs of labels works great - the slick side). If you know your Fedex driver, ask him for a pad of the forms they use to stick on your doorknob for a signature - there is a piece of label backing that is great for 2x3" or so boards.

I learned something today...

Kevin

psycho
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Re: Funky Pot Problem

Post by psycho » Tue Apr 27, 2010 6:32 pm

Well, I had to order parts for another project (tactile buttons for our oven's control board). I ordered up a couple pot's, too and the whole racing wheel/pedals project works great! I am more than happy with the way it turned out and I wanted to update you all and say thanks again for your input!

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CeaSaR
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Re: Funky Pot Problem

Post by CeaSaR » Wed Apr 28, 2010 7:07 pm

Most excellent Kevin! May your races never falter and may you best them all.

CeaSaR
Hey, what do I know?

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