CTS Potentiometer

http://www.alliedelec.com/Catalog/Indices/Products.asp?N=4294966088&sid=46B664802ABAE17F

Well done that man!

You are to be commended an all levels, so many people wouldn't have the wit to get as far as you have..all your assumptions sound just fine to me...hope you get the pot' ok. (or one near enough to work, it's possibly not hyper-critical, they are often only 10 or 20% tolerance anyway)

You get my award for most accurate and comprehensive question (dunno what it will be, maybe I can manage to find a spare mouse).

Good luck

Del

I concur with Del! Nice treat that the originator provide 'nuff info to go somewhere coherent and understand the question!

Of course, with an in-house replacement one loses the original equipment manufacturer's warranty on the new board and possibly the remainder of the equipment, which may or may not be important. The heat dissipation is well within the capabilities of a small pot. 10K is a very common value, and 10K pots can be found almost everywhere.

If you can handle a smouldering iron (sorry, that should say soldering iron), then a 10k pot from the local electronics hobby shop should be an easy thing to substitute for the damaged one. In Britain I would expect to pay less than £5 for one new.

Another possibility is to strip one from an old radio or television, and use that as a replacement. Bear in mind that 'volume' potentiometers have a logarithmic characteristic and that a linear characteristic may be better for this particular application, though both will work after a fashion.

Another possibility is to insert a resistor of, say, 2.7kΩ into one of the two outer legs of the existing pot so as to move the setpoint to a different point on the scale that isn't cracked, and reset the setpoint on the knob to that place. The markings on the outer bezel of the equipment may have to be re-marked.

Some pots will respond to careful dismantling, cleaning with a suitable solvent, and careful reassembly. Twisting the wiper contacts gently so that they have a different bearing pressure on the resistive material's surface can also work wonders on occasions.

Another possibility is to insert fixed resistors in place of the pot, so that the temperature setpoint is always the same. After all, if it hasn't been re-set for some time then why not just do away with the adjustment facility?

It's play time!

great tips!

Wow, you guys are great. I thought that I was on the right track, thank you all for verifying my assumptions. I had thought of adding a resistor to move the linear point a bit to get out of my dead spot, but thought that replacement would be a better idea. I did see a small 10K linear potentiometer on the web page referenced, and think that once I find one that is a reverse shaft I will replace it. I also agree that a linear curve would perform better than a log curve, it would more accurately represent the intent of the setpoint pot, ie. to change the temperature of the chilled water in a linear fashion. Also, the equipment is 17 years old, so there is not fear of voiding any remaining warranty as there is none.

As usual you folks are spot on! Thanks again!

Dave

Potentiometers can often be restored to full efficiency by shooting them full of WD40, try it!

WD40 works well as a cleaner and lubricant for controls, and I have used it in this way for years. I used to worry about the waxy residue left by WD40, but it seems not to matter. Silicone spray also works well, and the lubricating residue it leaves is less likely to collect dust.

The WD40 advice seems counterintuitive. I was always told that this product was not a good mix with electrical components. Is it a matter of lubrication on the pads that assists in restoring the potentiometer to function? I have not attempted to 'crack' this pot open yet, but am wondering how WD40 will work if the parts are pitted or have surface corrosion?

Good point nonetheless, and will certainly try to shoot it with WD40 before I go the desolder it out and replace.

You learn something new every day, especially in maintenance and repair. The potential ( no pun intended!) for working on something new every day is endless!

Thanks Again,

Dave

Your math is correct and your on the right track. Since this is a controls issue... any pot should do the trick as long as its resistance is less than the controller's input impedance. The power calculation is only relevant if you know the controller's input impedance (which is usually very high).

If this were an issue of a potentiometer powering a circuit... then you'd have to take in consideration the power rating of the potentiometer. Think about it... you have a 10k pot... at 24V that is only 2.4mA and 58mW... BUT... as soon as you turn it to half... that's 115mW.

As I said previously... this is a signal control issue... so you should be fine.

So far, I haven't found a relavent reference to this pot. Does is have a 1/4" shaft maybe with a screwdriver adjustment? Metal can body? If so, some of those mfr'd are sealed so zapping them with a contact cleaner would not work (look around the terminals (should not have to remove the part): do they seem to have an physical opening or anywhere around the body? If exist openings, then use something like Channel Master Contact Shield, silicone base or Radio Shack equiv. (they used to sell this). These have a lubricant which is important, otherwise any that remains internally might be stripped and the part could be trashed (worse than it was).

Moving right along: Since you said that it was "reverse" then it almost surely a pc board mounted fella. You don't need to necessarily replace it with a reverse type (only the rotation of the replaced part will change the (temp?) in the opposite direction. That should not be a problem. PCB pots will not have any method of zapping with a contact cleaner. Measure some of the physical attributes of the part and go look at perhaps http://www.digikey.com/scripts/DkSearch/dksus.dll?Cat=262965;keywords=potentiometer. look at some of the data sheets available for side/top adjustment/pin-out dimension. It's probably linear. Note that # of turns may be of importance. I'd recommend something in the middle of choices and you should be fine.

Good luck!

Bill, the shaft is smaller than 1/4 inch, and it is a round shaft with a small knob with set screws holding it on. The shaft is metal, as is the body of the pot. It is mounted on a pcb, so I will have to go open the chiller up again and get some good measurements of the physical size, locations of the pin out's, etc...

Thanks for the input, you guys are great. I appreciate the input on this. My boss about flipped when I told him that the replacement of this pcb was over $600, just to replace a flaky $5 part!

I'll post up again with physical dimensions when I get them, but I have been pouring over mouser's catalogue, as well as digi-key. I'll find something!

Dave

Okay Dave, thanks for your response. Off today, so I can be more-so attentive.

Just maybe, once given your dimensions etc, I might have something suitable in my beloved junkbox, and where I would be glad to pop the baby into the mail to you. A freebie.

If this fails, then you'll have more parameters to find a suitable pot. Warning: pots are quite more expensive today than they were years ago. I am appalled. Must be gas prices, huh? A basic pot today is $10++, but not at all near $600 (yet?). Used to be a few dollars. Also, try and determine if it a multi-turn type (stop-to-stop, it might be). Make sure you mark the starting point so that you can return back to the same setting.

Bill

Bill, you must have this on your email response notification too! The pot is a single turn, somewhere near 270 - 300 deg sweep. It does not go all the way around. If it cools down any I can go over and shut the chiller down and take a look see for dimensions. With the way the temps are running, that will have to be at 7 am tomorrow morning when it will only be 75 -80 degrees!

Thanks for your help,

Dave

Measure the mounting shaft diameter while you are at it. It's probably about 3/8" and mounted with the appropriate hex nut.

By the way, if it is somehow really a reverse running pot (seem to remember they do that with ball bearings around the shaft), you can use a "normal" type pot and reversing the two outer wire connectons (still assuming it that type and not pcb soldered in to board).

If handy, also take a digital photo of the baby and import it here. A picture is worth a thousand words.

A mentioning: many of the resistor/potentiometer co's have merged/bought out/changed names. Think CTS is still around tho. and where they have discontinued this particular "baby". Not being deemed today's technology, mfr's tend to axe certain product lines.

Dear dave,

We are Omeg potentiometers in England and are not registered with this site.

We believe that our 20mm diameter potentiometer 10K linear should do the job. We are confused with your words 'reverse shaft'. Our unit is a nominal 0.25W please look at our website ( http:\www.omeg.co.uk ) as we have a number of options for terminals and shafts. If you can identify the exact configuration from our website we would be happy to send you a sample, good luck and we hope we have been able to assist you on this occasion.

Better still, email us a photo of what you have...

regards, Graham H.

Graham,

Thank you for the offer. I will try to get into that today and take some digital photos of the part. The reverse shaft nomenclature is from the CTS applications engineer that I queried regarding this pot. He says and I'll cut and paste here,"

Actually, the AW-5403 is old original CTS sample number dating back into the 70's.

I can't say I know who was the original owner (custom specs) these belong to, but at since 1989, this has been listed as a Megatech Electro product under their PN HT203988-1.

(your 1378912 actually means that CTS built this the 12th week of 1989.)

I can tell you that this is a 10K linear, it has a reverse shaft, meaning that shaft comes out the bottom of the part rather that panel mounted."

When I looked at the pot back in July it did not appear to me to be anything out of the ordinary. I'll try to get into the panel and post up some photos in order to better show what I'm looking for.

Thanks Again, you guys are great!

Dave