I have made an interesting discovery about pots, specifically the CTS made ones, either older ones stock in Gibsons prior to 1998 or the ones currently available from AllParts... This will be a long post so be warned, but this might be useful info for some...

Some of you may have purchased some replacement 500K pots only to measure them and find they were somewhat low, 450K, maybe even lower getting down towards 400K. You may have basically said "Ahh Sh*t" and either lived with it or maybe tried to exchange them. I have discovered that these lower than desired values are NOT permanent, there is actually something you can do to change the value of the pot, increasing the resistance...

I sort of accidently stumbled onto this, don't know why I didn't consider this before. I had done the nail polish mod on one pot and then later for a certain reason I wanted to undo that, and desolved the clear nail polish with acetone, finally cleaning things up with a small soft bristle brush, a toothbrush. To my surprise I noticed the value of the resistive element was a little higher than before. I pondered the situation and tried some things, and eventually found I could permanently increase the value of the low value pots up to and well beyond 500K. I came to the conclusion that new 500K pots that read excessively low really don't have something that is fundamentally out of wack. I found that some of them have some extra of that carbon semiconducting material of the resistive element, in particular the extra is ON THE SIDES of the resistive element. In other words in the manufacturing process maybe a little too much carbon material gets bounded to the track and overflows onto the sides. That track is only about 1/32 of an inch thick or so, but the extra on the sides looks to makes a difference.

An analogy might be some road crew putting down ashphalt for a three lane road, but they put some extra down stretching it far enough to make four lanes. Four lanes offers less resistance to traffic flow than three lanes. The extra carbon semi-resistive material means there will be a somewhat lower than intended overall resistance.

 

Click on images to enlarge.

 

The fix is to disassemble the pot (yeah I realize not something just anyone is going to want to do but it can be done) to get at the resistive element, then take a small sharp knife, and carefully I repeat CAREFULLY scrape the inner and outer SIDE EDGE of the resistive element, basically just cleaning it up. I am NOT referring to the flat surface of the resistive element where the wiper rides against and makes contact, I'm talking about the side edges about only 1/32" thick. With some pots it was scraping the inner edge that made more of a difference, some the outer edge. If you connected a multimeter to the outer terminals of the resistive element and monitored the reading as you carefully cleaned up the edges, you could literally watch the reading rise. I had a couple of new 500Ks that measured down well below 450K, I think one was 439K and one was down in the 420K range. I gave these the cleanup scraping treatment of the inner and outer track edges and I had no trouble getting these up to values in the order of 550K!! I figure with the original situation desolving the nail polish with acetone, maybe the acetone managed to also dislodge some of the excess carbon material of the resistive element, so it might be a good idea to go real easy on acetone and pot inerds.

I don't think this scraping should compromise the performance or life of the pots as I did not remove material where the wiper makes contact, only just a little bit on the edges. Once again I think all I really was doing was scraping off some excess carbon material that was left sort of drooping over the sides as the resistive element was originally manufactured.

I certainly wouldn't think you could get really carried away and turn a 250K pot into a 500K, but you probably could get a pot that was below it's stated value up to and a little beyond that design value.

Followup to The "Brian Mod":

I thought I would mention that for anyone who hasn't yet tried taking apart a recent CTS pot such as the ones from AllParts, you just have to carefully bend back the four tabs on the metal can/body. These four tabs secure the can to the top portion of the pot. Once you get the tabs spread far enough you can get things apart. However if you are disassembling one of the CTS pots made for/used by Gibson prior to 1998, the design is slightly different and you have to make one more move to get things apart. With that version of the CTS pots, with the brass shaft and threaded bushing, there is a circular clamp or ring around the shaft, much like a C clamp, it sits in a slight groove in the shaft just below the knurling. You have to gently open up that ring clamp just enough until the circle is large enough to allow the shaft to slide through it, to be able to remove the shaft and wiper assembly from the rest of the pot to then be able to get at the resistive element.

After you mod the resistive element and are putting things back together, gently but firmly bend all four tabs back into place, whether with needle nose vise grip pliers or some other delicate but firm action. I don't know how much unbending and bending back that metal will take without breaking, if your technique is good it looks like you can certainly do it at least once no problem, I've gotten away with repeat reassembling a couple of times.

As to the issue of the current Gibson pots versus the CTS made ones, a little while back we had a discussion thread going where I posted some pics comparing the insides, and DaveP also had some pics. Those pics really helped to illustrate the different materials and design and construction details. From what I'm seeing I wouldn't be surprised if the Gibson cheaper plastic inerds pots had a higher inherent capacitance. If you were to swap between the Gibson and CTS pots and keep everything else equal, and you noticed a change in the tone, would it be that the CTS pots produced an increase in the high end?, I thought a few have mentioned that observation. In other words electrically speaking the current Gibson pots may be functionally equivalent to using a CTS pot but with a small capacitor (10 or 20 picofarad?) connected across the outer two of three pot terminals, from input to ground. An excessive inherent built capacitance in a pot might be negligible if it was used in a low impedance circuit, but with a moderate impedance passive guitar pickup it could affect the very top end response, like having a small tiny tone cap always connected.

That's just a theory, if some out there worked in an area that involved access to sensitive electronics test gear such as a good capacitance meter, maybe he could try measuring the built in capacitance of a couple of different makes of pots.

Followup questions and answers:

Forum member: I saved at least 6 new CTS pots from the bin. The only problem is I want to keep going just to see how high I can get them!

I think an argument could be made that maybe the pots that start out with a low reading are quite favourable to use because that might suggest the overall coating of the resistive element is a bit on the heavier side, could help in terms of holding up to the long term wear and tear. As to how high you can get them, I bet if you gave both the inner and outter edges a good firm scraping, square/perpendicular to the surface of the resistive element, maybe even taking a few thousands off the side of the resistive element, you could probably get a pot up close to 600K. Once again as mentioned in my original soliliquy, the wiper rides in the middle area so it probably should not have any impact if one shaves off just a bit more of the edges.

Forum member: Is it possible that the pots would "wear" into the stated value over time?

I would rather expect not. If the value were to move up at all from wear, it would likely be from the area of the wiper contacting the resistive element and that would not be such a good thing to happen. You even might start encountering some intermittent scratchy noise if the resistive element became that worn. I've disassembled CTS pots from earlier 90s LPs and they were still in very good shape, only some slight sign of wear, but still measuring way down below 450K.

Forum member: I found that by holding the exacto blade at a 90 degree angle along the edge of the resistive element, as oppose to flat against the edge, it works better.

It might be easier to illustrate with a photograph of the procedure, but I think we are on the same page. Let me try describing it another way: Picture a car tire lying on it's side. The side of the tire is analogous to the surface of the resistive element. The tread area of the tire that comes in contact with the ground is analogous to the "side" of the resistive element, the area we want to scrape. If you could visualize this, either a miniature tire or a giant pot resistive element, we want to scrape the tread area of the tire, perpendicular to the side surface of the tire. An exacto blade is an excellant choice for the job, and as you scrape the blade would always point to the center of the "tire". How's that for mental visualizations/gymnastics?

Forum member: I think the American Fender Strat tone control is out of the circuit at full open.

The infamous nail polish mod is another way of achieving this same effect. Look for a picture on the forum home site that helps to illustrate this. The nail polish mod puts some insulation on the track at one end so as the tone control is adjusted from about 9 to 10 the wiper loses contact with the resistive element and the tone control disconnects completely. I heard with the Fender tone pots that the pot track has a gap or slice at the end of the track. The nail polish insulation achieves the same result as cutting the track *but* it allows the mod to be undone.