Possible to repair a loose potentiometer leg?

[dchang0]

I've just tracked down a rustling noise in my brand new build with a brand new pot to one of the legs being loose (not firmly connected to the carbon trace).

This is the second time I have run across this kind of problem (first it was an Alpha brand pot in a different amp, now it is a Bourns brand pot with push-pull switch).

Is there a way to fix this? I'm assuming some special kind of crimping tool is needed to crimp the metal legs onto the non-conductive board. Can it be fixed with a dab of solder? (I doubt that carbon will take solder, but maybe it will.) Any tips?

[Jazz P Bass]

You may be able to get away with 'popping' the rivet with a tapered center punch tool.

Maybe not.

[dchang0]

Hmm. That might work. I was thinking I needed a sort of crimper, but maybe if I disassemble the pot and remove the flat board, I can try what you suggested with a metal anvil or vise on the underside. It just needs to grab the carbon trace, I think. Or maybe conductive glue...

I'll ask Bourns to send me a replacement. Kinda annoying that this has happened twice in two builds with brand new pots...

Sadly, CTS's 450 series SPST push-pull pot has gotten one scathing review on StewMac saying it's got a wobbly shaft and gritty feel. And the CTS DPDT push-pull got several reviews saying the shaft pulls right out of the pot! Guess I'm stuck with Bourns or Alpha for push-pulls.

[daz]

Hmm. That might work. I was thinking I needed a sort of crimper, but maybe if I disassemble the pot and remove the flat board, I can try what you suggested with a metal anvil or vise on the underside. It just needs to grab the carbon trace, I think. Or maybe conductive glue...

I'll ask Bourns to send me a replacement. Kinda annoying that this has happened twice in two builds with brand new pots...

Sadly, CTS's 450 series SPST push-pull pot has gotten one scathing review on StewMac saying it's got a wobbly shaft and gritty feel. And the CTS DPDT push-pull got several reviews saying the shaft pulls right out of the pot! Guess I'm stuck with Bourns or Alpha for push-pulls.

I do what jazz described tho i often use a small phillips partly because thats what is usually handy but also v]because the 4 blades dig in and require less of a hit. But it works just fine. Tho some have different types of crimps than rivets and require a different tool, maybe a flathead or whatever works. And depending on the pot it may be best if not required to remove the wafer to do it, but most of the time i'm able to do it w/o disassembly.

[J M Fahey]

Carbon track is way thinner than paper and very fragile, so unless just deposited and cooked, it won´t stand recrimping.

Apply a drop of conductive siver paint, which wets not only the terminal but surrounding carbon surface.
First crimp/press it, not for better electrical contact but to fix it in place, any movement will crack fragile silver paint.

[dchang0]

Thanks for the help, guys!

[dchang0]

!@#$!@#$!! Found another rustling pot with a loose leg--that's two in one amp (also Bourns). This is getting stupid!

[dchang0]

Taking J M Fahey's advice to heart, I searched for a gentle way to recrimp the legs without damaging the traces, and I think this will work for the long term.

It is a Knipex tool for removing locking/retaining snap rings that just happens to be shaped in such a way that it can crimp the leg back onto the trace. Ordinary needle nose pliers don't work because the flat circuit board is too thick and prevents the tip of the pliers from crimping the metal parts. Larger pliers that are shaped acceptably have teeth that would imprint in the metal and not press evenly.

I do not know if other brands of snap ring pliers are shaped like this, but they might be.

Here's a blurry photo of the tool in action:



I made sure to squeeze only on the side of the center hole that touches the carbon trace. So far, it seems to have solved the problem; we will see if it stands the test of time.

[Pedro Vecino]

That procedure, to me at least, does not give me any confidence. Then will face to the heat of the welding. If the amp will step on a stage, uses a new potentiometer.

[dchang0]

Good point.

Perhaps if the crimping is done when the leg is hot (while holding a soldering iron to it), it will shrink further when it cools down and grab the carbon trace more tightly...

I will order another pot. Hopefully it will not be defective like this one (also brand new).

[J M Fahey]

Buy from another brand.

Anybody can get a bad part, even if 1 in 1000 or one in a Million fails, *somebody* will get that one, so that alone is not a cause for concern.

Now geting TWO bad ones in, say, 10 or 20 is horrifying, even worse because they were not from the same batch (say the machine had a problem and pots from 23147228 to 23147328 were a bad run) but one original and another bought somewhere else.

OR: that factory likes lo lightly crimp pot legs, for whatever reason.

First case speaks of poor QC; second says poor design, both attributable to that brand and model, so avoid it.

There´s a third possibility though , that you are somewhat, ummmmm .... "rough" handling them

[Leo_Gnardo]

What Juan said ^^^. I still use Bourns "blue cube" trimpots, never had a problem. OTOH I've seen complaints about their panel mount pots over the last couple of years. Also about 10 years back I bought a couple batches of "guitar pots", presumably made extra tough by Bourns, rated for about half a million turns, with splined shafts to take push on knobs. For all their claims of "specially engineered" these didn't work out very well and I'm not about to buy any more. I dunno what's going on at Bourns, they were at one time a top quality supplier but those days seem to have gone. I wonder whether their costly conductive plastic pots are better, but hesitate to spend $12 each to find out.

Alpha pots, which were crap 30 years ago, seem to have improved quite a bit, the only problem left it's hard to solder to the metal cover even when scraped/sanded, proper solder, good iron temp plus flux. Lots of Alphas used in x-pensive booteek amps lately.

I've gotten some CTS lately too. Some work fine, some not so much. Another company that had an excellent reputation. You can read reviews from purchasers on Stewart-McDonalds' website, there's some real zingers in there. CTS another company that's apparently let QC lapse while giving full funding to their sales/promotion department.

Whatever happened to Stackpole? Lots of 40-50 year old Stackies still working in Fender amps from back then.

[dchang0]

It doesn't seem like there are any brands left to choose from other than Alpha, Bourns, and CTS... At least not in the guitar/guitar amp designs, values, price range AND stocked by the stockists like Mouser, Digikey, etc.

To Leo_Gnardo's point about the Bourns "blue cube": I am using the Bourns panel mount pots but COULD step up to the blue sealed pots, most of which run $15 each. Those probably are designed and manufactured to much higher quality. Many of the conductive plastic pots appear to be made very well too.

To J M Fahey's point about "rough" handling, as far as I can tell, I'm not doing anything out of the ordinary. I have two suspicions: maybe it's the heating up/cooling down of the controls that is loosening the pot legs, or maybe the power is exceeding the rating of the carbon traces (I have noticed that the audio tapers always have much lower maximum power ratings than the linear tapers and also that the designed-for-guitar pots always have lower maximum power ratings than the standard pots). But other people seem to be able to use the same pots in the same amp designs without trouble...

From what I can tell with my one CTS 3M reverse log pot (not defective), the legs are designed better--they really "grab" on to the board with a different type of crimping. But this may be specific to this series of CTS pot--it does not look like a typical CTS pot that I have seen in guitars. I think the 3M reverse log is a rare value only found in whatever special series this is.

Re: Stackpole, I guess maybe I'll be buying NOS pots now along with NOS tubes, because they don't make them like they used to. But then again, maybe this re-crimping trick will work and I won't have to replace these pots again.

[pdf64]

Such damage could also be due to poor packing / shipping / handling practices upstream of the customer. Standard pot terminals can be quite fragile.

[dchang0]

In this case, I think the packing was not at fault. Mouser wrapped each pot in copious amounts of bubble wrap. In the prior amp build, the pots came basically loose/bulk packed with no bubble wrap at all. I forget the supplier--it was either tubesandmore.com or tubedepot.com. They certainly get their pots in huge quantities. Mouser does too--it may be that while on Mouser's shelf, the pots are all in a big bin bumping up against each other, and then when they ship it, they wrap it nicely.

Here's one thing that makes me think it's poor quality/design on Bourns' part.

Due to my poor experience with Alpha in my prior build, I ordered exclusively Bourns pots for this build. Aside from the loose pot leg on one pot, one of the other Bourns pots is much lighter than the other pots in terms of rotational torque. I checked the abbreviated spec numbers stamped on the pots--the pots are all from the same series (PDA24) and have the same torque rating (Standard = SRT, versus High = HRT). They are all supposed to be 20 to 40 g-cm torque to turn. But this one pot is far too easy to turn. Either they mismarked it (the PDB241-GTR guitar series pots have a lower minimum but wider range of 10 to 45 g-cm), or this is another defective pot in my small sample set.

I suppose the third possibility is that all the pots are within the Standard torque range, but all the other pots happen to be at the upper end of the range and this one pot happens to be at the lowest end of the range (since I cannot measure the torque, I cannot say for sure this pot is out-of-spec). But it is highly unlikely that the "good" pots would all be so similar to each other and then have a single outlier so far away (imagine a very narrow bell curve with an outlier several sigma away).

Important to note: rotational torque is not likely to be affected by physical damage to the pot on the shelf or during shipping--this is likely a manufacturing flaw.

Next build, I'm going with CTS, even though CTS gets roasted on the ultra-poor design of their new made-for-guitar DPDT push-pull pots.