Possible to repair a loose potentiometer leg?

[J M Fahey]

I was accusing nobody of anything specific , just trying to leave no stone unturned

I think you attach your pots with nuts to the panel and then wire them with wires to some kind of board, the "suffering" I have seen sometimes was when a front panel pot was directly soldered to a PCB.
Unless perfectly matched, that does cause problems: maybe not properly spaced PCB holes, or itīs slightly higher or lower than expected so when mounted it twists or bends pot legs or isnīt firmly (or at all) supported andvibrates or ....

[dchang0]

Oh, nothing to worry about. I didn't take what you said as an accusation. Even if it were, I would see it as help/assistance in finding the source of a recurring (and super-annoying) problem.

You are correct--so far all of these problematic pots have been "loose" panel mount, in that there is no PCB to align, etc.

[Enzo]

We don't know the batch size, do we? Two ordered at different times from say Mouser could still be part of the 10,000 lot they ordered on whatever date.

It has been a long while ago now, but Fender's Bassman series of solid state small combo amps had a bad run of pots. Same sort of failures we are discussing. Bassman 250 and similar amps. They sent out a bulletin to just replace all the pots in each amp, and file a warranty claim.

[rjb]

Semi-Incoherent Rant (Waaanh!)

Having just wasted an inordinate amount of time, and trashed I-don't-know-how-many push-pull pots, just to wire up a damn Telecaster, I can state unequivocally that the Bourns GTR pot legs do not hold up to standard assembly techniques as well as those on old-school 1"/25mm pots.

The pots I used were PDB183-GTR (17mm with push-pull DPDT). Bourns does not sell "pro audio" push-pull pots with a larger body.

I initially thought the problem was mechanical frailty- that the legs became loose with the slightest hint of external torque (e.g. bending terminal 1 to the pot body- which you can't do anyway, due to the physical configuration). I believed this because the loose legs appeared after I had tacked some test leads (individual 30AWG wires from a ribbon cable), even though I looped the leads over the control plate to provide strain relief.

But now, having read Bourns' datasheets (RTFM after all else fails), I'm pretty sure the issue is intolerance to soldering heat.

[Background: Since my trusty old Weller variable temperature soldering station died, I've been using a passed-on WTCPT station. These are "variable temperature" in that you can change tips for different temperature ranges- 600F, 700F, and 800F. I have a baggie of 800F tips for use with Solderon magnet wire, and I think the original PTA7(700F) tip is in the iron. My soldering m.o. is "hot and fast", and I usually have no problems with passive components.]

The first thing I noticed is that the spec for "soldering condition" in the latest datasheets is more stringent than in previous datasheets- not a good sign. The old spec for 17mm pots was 350C max for 3 sec max Manual; 260C max for 5 sec max Wave. The latest spec is 260C max for 3 sec max, Manual or Wave.

Well, sir, 260C is only 500F. So, according to Bourns, the coolest tip available for my soldering station is 100F too hot for their delicate pots. Moreover, I've been applying somewhere between 200F and 300F over their spec. D'oh. I never realized that pots could be so wimpy. Next time, I'll either try an 8W iron or break down and buy a cheapy adjustable temp station.

BTW, just for comparison, here's the spec for the polymer "vintage premium" 82 series:
Recommended hand soldering using Sn95/Ag5 no clean solder, 0.025 ” wire diameter.
Maximum temperature 399 °C (750 °F) for 3 seconds.

That's more like it.

-rb

Apply a drop of conductive silver paint, which wets not only the terminal but surrounding carbon surface....

I tried this, and it seems to work (even though the paint in the opened container had started to thicken). Got a reading of ~17 ohm between the wiper and the previously floating leg. Good enough for rock 'n' roll. Not sure about long-term reliability, though.

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.

Most of the "guitar" pots (ones with "GTR" in the model number) are only rated for a rotational life of 15,000 cycles. The longer life pots are the "premium" 82 and 95 (100K cycles), PDA241 (200K), and the PDB241 with "high rotational life" option (500K). I think the only features that make a GTR pot a "guitar" pot are the splined shaft and 3/8" bushing (some others have M8 bushing for 24mm body and M7 for 17mm body).

...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.

You're right; PDA241 pots with "standard" torque option should all be within 20-40 g-cm range. But most other Bourns pots don't have rotating torque options- and have huge tolerance ranges. For example, the latest datasheets at Bourns' site say 20-200 g-cm for PDB241 and 10-150 g-cm for PDB181-GTR.

I guess if you list it in the datasheet, its not a problem; its a feature.

We don't know the batch size, do we? Two ordered at different times from say Mouser could still be part of the 10,000 lot they ordered on whatever date.

The pots I used were definitely from different batches. Some came from Mouser years ago; some were recently purchased from a Reverb vendor. The older ones have brown epoxy around the base of the switch terminals; the newer ones have red epoxy.

[dchang0]

I'd say your excellent post is pretty coherent and almost certainly spot-on correct.

You and I had the same trouble with the same pot, it sounds like. Mine is Bourns p/n PDB183-GTR22-504A2.

I have a Hakko N454 25W pencil iron with a fixed temp of 450 deg. C. That's waaaay over the Bourns spec you found. I was definitely holding the iron to the trace over 3 seconds, as the push-pull wiring is slightly more complex than just one wire per leg.

Does the heat cause the solder leg to expand too much, from which it fails to contract fully? Or is it that the heat damages the carbon traces? Or something else or a combination of factors? Put another way, I am wondering if re-crimping will permanently solve this...

So far, the re-crimping trick has worked for me, but I am not using this amp often enough to thermally stress the pots with repeated power-ups and power-downs.

Re: "I think the only features that make a GTR pot a "guitar" pot are the splined shaft and 3/8" bushing..." I think the real feature that graces the Bourns GTR series is cheap construction and low quality.

[rjb]

I was definitely holding the iron to the trace over 3 seconds, as the push-pull wiring is slightly more complex than just one wire per leg.

That's another complaint of mine. Why are the pierced switch terminals so minuscule? They don't seem to have considered that someone might want to connect more than one 30AWG wire. For my layout, I had to get "creative" in wrapping wires and/or component leads around the terminals. I didn't burn up any switches- but every time you replace the pot, you have to re-do the switch wiring.

Does the heat cause the solder leg to expand too much, from which it fails to contract fully? Or is it that the heat damages the carbon traces? Or something else or a combination of factors? Put another way, I am wondering if re-crimping will permanently solve this...

I dunno. Is the soldering temperature high enough to anneal the rivets? We do know that the legs get loose.

So far, the re-crimping trick has worked for me, but I am not using this amp often enough to thermally stress the pots with repeated power-ups and power-downs.

Is that really an issue? Does the amp get as hot as a pizza oven (500F)?
(Not being sarcastic. I truly don't know.)

Re: "I think the only features that make a GTR pot a "guitar" pot are the splined shaft and 3/8" bushing..." I think the real feature that graces the Bourns GTR series is cheap construction and low quality.

Yea, that too.

-rb

EDIT: Before giving up on Bourns pots, maybe we should try soldering at the "right" temperature . I do wish someone had told me that they're more sensitive than germanium transistors. And I still hate the tiny switch terminals. But, although 15,000 spins doesn't seem like much to someone who does a lot of fake pedal steel volume swells & "boo-wahs", I haven't seen rotational life data for any other brand.

[dchang0]

That's another complaint of mine. Why are the pierced switch terminals so minuscule? They don't seem to have considered that some one might want to connect more than one 30AWG wire. For my layout, I had to get "creative" in wrapping wires and/or component leads around the terminals. I didn't burn up any switches- but every time you replace the pot, you have to re-do the switch wiring....

I DID burn up the switch (as well as one of the pot legs). But I didn't mention the switch because I simply moved to the other pole, and I could always use the crimp trick to fix it.

I dunno. Is the soldering temperature high enough to anneal the rivets? We do know that the legs get loose.

Yes, they definitely get loose. Mine was clearly able to rotate several degrees, maybe 10 to 15 degrees, until re-crimped.

Is that really an issue? Does the amp get as hot as a pizza oven (500F)?
(Not being sarcastic. I just don't know.)

LOL--nope, nowhere near as hot as a pizza oven. HOWEVER, we cannot assume that the pot meets the spec. I didn't mention how the new Alpha pot I had failed--it started out great and then slowly got fizzy/crackly over a long time. That was the amp I used the most (my practice amp, a 5F1 Champ, where the heat rises directly into the controls and where the control panel gets surprisingly warm and almost hot to the touch if left on for a few hours). I didn't put a thermometer in there, but my bet is that it was still much lower than a pizza oven. Yet, the pot did fail with a loose leg, and it was in a situation where the leg could not have been mechanically loosened (pot was not loose in the control through-hole and would not have turned, even if it did turn, I usually leave enough extra wire that it could turn without stressing the legs).

I will have to go look up the temperature spec for that Alpha pot to be sure. It is the most common Alpha pot model that just about everybody sells.

EDIT: Just checked the Alpha website and the retailer's website, and neither one mentions soldering temp maximums:

This is what I bought (500K audio):

https://www.tubesandmore.com/product...dio-38-bushing

This is what I think it is on the Alpha site, but the pots I received do not have slots in the solid shafts:

http://alphapotentiometers.net/html/24mm_pot_12.html

Alpha does mention 10,000 cycles as the rotational lifetime.

[rjb]

Alpha does mention 10,000 cycles as the rotational lifetime.

Well, poop.

[dchang0]

Maybe we have to think about this stuff as if they were consumables like tires on a car. Install them so they can be replaced easily (possibly without soldering, such as EMG is doing with their solderless wiring system).

[MarkusBass]

I have a Hakko N454 25W pencil iron with a fixed temp of 450 deg. C.
... I think the real feature that graces the Bourns GTR series is cheap construction and low quality.

I use a soldering station with temperature set to 250 deg Celsius and never had such problems (this applies also to Bourns GTR pots - I use them a lot). I also do it quicker than 3 seconds, somewhere between 1 and 2 seconds.

Mark

[dchang0]

Thanks, Mark. I think we can say rjb nailed it, then. Hopefully more people find this thread and save themselves a lot of grief when dealing with this new generation of low-spec pots!

[g1]

although 15,000 spins doesn't seem like much to someone who does a lot of fake pedal steel volume swells & "boo-wahs", I haven't seen rotational life data for any other brand.

Don't know if this is a valid comparison, but the Dunlop 'Hot Potz' literature claims 1 million cycles.

[rjb]

Don't know if this is a valid comparison, but the Dunlop 'Hot Potz' literature claims 1 million cycles.

With my current pickups, a 100K volume pot might work... but I'd have to give up the series/parallel switch. On the other hand, who needs 14 wiring configurations (4x3 +2), most of which sound pretty similar.... Nah. I'll wait till the Bourns wear out.

-rb

[Pedro Vecino]

3 seconds in a weld is an eternity

[dchang0]

3 seconds in a weld is an eternity

One can exceed 3 seconds easily if one is soldering two or three wires (such as two wires plus one capacitor leg) to the same solder lug in immediate succession. Both rjb and I have done so due to the fairly complex and cramped wiring on the push-pull switches.