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 Post subject: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 3:11 am 
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Location: Mesa, AZ 85206
Hi Folks

A 300Ī© 50W line adjust pot has two distinct points of discontinuity along the wire wound track.

It strikes me that I could bridge these points with little if any loss of resistance along the track. Finding another of these would be difficult.

What is the best way to do this? I attempted heating the point, but I could not get solder to stick to the coiled wires at the temps I was using.

Higher heat? Some sort of conductive epoxy?

Thanks!

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 4:49 am 
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It might be possible but I would be concerned about reliability. For ten bucks it could be replaced.
As rheostats go, this seems to be a very good price
Cheers
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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:17 am 
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Sadly, It doesn't have the center tap...I have 50W 300Ī© pots, just none with the center tap...

There are two things that make this pot hard to replace...the center tap, and it's elongated shaft...my other 50W wouldn't fit as it needs to be raised above another pot close by...

Steve

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:39 am 
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You can't solder to the resistance element. I have repaired pots in the past but it's a bit hokey.

One way is to bypass the broken spot with a piece of metal or wire, using a clamping arrangement or screws. Depending on the wiper size you may lose contact as you go through the break. You can also put a piece of foil over the break such that the wiper can go over it, and cement or clamp the foil in place.

I did this on a GR impedance bridge and managed to get it working nicely with minimal disturbance as I moved over the break area. It was large enough that I could use screws to hold the jumper part.

If you can unwind the element enough to bring the broken ends together and twist them, that could work.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 6:21 am 
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Thanks, Bob..I will stop thinking of soldering.

I was thinking of two possibilities...

drill a small hole between the broken windings on the side and epoxy a small brass tack that bridges the elements,

use a conductive epoxy that would bind to the ceramic between the broken windings..

Both of these are similar to what you suggested.

In both cases, I would only lose a couple of ohms.

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 8:44 am 
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drworry wrote:
Thanks, Bob..I will stop thinking of soldering.

I was thinking of two possibilities...

drill a small hole between the broken windings on the side and epoxy a small brass tack that bridges the elements,

use a conductive epoxy that would bind to the ceramic between the broken windings..

Both of these are similar to what you suggested.

In both cases, I would only lose a couple of ohms.

Steve

That is the first rheostat I've seen with a center tap. What is its function in the circuit?
Also, does the rheostat get very hot when in use? Thanks

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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 3:02 pm 
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What about using a new pot and creating a center tap on it? That might be easier than repairing a break in an old one.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 4:25 pm 
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The spare 50W 300ohm pot I have won't let me drill such a hole without drilling through teh windings...I may have to look for another one.

Moderator: this thread is being duplicated by me (unintentionally) with the 10-60 thread...please lock, remove, or let alone at your discretion.

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 4:40 pm 
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Location: Tucson, AZ
Given that it is a center tap, would be pretty unusual to find a replacement, you might try a silver conductive epoxy to connect the broken wires. How well this will hold up, and the current/heat may be an issue.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 4:46 pm 
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Just a thought.

Maybe a tack/spot welder to attach a small shaving of shim stock between windings?

might be able to make one from a Weller D550 gun or something.

It would take experimentation

-Steve

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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:10 pm 
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You can also try using a repair kit for broken window defroster traces by Permatex to join the broken wires.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:22 pm 
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I am going to start with using a nickel conductive pen to see if in fact I can obtain a functional unit by bridging the broken windings. If that is successful, I will probably pony up the $50 for silver epoxy and see how it lasts.

If my initial efforts are successful, I might be able to drill a small hole on the shoulder of the original and place a conducting shim on the side.

If one looks at the attached pic of the spare pot I have, the green winding covering on the side might be conformal covering...if I can remove a small section of the covering with acetone/lacquer thinner, I might be able to fashion a pressure fit tap drilling into the shoulder.

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:34 pm 
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The wire used for these can't be soldered and I doubt any conductive epoxy or similar would bond and hold up. The only repair I can imagine would be to spot weld things back together and then grind it smooth.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 5:42 pm 
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my hope would be that the conductive epoxy binds to the ceramic between the windings maintaining electrical connectivity between the broken windings.

Sadly, spot welding is not in my skill set..wouldn't the heat of welding crack the porcelain?

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 6:22 pm 
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Actually, spot welding is probably the best solution. It's not difficult. You just lay two metals together and discharge a capacitor across them. Practice on unimportant stuff and then zap the wires (with an appropriate bridging metal).


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 6:52 pm 
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Steve, you can make your own spot welder inexpensively. No, you aren't repairing battleships with a home-built one but I am sure you can perform this sort of repair.

I did build one quite some time ago {12 years ago?} to rebuild the rechargeable battery packs for my cordless drill. I obtained a big transformer from a dead microwave. Chopped and cut out the secondary and put in 1 very heavy turn of wire, it just barely fit inside the frame. That one turn was run to good quality copper tips used for the spot welding. For good current to pass the joints between cells in a battery pack, they are spot welded. So the tips you use for welding need to be decent and well connected to your secondary.

For info, I suggest you take a look at videos of guys rebuilding their battery packs and/or homebrew spot welders.

The beauty of this is the duration of you applying the tip to weld is very short. So the only real heat is very short in duration and only at the metal joined due to the heavy electrical current flow. After all, rechargeable battery cells cannot survive the heat of soldering and solder is not a good heavy current conductor. I am quite sure that if you experimented a bit with say the old variable filament dropping potentiometers used in battery radios {Ones that predate electricity in houses} or another high power pot that has an open in it, it would give you a feel about making good contact with the one side of the welder and locating the metal jumper and then performing the spot weld.

And your weld would not be along the track of your pot, but the bottom or side. This way, your wiper is not going over a bump, etc.

Check it out, this may be a perfect solution. Inexpensive, potentially you just need to buy high quality copper tips.
John S.


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 7:28 pm 
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this seems like a good skill to have...

https://www.amazon.com/Portable-Machine ... NrPXRydWU=

Probably would do it!

Steve


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 9:29 pm 
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Nichrome resistance wire can be soldered using a plumbers liquid acid flux, Oatey. Do not get the grease type.

Use an iron with a large soldering copper and preheat the body of the pot in an oven at 125 deg F.

If a small iron is used there is not enough stored calories. If a modern iron with an iron plated tip is used the acid will begin a rapid erosion of the iron plated tip...

Rinse the pot with water after soldering a bridge across the open windings.

Chas

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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 10:04 pm 
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For $14.30 you can get the correct 25 watt pot that will fit your tester correctly. And it is delivered free.
https://www.amazon.com/Wirewound-Potent ... B07MG5472Q

DM


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 Post subject: Re: 50W wirewound pot repair
PostPosted: Sep Tue 14, 2021 10:13 pm 
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Alas, nope, DM....no center tap...

I already have spares of those for my 10-12.

The 10-60 uses this pot to manage voltages and currents for VR tubes, and so requires a higher wattage..the tap is needed (presumably) because the 10-60 needs to span 70-140 volts for VR tube testing.

Steve


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