Vintage potentiometer repair: What compound to use?

[userx2]

Hello,

I am trying to repair some vintage potentiometers made by VDO.
The problem I have is that tge wire coil was secured in a ceramic slot with some white cement looking stuff. Without that, the wire coil gets literally wiped (!) out of the slot by the potentiometer wiper.
What product would be suitable to affix the coil back in the slot?
It gets quite hot, hence the ceramic housing.

Best regards
X

 

[BradtheRad]

'JB Weld' has a line adhesives useful in automotive repairs. Durable. High heat tolerant. Epoxy mix 2 tubes. Even though 'steel' might be on the label I don't think it's electrically conductive but it's a good idea to check what the instructions say.

 

[userx2]

Hi, yes, I saw that and a few others.
My worry is whether they conduct or not. Especially when it says can fe machined etc.
I ban't just buy them to try because all have to come from overseas $$ as nothing seems ailable in Australia.

If anyone perhaps knows of a specific product that will work, I will certainly get that.

X

 

[dick_freebird]

How hot the wires get, will matter. I've had JB-Weld
regular up to 300C for long periods.

If the "potting" seems more mineral-like than organic
it might be what they call "frit". Kinda like grout, used
in incandescent bulbs and industrial grade ceramic,
non-hermetic IC packages (the white stripe in the center
of those types' "Oreo").

Don't know offhand where that might be found but it's
an old timey industrial feedstock.

If those are really "rheostats" meant to control power
by being dissipative, you could see some significant
heat thrown and might need the ceramic. Might keep
an eye out for any thermal conductivity info.

There's an outfit that specializes in high temp adhesives
and materials but I can't remember it right now. See them
in the back of mags like Machine Design, Tech Briefs etc.
If I run across it I'll post it. Probably industrial-spendy. A
small amount of frit is probably dirt cheap.

--- Updated Apr 21, 2024 ---

Here is a source, evidently much use in ceramic arts as
well as industrial. Looks like finishing the job wants a kiln.

https://www.theceramicshop.com/store/category/7/9/Frits/

Here is something more in line with your application, frits
and glass pastes for electromechanical uses

Glass Frit,Glass Pastes and Glass Lid for Hermetic Sealing ｜ Products ｜ AGC

Materials for electronics applications used for the purposes of insulation, hermetic sealing, etc.

www.agc.com

 

[userx2]

Thank you, I think that frit stuff is likely it. Yes, grout like.
I will study tour links.

Yes, this is a dash light dimmer. It goes in series with about 16W load.

Best regards
X

 

[FvM]

Ceramic potting compounds and cements are often using Al2O3 powder as filling, interesting part is the "glue" component, can be e.g. water-glass. Important parameter is curing temperature.

Search keyword is "ceramic cement".

 

[userx2]

Ok, thank you.
So what I can see looks like they filled the slot about 1/3 with somethin and tgen ket the wrie settle in that.

I do not think it was fired after this as there are brass bits that would have melted.

 

[dick_freebird]

You might like to look at those brass grommets, as the
resistor wire green color to me says corroded nichrome.
You may have contact-resistance problems from oxidation.
Might even need to braze a connection or replace the
existing brass with new (look to textile snap or grommet
assortments for something that would swage in properly?).

Should be sub-ohm resistance from brass to closest wire
point. Might have to scrape the wire to even get a reading.

 

[userx2]

You might like to look at those brass grommets, as the
resistor wire green color to me says corroded nichrome.
You may have contact-resistance problems from oxidation.
Might even need to braze a connection or replace the
existing brass with new (look to textile snap or grommet
assortments for something that would swage in properly?).

Should be sub-ohm resistance from brass to closest wire
point. Might have to scrape the wire to even get a reading.

I am aware of those problems and they are present in these parts. I will however fix that but the main issue is the wire coming out of the groove. The brass you mean is rivets.

 

[D.A.(Tony)Stewart]

Epoxy outgassing is toxic when too hot.

Consider ceramic tile grout.

Also reshape those coils to have a flat top surface to avoid discontinuity arc.

 

[userx2]

Tile grout! That is a really good idea. Now that we are on this track, how about plaster of paris/ gypsum ?

The great thing about those is that I may actually have some of that somewhere.
I am trying to keep the cost of this managable since I am doing it as a favour for someone.

Best regards
X

 

[D.A.(Tony)Stewart]

Tile grout! That is a really good idea. Now that we are on this track, how about plaster of paris/ gypsum ?

The great thing about those is that I may actually have some of that somewhere.
I am trying to keep the cost of this managable since I am doing it as a favour for someone.

Best regards
X

Fine-grained plaster might shrink and detach more than tile grout. Moisture content must be low to survive and that might be a challenge.

You do it trial and error but it all depends how many watts are dumped into these wires and the junction temp. Contact resistance and top surface flatness is another challenge. I might expect 5 Watts and 250 'C but who knows.

Much like JB weld, it shows it to be a good insulator inspite of ceramic and metallic fibres.

https://www.cotronics.com/vo/cotr/pdf/08%20-%204700%204703.pdf

 

[dick_freebird]

High moisture is not your friend here, and portland cement binds plenty of that, and will let it go when hot, ruining its adhesion and maybe more corrosion.

Frit is mineral-fluxed glass and aside from the borax's bound water (which will drive off) will "freeze" really chemically stable. I think you want this.

I did not pick up what the range of frit working temps is and some glasses (esp. w/ lower MP admix) could be brass-safe. Use in IC packaging means seal temps below aluminum and gold.
Low heat has got to be an industrial process interest I'd think.

 

[userx2]

High moisture is not your friend here, and portland cement binds plenty of that, and will let it go when hot, ruining its adhesion and maybe more corrosion.

Frit is mineral-fluxed glass and aside from the borax's bound water (which will drive off) will "freeze" really chemically stable. I think you want this.

I did not pick up what the range of frit working temps is and some glasses (esp. w/ lower MP admix) could be brass-safe. Use in IC packaging means seal temps below aluminum and gold.
Low heat has got to be an industrial process interest I'd think.

I understand, but not sure where to get or how to make that type of stuff (yet).

 

[dick_freebird]

Got another engineer magazine and found the high temp
adhesives vendor I was recollecting:

Cotronics Corp
www.cotronics.com

See some pics in the ad whih look like the problem at hand
(potting resistor coils into alumina or porcelain body). Up to
4000F, on certain products, "maintains resistance and dielectric
strength up to 2800F" ("RESBOND 920" product).

 

[userx2]

Got another engineer magazine and found the high temp
adhesives vendor I was recollecting:

Cotronics Corp
www.cotronics.com

See some pics in the ad whih look like the problem at hand
(potting resistor coils into alumina or porcelain body). Up to
4000F, on certain products, "maintains resistance and dielectric
strength up to 2800F" ("RESBOND 920" product).

Hi, thank you so much for remembering this thread.
That is great info.

Best regards
X