1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?


PMF
 

I did a quick search and found some cryptic comments in one or two instances of someone using a particular glue or epoxy to repair the above. Alas, the correspondent made no mention what he was using, and my attempts to use JB Weld and then Ultragel Superglue both met with failure.

I had gone through one set of messages that initially thought the material used was Nylon until heat tests disproved that theory. Others mentioned a chemical scoring or scouring of the surface so that whatever epoxy was ultimately utilized would adhere. Last thing to mention is that because of the structure of the bracket holding this potentiometer, there is no room for anything wider than the diameter of 1/2" PEX-b to be used as a support structure around that control cross - should a suitable adhesive be named. This rules out a metal ring with a set screw so that it could hold the control shaft in place of the broken plastic cross.

After all of this hassle, I also wonder if anyone has come up with an alternative potentiometer for this? It would be a daunting task because of the position of this control and its several functions via the Variable knob and shaft, but if no adhesive will hold, is there an alternate potentiometer?

PMF


Dave Wise
 

How about a spring-type hose clamp? Cut down to height, drill a hole to access the setscrew in the original plastic, place it, cut off the tabs, and cover with JB to lock it. Amazon sells a baggie for less than $10, you pick the diameter. I’d say 11mm (0.43 inch) or maybe 10mm (0.39 inch). Search for “spring hose clamp”, or here are a couple of links:

https://www.amazon.com/Fawcotu-Spring-Manganese-Clamps-Fastener/dp/B09MQMHYLY/ref=sr_1_10
https://www.amazon.com/Pressure-Clamp-Spring-Silicone-Vacuum/dp/B09KTHSJ78/ref=sr_1_8

FWIW,
Dave Wise

From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of PMF via groups.io
Sent: Tuesday, May 17, 2022 1:51 PM
To: TekScopes@groups.io
Subject: [TekScopes] 1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?

I did a quick search and found some cryptic comments in one or two instances of someone using a particular glue or epoxy to repair the above. Alas, the correspondent made no mention what he was using, and my attempts to use JB Weld and then Ultragel Superglue both met with failure.

I had gone through one set of messages that initially thought the material used was Nylon until heat tests disproved that theory. Others mentioned a chemical scoring or scouring of the surface so that whatever epoxy was ultimately utilized would adhere. Last thing to mention is that because of the structure of the bracket holding this potentiometer, there is no room for anything wider than the diameter of 1/2" PEX-b to be used as a support structure around that control cross - should a suitable adhesive be named. This rules out a metal ring with a set screw so that it could hold the control shaft in place of the broken plastic cross.

After all of this hassle, I also wonder if anyone has come up with an alternative potentiometer for this? It would be a daunting task because of the position of this control and its several functions via the Variable knob and shaft, but if no adhesive will hold, is there an alternate potentiometer?

PMF


Joel B Walker
 

You may have to search here a little harder. Or possibly on the Antique Radio Forum. I remember reading a post about this problem and the author made or jadd someone make brass or steel rings that were a very tight push fit over the cross section of the rotor on the pot. A hole was provided for the set screw to be accessed. They were very pleased with the results. I understood that the author or his source was going to make them for sale.


 

PMF,

I have several Type M plug-ins with the same problem of Tek-made wire-wound pots cracking where the shaft is secured in the cross-shaped rotor. I have not managed to fix these yet. The "best" solution advised by the group were machined metal rings that slip over the rotor and hold the cross-shaped pieces together. These have been referred to as "Adney Rings" in honor of Jim Adney. He was talking about making up a new batch, but needed to have a minimum quantity to order. I have not heard more about that in several months.

The reason that you can't glue these things is that they are made from a particularly impervious plastic called "Delrin" that has some desirable properties (easy to machine, maintains dimensions). Sadly, the plastic does not hold up to continuous mild stress over half a dozen decades. Either the plastic has become brittle with age, or the sharp corners where the arms of the cross meet was a serious design flaw (or both). Maybe some of the set screws were over-tightened too.

In the absence of Adney Rings I have considered just wrapping the cross in a couple loops of bus wire and twisting the ends together to tighten it. I was considering filing a small groove in the ends of the cross arms to hold the bus wire in place. I have not tried this yet.

Here is a recent thread on this topic that covers the ground pretty thoroughly: https://groups.io/g/TekScopes/topic/84897039#186726

-- Jeff Dutky


 

PMF,

If I did not make it clear in my message, you are probably not going to be able to effectively glue Delrin -- basically nothing sticks to Delrin -- so a mechanical solution is the best way to go. The Adney rings would be flush with the circular base of the rotor, so they would not interfere with the mechanics around the pot. The bus wire approach would be even less likely to interfere, so long as you bent the twisted ends out of the way.

Instead of metal Adney rings, you might be able to make something similar from PVC tubing, or cast something out of epoxy that would fit over the cross-shaped end of the rotor and hold it together.

-- Jeff Dutky


Jim Adney
 

I came up with the solution shown here and in the next photo, which I think is small enough to work anywhere.

https://groups.io/g/TekScopes/photo/262268/3196738?p=created%2C%2Cadney%2C20%2C1%2C0%2C0

The ring is threaded, so this repair does not depend on the plastic threads to still be good.

The setscrew shown worked for me, but a shorter one would probably be better, if your clearances are tight.

I wanted to have a batch made, but in my mind I set a price limit of $5 each, including the setscrew. All the quotes I got back would have priced them well above my price point, even for a batch of 200, so I haven't done anything more with this. Too many other projects to contend with. I have one other place I should try for a more reasonable price; I just have to do that.

If people are willing to spend $8-10 each for these, let me know, but I think that's an unreasonable price. Let me know if I'm wrong. Or if I can justify buying more, that would make a difference, but I was barely able to get commitments for 100 here earlier.


Joel B Walker
 

This is the solution I remembered!


Joel B Walker
 

Would it be inappropriate for me to ask the dimensions of the Adney ring?


bill koski
 

Great solution!

I'm throwing out a few suggestions for other future repairs.
I've wrapped cracked barrels on knobs and couplings tightly with non-stretch fishing line like Fireline and you can add a coat of JB over it as well.

3M makes a line of automotive products called Super Fast Plastic Repair. Comes in a couple different formulations depending on what you are trying to repair.
A couple draw backs are that it's expensive (about 50 bucks for a cartridge) and it's a 2 part system and you need a 2 piston cartridge gun to use it. I used a large caulk gun and a pusher made out of a piece of 1x with 2 large dowels attached to it. It's pretty well set in about a minute and I think reached full hardness in about an hour but you can speed that up by adding some heat.

JB makes a 2 part plastic repair that comes in a smaller disposable 2 piston syringe . Haven't tried that myself but it might be handy for some repairs. I'm guessing that it's similar to one of the 3M products but it appears to have a longer set time. The 3M stuff sets really fast.
JB also has a 2 part plastic repair putty. I also have not tried that but putting it out there for consideration


Dave Wise
 

Unlike Type CA, there’s plenty of radial clearance around the 1A4 pots, so the Adney ring should work there.

Dave Wise

From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Jim Adney via groups.io
Sent: Wednesday, May 18, 2022 5:14 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?

I came up with the solution shown here and in the next photo, which I think is small enough to work anywhere.

https://groups.io/g/TekScopes/photo/262268/3196738?p=created%2C%2Cadney%2C20%2C1%2C0%2C0<https://groups.io/g/TekScopes/photo/262268/3196738?p=created%2C%2Cadney%2C20%2C1%2C0%2C0>

The ring is threaded, so this repair does not depend on the plastic threads to still be good.

The setscrew shown worked for me, but a shorter one would probably be better, if your clearances are tight.

I wanted to have a batch made, but in my mind I set a price limit of $5 each, including the setscrew. All the quotes I got back would have priced them well above my price point, even for a batch of 200, so I haven't done anything more with this. Too many other projects to contend with. I have one other place I should try for a more reasonable price; I just have to do that.

If people are willing to spend $8-10 each for these, let me know, but I think that's an unreasonable price. Let me know if I'm wrong. Or if I can justify buying more, that would make a difference, but I was barely able to get commitments for 100 here earlier.


PMF
 

Thanks for all of the suggestions. While I hadn't known the name, the Adney Rings had inspired my (feeble) attempt to surround the cross of the Variable control pot with a ring made of PEX-b that I had mentioned in the OP.

After reading up on Delrin, I have to wonder if even the dear old Loctite 401 + 770 primer would suffice, but as the bracket in the 1A4 is so much closer to the pot than the illustration of the set-up that Mr. Adney had used, I might have no choice. (The bracket consists of two 'fingers' in close proximity to the potentiometer. These hold it in place and allow it to move back and forth on the shaft that activates the 'Invert' slide switch, which is behind this potentiometer. The clearance between these and the pot would admit a ring that adds approximately an eighth of an inch to the overall diameter of the control but no more, and the set screw cannot extend beyond this ring without hitting these 'fingers.')

On the other hand, the stability of a thinner version of the Adney Ring is most desirable because of the instability of the bloody Delrin.

PMF


Dave Voorhis
 

On 18 May 2022, at 13:14, Jim Adney <jadney@...> wrote:

I came up with the solution shown here and in the next photo, which I think is small enough to work anywhere.

https://groups.io/g/TekScopes/photo/262268/3196738?p=created%2C%2Cadney%2C20%2C1%2C0%2C0

The ring is threaded, so this repair does not depend on the plastic threads to still be good.

The setscrew shown worked for me, but a shorter one would probably be better, if your clearances are tight.

I wanted to have a batch made, but in my mind I set a price limit of $5 each, including the setscrew. All the quotes I got back would have priced them well above my price point, even for a batch of 200, so I haven't done anything more with this. Too many other projects to contend with. I have one other place I should try for a more reasonable price; I just have to do that.

If people are willing to spend $8-10 each for these, let me know, but I think that's an unreasonable price. Let me know if I'm wrong. Or if I can justify buying more, that would make a difference, but I was barely able to get commitments for 100 here earlier.
I think $8 - $10 USD is a reasonable price, given it’s a reasonable solution without a reasonable alternative.

Put me down for 10 of them if and when you get them, plus whatever it costs to ship to the UK.

I only really need one at the moment, but I like to be prepared, and the original Tek plastic bits are only getting more likely to crack with age.


n4buq
 

Out of curiosity, what are the ID and OD dimensions for that ring?

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Voorhis" <voorhis@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Wednesday, May 18, 2022 2:32:02 PM
Subject: Re: [TekScopes] 1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?
On 18 May 2022, at 13:14, Jim Adney <jadney@...> wrote:

I came up with the solution shown here and in the next photo, which I think is
small enough to work anywhere.

https://groups.io/g/TekScopes/photo/262268/3196738?p=created%2C%2Cadney%2C20%2C1%2C0%2C0

The ring is threaded, so this repair does not depend on the plastic threads to
still be good.

The setscrew shown worked for me, but a shorter one would probably be better, if
your clearances are tight.

I wanted to have a batch made, but in my mind I set a price limit of $5 each,
including the setscrew. All the quotes I got back would have priced them well
above my price point, even for a batch of 200, so I haven't done anything more
with this. Too many other projects to contend with. I have one other place I
should try for a more reasonable price; I just have to do that.

If people are willing to spend $8-10 each for these, let me know, but I think
that's an unreasonable price. Let me know if I'm wrong. Or if I can justify
buying more, that would make a difference, but I was barely able to get
commitments for 100 here earlier.
I think $8 - $10 USD is a reasonable price, given it’s a reasonable solution
without a reasonable alternative.

Put me down for 10 of them if and when you get them, plus whatever it costs to
ship to the UK.

I only really need one at the moment, but I like to be prepared, and the
original Tek plastic bits are only getting more likely to crack with age.




Shaun M
 

Another post failure solution:

Checking one of my CA units showed that sometime in the past someone experienced this problem and epoxied the metal shaft to the ID of the Delrin rotor. One of the arms of the Delrin rotor (the threaded one) is missing so I can easily see the clear epoxy used to make the repair. It has worked fine for several years now.

Shaun M


Dave Wise
 

I did the same thing, so another type plugin, I forget which. Just a blob of JB adhering the shaft to the pot and hope I never have to remove it. I would do it differently now.

Dave Wise

From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Shaun M via groups.io
Sent: Wednesday, May 18, 2022 2:25 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] 1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?

Another post failure solution:

Checking one of my CA units showed that sometime in the past someone experienced this problem and epoxied the metal shaft to the ID of the Delrin rotor. One of the arms of the Delrin rotor (the threaded one) is missing so I can easily see the clear epoxy used to make the repair. It has worked fine for several years now.

Shaun M


Jim Adney
 

On Wed, May 18, 2022 at 03:33 PM, n4buq wrote:

Out of curiosity, what are the ID and OD dimensions for that ring?
https://groups.io/g/TekScopes/photo/262268/3196738

I don't have the exact dimensions handy, but they come directly from the Tek pot. The Adney ring in front of me looks to measure ~7/16" ID, ~11/16" OD, and ~3/16" wide. Don't use those measurements to make one, measure your pot and make it to fit exactly.

If I can find someone to make a run of these at a reasonable price, I will include a shorter SST setscrew that won't protrude above the OD of the ring.
I realize that $10 may not seem like much if you just need one, to fix one plugin, but for those of us who have large plugin collections, that starts to get pricey.


n4buq
 

I was asking because rings like this are available for a very reasonable cost. If 7/16" is accurate, then off-the-shelf rings might work; otherwise, an off-the-shelf ring could be easily modified.

https://www.mcmaster.com/shaft-collars/shaft-collars/shaft-collars-6/for-shaft-diameter~7-16/

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "jadney" <jadney@...>
To: "tekscopes" <TekScopes@groups.io>
Sent: Thursday, May 19, 2022 9:29:07 AM
Subject: Re: [TekScopes] 1A4 Cracked Variable Potentiometer Repair - Which Epoxy to Use?
On Wed, May 18, 2022 at 03:33 PM, n4buq wrote:

Out of curiosity, what are the ID and OD dimensions for that ring?
https://groups.io/g/TekScopes/photo/262268/3196738

I don't have the exact dimensions handy, but they come directly from the Tek
pot. The Adney ring in front of me looks to measure ~7/16" ID, ~11/16" OD, and
~3/16" wide. Don't use those measurements to make one, measure your pot and
make it to fit exactly.

If I can find someone to make a run of these at a reasonable price, I will
include a shorter SST setscrew that won't protrude above the OD of the ring.
I realize that $10 may not seem like much if you just need one, to fix one
plugin, but for those of us who have large plugin collections, that starts to
get pricey.



Jim Adney
 

On Thu, May 19, 2022 at 09:48 AM, n4buq wrote:

I was asking because rings like this are available for a very reasonable cost.
If 7/16" is accurate, then off-the-shelf rings might work; otherwise, an
off-the-shelf ring could be easily modified.

https://www.mcmaster.com/shaft-collars/shaft-collars/shaft-collars-6/for-shaft-diameter~7-16/
I've looked at McMaster as well as the stock offerings from Ruland and Stafford but they are all much larger in OD, width and setscrew size. Certainly one could be modified, but that would actually be more work than starting from bar stock, and it would not end up being as nice.


c n
 

On Tue, May 17, 2022 at 01:51 PM, PMF wrote:


I did a quick search and found some cryptic comments in one or two instances
of someone using a particular glue or epoxy to repair the above. Alas, the
correspondent made no mention what he was using, and my attempts to use JB
Weld and then Ultragel Superglue both met with failure.

I haven't tried it with Delrin but JBWeld Plastic Bonder (not Weld!) works well with almost all the plastics I have tried it on. It is a two part urethane used to repair bumper covers. It comes in a cream color and very dark gray - nearly black.
JBWeld Plastic Weld is an epoxy with which I have had very little success.
Being the same brand they are easy to confuse.

--
Chuck N.
547 453 475


stevenhorii
 

I have used Master Bond epoxies in the past with excellent results. They
are more expensive than the typical epoxies you find in the big box
hardware stores. I used one of their epoxies to repair the plastic handles
of a pair of scissors. The break resulted in a roughened but fairly small
cross-sectional area. I didn’t have much hope for gluing them, so I tried
one of the Master Bond high-strength epoxies (note that it was not a
fast-cure epoxy). The scissor handles are still working fine after several
years of post-repair use and have stood up to the pressure of cutting
thicker card stock or multiple sheets of paper. The Master Bond folks have
some epoxies designed for acetal plastics like Delrin (to be honest, I
thought it was a fluoropolymer given its kind of slippery feel) though they
recommend a surface treatment first:

https://www.masterbond.com/applications/bonding-acetal-plastics-industrial-adhesives#:~:text=Master%20Bond%20formulates%20epoxy%20adhesives,Delrin%C2%AE%20to%20dissimilar%20substrates
.

I have no CoI with Master Bond.

Steve H.

On Fri, May 20, 2022 at 10:48 c n via groups.io <shermantecumseh=
yahoo.com@groups.io> wrote:

On Tue, May 17, 2022 at 01:51 PM, PMF wrote:


I did a quick search and found some cryptic comments in one or two
instances
of someone using a particular glue or epoxy to repair the above. Alas,
the
correspondent made no mention what he was using, and my attempts to use
JB
Weld and then Ultragel Superglue both met with failure.

I haven't tried it with Delrin but JBWeld Plastic Bonder (not Weld!) works
well with almost all the plastics I have tried it on. It is a two part
urethane used to repair bumper covers. It comes in a cream color and very
dark gray - nearly black.
JBWeld Plastic Weld is an epoxy with which I have had very little success.
Being the same brand they are easy to confuse.

--
Chuck N.
547 453 475