Type M Cracked Channel Position Pots


 

I acquired a Type M plug-in (because I am obsessed with having a 4 channel plug-in for every scope I own that can have one) and it turns out that the shaft couplings for the channel position pots have mostly cracked from age so that the shafts and their knobs turn freely without turning the pot. This is similar to the failure of other shaft couplings in newer instruments, especially where the shaft coupling has been made of plastic, but this coupling is directly part of the pot itself.

I'm going to try removing one of the affected pots and see if I can epoxy the coupling back together. There's plenty of space around the coupling for epoxy, but I'm still worried that the epoxy won't hold when the set screw is tightened. Has anyone else fixed this failure on a Type M plug-in?

-- Jeff Dutky


Dave Wise
 

What shaft couplings, Jeff? Did you mean “gain pots” instead of “position pots”? If so, it’s been cussed and discussed for years, with at least a couple of inventive solutions in the archive. I’d try “tek-made pot” for a search string.

Regards,
Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jeff Dutky via groups.io
Sent: Saturday, August 14, 2021 10:12 PM
To: TekScopes@groups.io
Subject: [TekScopes] Type M Cracked Channel Position Pots

I acquired a Type M plug-in (because I am obsessed with having a 4 channel plug-in for every scope I own that can have one) and it turns out that the shaft couplings for the channel position pots have mostly cracked from age so that the shafts and their knobs turn freely without turning the pot. This is similar to the failure of other shaft couplings in newer instruments, especially where the shaft coupling has been made of plastic, but this coupling is directly part of the pot itself.

I'm going to try removing one of the affected pots and see if I can epoxy the coupling back together. There's plenty of space around the coupling for epoxy, but I'm still worried that the epoxy won't hold when the set screw is tightened. Has anyone else fixed this failure on a Type M plug-in?

-- Jeff Dutky


Ananda
 

After usg epoxy, can you put a sleeve on the outside with a hole for the screw? You can get some thin gage tubing for this.


 

Dave,

You are right, my mistake: it's the variable gain pots. I will try the search that you suggest ("tek-made pots")

Thanks

-- Jeff Dutky


Dave Wise
 

“adesilva” came close to what I think is the best fix: a metal ring that slips over – or replaces – the plastic cross, drill and tap a hole through it, and run in a new long setscrew. Not my invention, I just like it. I’ll try it when one of my pots becomes difficult.

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jeff Dutky via groups.io
Sent: Sunday, August 15, 2021 10:35 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type M Cracked Channel Position Pots

Dave,

You are right, my mistake: it's the variable gain pots. I will try the search that you suggest ("tek-made pots")

Thanks

-- Jeff Dutky


Harvey White
 

Be careful on which controls you use this.  Some of those control bodies are at elevated voltages.  Likely, though, TEK replaced the pot shaft with a non-conductive fiber one.

Harvey

On 8/15/2021 2:47 PM, Dave Wise wrote:
“adesilva” came close to what I think is the best fix: a metal ring that slips over – or replaces – the plastic cross, drill and tap a hole through it, and run in a new long setscrew. Not my invention, I just like it. I’ll try it when one of my pots becomes difficult.

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jeff Dutky via groups.io
Sent: Sunday, August 15, 2021 10:35 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type M Cracked Channel Position Pots

Dave,

You are right, my mistake: it's the variable gain pots. I will try the search that you suggest ("tek-made pots")

Thanks

-- Jeff Dutky







 

Dave,

Yes, adesilva’s solution came right up once I had the right search terms. I’m considering my options for what I can manufacture. The rings that adesilva made were quite beefy; I’m wondering if a metal band, or even some wire, would do as good a job.

— Jeff Dutky


 

Harvey,

These controls have steel shafts, so I doubt that there are any dangerous voltages involved.

I also had an idea that I could simply epoxy half of a shaft coupler on top of the cracked plastic cap, since there is plenty of space in this portion of the Type M plug-in.

— Jeff Dutky


Harvey White
 

If you can find a bit of thick wall tubing, ID to fit the shaft, then for a non-flexible coupling, you can just drill and tap after cutting off a piece.  For flexible couplings, perhaps a hobby shop for boat propellers might be useful.  I'm assuming you don't have a lathe.

(If you ever want one, there are some good lists around, and I do have one, so can give you recommendations.  Won't be cheap, of course, they never are.

Harvey

On 8/15/2021 3:54 PM, Jeff Dutky wrote:
Harvey,

These controls have steel shafts, so I doubt that there are any dangerous voltages involved.

I also had an idea that I could simply epoxy half of a shaft coupler on top of the cracked plastic cap, since there is plenty of space in this portion of the Type M plug-in.

— Jeff Dutky





Jim Adney
 

I don't think there's any kind of adhesive that will stick to that plastic. Here's a photo of my solution to this unfortunately common problem:

https://groups.io/g/TekScopes/photo/262268/3196738 (see also the next photo to the right)

I make these in batches. I've used a few and sold a few. They work great and are a permanent solution, but I hear that it's a tight squeeze getting them into place in some modules. They aren't crowded once in place.

Let me know if you are interested and I'll see if I have any left. Otherwise, I'll have to make more.


Dave Wise
 

Adney, Adney, that’s the guy I was thinking of. Thank you, Jim, that ring is a freaking work of art. We should call them Adney Rings.

Dave Wise

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jim Adney via groups.io
Sent: Sunday, August 15, 2021 1:51 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type M Cracked Channel Position Pots

I don't think there's any kind of adhesive that will stick to that plastic. Here's a photo of my solution to this unfortunately common problem:

https://groups.io/g/TekScopes/photo/262268/3196738<https://groups.io/g/TekScopes/photo/262268/3196738> (see also the next photo to the right)

I make these in batches. I've used a few and sold a few. They work great and are a permanent solution, but I hear that it's a tight squeeze getting them into place in some modules. They aren't crowded once in place.

Let me know if you are interested and I'll see if I have any left. Otherwise, I'll have to make more.


Ananda
 

Jeff,
I did not make them. It just came to my mind. And to think that someone else has already done it!!!!
I am sure one can think of many alternatives for this sort of thing. Isn't that why we are engineers 🤔


 

My mistake. I seem to be operating at less than full capacity this weekend.


 

Dave,

Any idea what the plastic was that Tek used for these parts? I'd like to investigate what solvents might work on it, as I have access to a small supply of (now) exotic solvents that I have previously used to mend plastics that were resistant to gluing.

-- Jeff Dutky


Dave Wise
 

It’s the merest naïve speculation: nylon?

From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Jeff Dutky via groups.io
Sent: Sunday, August 15, 2021 8:19 PM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type M Cracked Channel Position Pots

Dave,

Any idea what the plastic was that Tek used for these parts? I'd like to investigate what solvents might work on it, as I have access to a small supply of (now) exotic solvents that I have previously used to mend plastics that were resistant to gluing.

-- Jeff Dutky


snapdiode
 

Most of the plastic parts Tek used were made of Delrin. Impossible to glue together with anything I can find locally. Delrin has very low surface energy so it needs to be activated. https://en.wikipedia.org/wiki/Polyoxymethylene


 

snapdiode,

Delrin is what I was expecting.

I've ordered replacement NOS pots, and hopefully their plastic hasn't already started cracking. When I've got the exiting pots out of the unit I'll do some experimentation my exotic solvent. If that doesn't work I'll see what I can fabricate to mimic Dave's solution. Whatever I make will have to be smaller than the rings Dave uses; it looks like the tolerances in the Type M are pretty tight, specifically on one side of these pots.

If worse comes to worst I can simply replace them with modern pots. The only thing I will lose is the continuous turning "feature" that I actually find quite disturbing. The best solution would be to find 200-300 ohm pots with an integrated switch, which would make the VAR knobs act like those on more modern instruments, but I'll settle for just turning the knob to one end of it's range to get the calibrated setting.

It also occurred to me that I could leave the unit as-is: I rarely use the variable gain feature on any of my scopes, and the 500-series scopes that this would go in are really just show pieces / shelf queens. Having everything 100% working is not really necessary, and the VAR knobs doubly so.

-- Jeff Dutky


 

snapdiode,

after reading through the wiki page on Delrin I doubt that the solvent I have will work, but I'd still like to give it a try, in case the plastic is something other the POM.

I'm wondering if my original plan to epoxy the parts might still work, not by epoxying the broken parts together at the cracks, but by potting the entire part in a shell of epoxy. I could add plugs to the shaft and set screw holes to keep them from filling with epoxy, and hope that the epoxy itself is strong enough to hold the piece together. Alternately I could reinforce the piece with a ring of bus wire before potting it, which would relieve the epoxy of most or all of the stress from the tightened set screw.

Finally, I wonder if the pots can be disassembled, and new caps constructed of some other material.

-- Jeff Dutky


Jim Adney
 

Re: Adney Rings

I made a few of these rings several decades ago, when a friend bought a 564 with plugins whose var pots had suffered this unfortunately common failure. Since then, I've had to make a few more to repair the var pots on some of the plugins I've bought. Plus, I've sold a few. Look here for 2 photos of the items in question:

https://groups.io/g/TekScopes/photo/262268/3196738

(see also the next photo to the right)

FYI, the next time I'll use a slightly shorter setscrew, so it won't stick outside the Adney Ring as far.

I've made a couple batches of these. I've used a few and sold a few. They work great and are a permanent solution, but I hear that it's a tight squeeze getting them into place in some plugins. They aren't crowded once in place.

I finally got a quote from a local machine shop for a larger number of them. I had hoped to be able to sell them for $5 each, including the stainless steel set screw, but I won't be able to meet that price unless I can sell quite a few, over 100. To make this feasible, I'd like to sell them in groups of at least 5, or preferably 10, for $5 each plus $8 shipping.

(BTW, I tried getting a quote from emachineshop.com, but their price was absurd. My local shop will do much better.)

Can I get a show of hands from anyone willing to buy some (how many?) If you work on these old plugins, it would be worthwhile to keep a few on hand. For myself, I've always hated seeing my last one get used. Or worse, when I'm down to my last one, and I need two. 8-p

And, to Dave Wise, thanks for the compliment. "Adney Rings" sounds good to me. I like it! ;-)


 

Jim,

I'm in for a batch of ten. I've got other 500-series plug-ins with the same kind of pots. They're not currently showing any signs of cracking, but best to have and not need than to need and not have.

Just to be clear, a batch of ten Adney rings (shipping included) would be $58?

-- Jeff Dutky