Cracking Corners on Tektronix TM500 Plug-In Faceplates - A Reinforcement Method

[xrunner]

It is commonly known that the older style plastic faceplates on Tektronix TM500 plug-ins can crack at the corners. This is due to pressure from the four screws that attach the faceplate to the top and bottom aluminum panels. Over the span of 30+ years the constant pressure leads to plastic fatigue. It can also be caused by repairs, in that the technician may have tightened the screws too much.

My test unit will be a DC504. I obtained this a few weeks ago. It did not have any electronic issues but it did have a crack in one lower corner. I will use this to test an idea for reinforcement; either for an already cracking part, or even better, to add to one that has not cracked yet, as prevention. I will be taking the plastic faceplate completely off to try this idea. I will be adding to this thread as I go along.

[bitseeker]

Good options for repairing corners that have broken off (probably similar to fixing cracks) or replacing ones that have disappeared (e.g., used gear that was broken prior to acquisition) would also be useful.

[xrunner]

Reference pics. Removed the aluminum front panel. You can see the 4 screws that cause the cracking. With the plastic panel removed the crack this one has is clear. The countersunk screws can really dig in and tend to not only put a force downwards but also outwards, which pushes against the plastic in more than one direction.

You can see where I dabbed a little MEK on the crack once before. But I want to add a reinforcement at the back.

Edit: Added pic of newer design which they changed (DC503 in this case). Notice the screws go into aluminum from the top - no cracks.

[timb]

 I've tried both superglue and epoxy to repair corners that have cracked. The former doesn't hold, the latter will hold if it's just a hairline crack and you can reinforce it a bit (by applying the epoxy over the crack as well, however this doesn't work if the crack extends below the lip).

I've considered scanning all the plastic faceplates of my modules and then having some new ones laser cut out of acrylic. They wouldn't have the lips, but they would work.

[mtdoc]

Excellent topic! 

[xrunner]

I've tried both superglue and epoxy to repair corners that have cracked. The former doesn't hold, the latter will hold if it's just a hairline crack and you can reinforce it a bit (by applying the epoxy over the crack as well, however this doesn't work if the crack extends below the lip).

This plastic melts together with MEK. That's what I use on this type plastic and am going to use here. Be back a little later ...

[David Hess]

Plastic welding cement is the thing to use for these.  Usually this is something like a small amount of MEK in methylene chloride.  When applied, it dissolves the surface and the separate pieces rejoin.  I often add polystyrene reinforcement where it will not show.

https://plastruct.com/technical-info/

[xrunner]

Plastic welding cement is the thing to use for these.  Usually this is something like a small amount of MEK in methylene chloride.  When applied, it dissolves the surface and the separate pieces rejoin.  I often add polystyrene reinforcement where it will not show.

https://plastruct.com/technical-info/

That's where I'm heading.

[timb]

I've tried both superglue and epoxy to repair corners that have cracked. The former doesn't hold, the latter will hold if it's just a hairline crack and you can reinforce it a bit (by applying the epoxy over the crack as well, however this doesn't work if the crack extends below the lip).

This plastic melts together with MEK. That's what I use on this type plastic and am going to use here. Be back a little later ...

Yeah, I'm sure that works well if there's just a small crack. Unfortunately a lot of the older modules are missing entire corners (they get bumped or sat down hard after being removed from a mainframe). I think out of 7 of my modules only one has all the corners (and they came like that). The faceplate seems to be the weak point of the design.

[David Hess]

You can see the 4 screws that cause the cracking. With the plastic panel removed the crack this one has is clear. The countersunk screws can really dig in and tend to not only put a force downwards but also outwards, which pushes against the plastic in more than one direction.

The mechanical design does not help but the plastic Tektronix used gets brittle with age contributing to the problem.

[David Hess]

That's where I'm heading.

The material safety data sheets available at that link list the ingredients and their rough proportions.  I like their Plastruct Plastic Weld which is as I described but it is cheaper if you buy the ingredients and mix it yourself.  The stuff has an annoying tenancy to escape even from tightly capped containers.

[xrunner]

The material safety data sheets available at that link list the ingredients and their rough proportions.  I like their Plastruct Plastic Weld which is as I described but it is cheaper if you buy the ingredients and mix it yourself.  The stuff has an annoying tenancy to escape even from tightly capped containers.

Yea, yea, I have Plastruct weld here.

If you've done this type of repair on these units let us see it. Why don't you post up a pic? 

[David Hess]

If you've done this type of repair on these units let us see it. Why don't you post up a pic? 

All of my TM500 modules were either undamaged or completely missing corners when I got them so there has been nothing to repair.

Below are photographs of two different 22xx covers.  The first one had a crack extended from the edge to the top.  The second one had three crushed corners.

I do not have a real satisfactory way to buff them so further work on the covers is on hold.

[xrunner]

I do not have a real satisfactory way to buff them so further work on the covers is on hold.

Looks strong but you need a small bottle of Tektronix blue enamel, not sure where to get it though.

So here's what I tried. I used 0.01" sheet styrene for modelling. I cut out a couple of squares. Then I wetted the area with MEK and clamped it down. After a few hours I cut out the holes with an X-Acto knife. I then applied more MEK to the edges. After that it feels very strong. It's not thick enough to make a noticeable difference once everything is put back together, and I think it's a good solution for reinforcement. When put back together I could not tell anything had been done except by looking at the corners from the back.

As I said in the title - it's for reinforcement so the cracking hopefully does not happen again. If you get an older one that does not have cracks I'd do this to prevent them from appearing. If you have a cracked one, fix the crack as well as you can by applying MEK and clamping. Then apply a backer like this to help stop it from cracking again.

You could possibly do it to the front too, but I'd check fit and finish to make sure the aluminum panel does not noticeably push out too far. If anyone has a suggestion to improve this procedure please add a comment.

[David Hess]

I do not have a real satisfactory way to buff them so further work on the covers is on hold.

Looks strong but you need a small bottle of Tektronix blue enamel, not sure where to get it though.

The covers are old enough to be nonuniform so no color is going to match.  I would have to paint each entire cover and I am dubious about this being worthwhile; I might just end up with a mess and I am mostly satisfied if they will work for their original purpose.  The reinforcement on the insides is not visible anyway.

I am thinking about bead blasting them to improve their finish.

[HighVoltage]

The best glue that I have found for these kind of plastics is Acrifix 117
It does not glue the plastic together, it kind of welds it perfectly and matches the color automatically.
For some reasons this works amazingly well for all test and measurement plastics.
I have repaired endless broken scope frames, covers and so many different parts with it.

I described it here in my Agilent PSU repair thread:
https://www.eevblog.com/forum/repair/brand-new-keysight-e3649a-psu-repair-and-teardown/msg716864/#msg716864