Broken button on DEC PDP-11/04 KeyPad

[intabits]

So this ham-fisted clown was cleaning the front panel keypad of my 11/04, and not knowing how fragile they were, broke one of the buttons off.

The buttons are part of the frame, all being a single piece of plastic, but they are joined to the frame body only by a thin bridge at the top of the button, which flexes when the button is pressed.

(There seems to be no benefit to removing the screws, as the switch matrix pcb appears to be glued to the frame)

I've made an unintrusive repair by using a thin wad of paper to support the key to be flush with the frame like all the other keys, and joined it to the frame using a small piece of duct tape, then removed the paper.

(top-right button)

It appears to work well, but I expect that at some time the tape will lift, and a more permanent repair will be required.
I was thinking of finding some membrane (suggestions?) to replace the tape, that can be glued using an acetone/solvent adhesive that will "weld" into the plastic.

Does anyone here know of a permanent repair technique for this damage, or have any good suggestions?

 

[1944GPW]

Since you can't get to the underside of the button frame, I would drill a row of sub-millimetre holes along the top of the button piece and also along the frame piece.
Then prepare some plastic rod to fir those holes by heating a piece of styrene sprue over a candle and pulling. Select the sprue sections that match the drilled diameter. Cut into lengths.
Cut a square piece of flexible membrane plastic and drill the same hole pattern. Glue the pins into the button first, add membrane, use point heat source (heated tip of paperclip wire) to rivet over the tops of the sprue pins.
Then place the button in and do the same with the frame. You will see welded plastic rivet pin and posts construction technique in things like laser printer keypads and the like.
If the black frame can be removed from the board matrix I would drill and bind with Kevlar thread instead.

 

[bobaboba]

Saved in my hints and tips file in case it’s handy sometime.

 

[intabits]

Since you can't get to the underside of the button frame, I would drill a row of sub-millimetre holes along the top of the button piece and also along the frame piece.
Then prepare some plastic rod to fir those holes by heating a piece of styrene sprue over a candle and pulling. Select the sprue sections that match the drilled diameter. Cut into lengths.
Cut a square piece of flexible membrane plastic and drill the same hole pattern. Glue the pins into the button first, add membrane, use point heat source (heated tip of paperclip wire) to rivet over the tops of the sprue pins.
Then place the button in and do the same with the frame. You will see welded plastic rivet pin and posts construction technique in things like laser printer keypads and the like.
If the black frame can be removed from the board matrix I would drill and bind with Kevlar thread instead.

Thanks for the reply. I think I understand what you mean there, though it does seem rather involved (and somewhat fiddly).

The solution I've come up with for the moment with the duct tape is perfectly adequate functionally - it's just a question of longevity.
All that's required is a small sheet of stuff like the duct tape piece I've used, that won't come unstuck.
I'll leave it as is for now to find out how long it actually does hold up, but want to be prepared for the time when it must be improved.

You don't mention a possible membrane substance, but using styrene for the pins makes me wonder about using that. Hobby shops sell sheets of it, but I'm not sure how thin it goes.
The deflection of the buttons is no more than 2mm on the bottom side, so a thin piece of styrene at the top side, where the duct tape is now, should work.
If the plastic used for the keypad assembly is itself styrene, or at least "weldable" with acetone/solvent glues like styrene is, that could be a winner.
I'll test the plastic in an unobtrusive place, and investigate styrene sheets.

 

[legalize]

Have you tried just supergluing the tab back to the main piece of plastic? Looks like it's a clean break with no little debris pieces.

 

[1944GPW]

You don't mention a possible membrane substance, but using styrene for the pins makes me wonder about using that. Hobby shops sell sheets of it, but I'm not sure how thin it goes.
The deflection of the buttons is no more than 2mm on the bottom side, so a thin piece of styrene at the top side, where the duct tape is now, should work.
If the plastic used for the keypad assembly is itself styrene, or at least "weldable" with acetone/solvent glues like styrene is, that could be a winner.
I'll test the plastic in an unobtrusive place, and investigate styrene sheets.

Rob, you're only "down the road a bit" in Vic so PM me your address and I'll get together a small kit of various plastics (styrene, membrane-type, sprues, Kevlar thread etc) in an envelope and post it to you. No need to spend money on these at a hobby shop as it can be quite expensive considering you only need small amounts. You're under no obligation to use any of it, but plastics of various types for modelling and engineering is a big interest of mine so I have this stuff lying around so to speak.

Definately a good idea to test with solvents to check its bondability, just along the frame edge of course to identify the type of plastic (could be ABS) as I don't think Resin Identification Codes were in use back then.

Legalize - As for supergluing a living hinge back together along the flexing edge (thin enough even without flexing), I would not think it would hold for very many actuations. It's going to need some flex backing or other repairs.

 

[intabits]

Thanks for the offer - PM sent!

I agree that a superglue bond would probably be far too brittle to withstand repeated flexing.

I've resolved to first try just a more robust version of my temporary fix, hopefully using a styrene membrane and solvent glue as outlined above...