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Model 4 membrane keyboard - anyone know how to fix it ?

TRS-Ian

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Sep 10, 2011
Messages
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Location
Melbourne, Australia
I have a membrane keyboard on a model 4 and to be honest they are a bit rubbish but I hate to throw out a whole keyboard because one or 2 key switches are non-responsive...

Has anyone had any luck fixing one of these types of keyboard and if so how is it done?

Cheers,

Ian.
 
This is a disassembled keyswitch from my Model 4:

TRS80_Model_4_Keyswitch.jpg


That particular switch had four solder points on it, so it took a little care to remove it.

If the rubber dome is intact then the problem is probably the little black disc on the underside of it. The black disk loses whatever magical properties are that make the circuit complete. Usually if you just press harder it works, but at normal typing pressures it is not enough. There are kits out there for restoring remote controls that consist of a chemical that you apply to the black disc to restore its conductive properties - find one that works, and then tell me. ; - 0

I have to do almost my entire keyboard .. that project has been on the shelf a long time because of the effort require to remove all of the key switches.
 
If those black disks are the same as the ones in the similar mechanism contained inside a Commodore PET keyboard what worked really well for me was swabbing the metal contacts clean with alcohol, dabbing the rubber disk clean with water, and finally before putting it back together *very* gently stroking the side of a sharpened pencil lead across the disk. Two keyboards that were essentially completely unresponsive after a "normal" cleaning were good as new after that.
 
That's the older style keyboard. Tandy released a newer version with the Gate Array and 4D if I remember correctly. Mine has the same as TRS-Ian does. And it a totally different beast. And I am not sure how to clean the contacts in them TRS-Ian. Good luck
 
Thanks Frank.

For people's reference, here is the keyboard chronology:

1983: Maxi-switch keyboard (see mbbrutman, above) with separate bezel for the first few thousand NGA M4s
1983-4: ALPS keyboard with separate bezel for all the rest of the NGA M4s
1984-5: membrane keyboard (so shitty the manufacturer didn't even put their name on it) with integrated bezel for all GA M4s
1985-6: membrane keyboard (ditto) with extra backspace key and integrated bezel for the first batch of M4Ds
1986- : back to ALPS keyboard with separate bezel for the rest of the M4Ds.

Here's someones video explaining how membrane keyboards work: http://www.youtube.com/watch?v=34IFo90MuoA

Looks like I'm just going to have to take one apart and see what I find...

Ian.
 
The black dot in the green plunger has no magnetic properties. It is conductive rubber.

If you get some Rubber Renue (used for resurfacing belts and rollers in VCR and audio equipment), dab a little on the "dot" with one end of a q-tip and wipe it off with the other, it will work fine. You might want to clean up those two contacts on the base with a soft pencil eraser.
 
The black dot in the green plunger has no magnetic properties. It is conductive rubber.

I don't think anybody said anything about magnetic properties. The phrase used was "conductive properties."

An application of a chemical designed to restore the conductive properties will work well. I have a container of "Keypad fix" which contains carbon and a binder in liquid form; apply it and leave it for 24 hours to dry. It's similar to the pencil lead approach above, except that the carbon won't be loose and floating around.

Is Rubber Renue conductive, or does it just make the rubber more pliable?
 
For the older style keyboard where a few keys don't want to respond try this. Take the plunger if it's still in good shape (In Mikes picture the Green part) and super glue some copper tape to the piece of carbon/conductive rubber. The glue seems to like the carbon/conductive rubber and adheres very well. Have used couple of years now this way in my older MIV and Works every time! But then I have said this before!
 

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For the older style keyboard where a few keys don't want to respond try this. Take the plunger if it's still in good shape (In Mikes picture the Green part) and super glue some copper tape to the piece of carbon/conductive rubber. The glue seems to like the carbon/conductive rubber and adheres very well. Have used couple of years now this way in my older MIV and Works every time! But then I have said this before!

I have done this mod with aluminium foil and super glue in calculators but I hadn't very lasting results.Maybe with copper works better..
 
It's similar to the pencil lead approach above, except that the carbon won't be loose and floating around

Yeah, in the case of the PET I decided I didn't care too much if the pencil lead eventually got loose because the design of the PET keyboard makes it really easy to get in there and clean/renew it. (Removing the PCB exposes all the plunger tips at once.) If you have to desolder the keyswitches and pry them all apart separately I'd recommend something more "professional".
 
Try looking at;

Caig Laboratories Part #: K-CK44-G

It works on remote controls and other
rubber type switches.

 
I don't think anybody said anything about magnetic properties. The phrase used was "conductive properties."

An application of a chemical designed to restore the conductive properties will work well. I have a container of "Keypad fix" which contains carbon and a binder in liquid form; apply it and leave it for 24 hours to dry. It's similar to the pencil lead approach above, except that the carbon won't be loose and floating around.

Is Rubber Renue conductive, or does it just make the rubber more pliable?

Sorry, I thought I saw magnetic in there somewhere.

Actually, rubber renue is a resurfacer. It takes off a thin layer of oxidized rubber and presents a new conductive surface since the rubber is conductive all the way through.
 
Here is a picture of the Model 4D membrane keyboard, from the top it looks like any other Model 4 keyboard:

membrane1.JPG

and here is what it looks like underneath:

membrane2.JPG

Ian.
 
How to disassemble and repair?

How to disassemble and repair?

Here is a picture of the Model 4D membrane keyboard, from the top it looks like any other Model 4 keyboard:

View attachment 9382

and here is what it looks like underneath:

View attachment 9383

Ian.

That's the same keyboard I have here - with a dead spacebar. For lack of any better information, I sliced all the plastic tabs off with a razor blade and diassembled it. All keys have a little coil spring sitting inside the plastic tower, so if anyone tries this: Be sure to have the keyboard sitting face down on a level surface when you pry off the phenolic back plate. Nothing looked amiss on either side of the printed membrane, but I wiped it down with a soft cotton cloth and non-residue cleaner, popped everything back together and - presto - all the keys worked.

How to hold it together is the current issue. There's not enough plastic to drill out for tap screws - not even close (the little posts project from an edge-on plastic ridge that's no wider than the pins themselves). First attempt is a little dab of plastic model cement on each pin, applied from the back. In 24 hours I'll know if bonding only to the top of the pin has any strength. If that doesn't work, I"ll fabricate some straps to go across the back.
 
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Epoxy? or maybe some strong tape? Just a guess Model glue might work if it is plastic to plastic.

Unfortunately, it's plastic to a phenolic material. The same tan stuff that cheap PCBs were made on during that period of time. The glue has to find purchase on the cut end of the pin, which I think is iffy - even with 50-odd pins.
 
Some guy told me he had success using those tiny screws which hold cassette-tapes together.

Yes, if you have a really small screw and the ability to drill an accurate hole dead center on the post there might be enough material there to grab with the threads. I don't feel that lucky, so I'm going to fabricate a few straps to go over the back. There's an enclosed area along the top and bottom edge of the keyboard frame and I can put a few screws through laterally where they'll get some bite.

The bottom really doesn't receive much of any force when typing. It gets pushed by one very thin spring on the key being depressed. The key never bears directly against it.
 
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