IMB Model M Membrane Contact cleaning

[Lex]

I inherited an old IBM PS2 keyboard Model M2 (made by Lexmark). Part # 1395300, and manufactured 15-Feb-1994. I have read many guides on how to clean it, and also looked at some of the videos, but all of these seem to deal only with the keys. If I understand it, this uses the buckling spring design, rather than the capacitive circuit used on earlier keyboards.

Once disassembled, connected to the microprocessor board are two clear flexible plastic membrane sheets with circuits etched on them. And below the sheets is a removal piece of silvered reflective plastic. I have found nothing that tells me exactly how these plastic sheets function. Does pressing them together under a key create the character, and if so, how are they kept apart normally? And how are they, and the reflective plastic cleaned?

I ask because a few of the keys do not seem to create a character when pressed, and a few others repeat the character endlessly.

Thanks for any help you can provide.

Harry

[fohat.digs]

There are 2 mylar sheets with contacts on them, and a 3rd sheet with no print and holes where the contact dots are found on the other 2 sheets.

The contacts are normally separated only by the thickness of the sheet between, and the foot ("pivot plate") presses down to slightly deform the upper sheet and make momentary contact with the contact on the lower sheet.

When I take a Model M apart, I wipe them lightly with water or a weak alcohol solution.

You're welcome,
Harry

[Snowdog993]

I'm out of this conversation.

[fohat.digs]

It's an M2 Fohat.

OH, I'm an idiot. Disregard everything that I said. I don't know anything about M2s.

[Snowdog993]

I am out of this conversation.

[Lex]

There are 2 mylar sheets with contacts on them, and a 3rd sheet with no print and holes where the contact dots are found on the other 2 sheets.

The contacts are normally separated only by the thickness of the sheet between, and the foot ("pivot plate") presses down to slightly deform the upper sheet and make momentary contact with the contact on the lower sheet.

When I take a Model M apart, I wipe them lightly with water or a weak alcohol solution.

You're welcome,
Harry

Well you guys may not know anything about the M2, but mine has the same 3 mylar sheets that you mentioned. I used the weak alcohol mix to clean both sides of all three, althrough they were not very stained.

Does yours have the silver sheet below the 3 mylar sheets? If so, what is it's functon? It has some minor stains on it that I could not get out.

Harry

[fohat.digs]

Does yours have the silver sheet below the 3 mylar sheets?

I wouldn't know, I have never opened an M2 up out of fear.

In a standard M, there is a rubber mat ("blanket" in Unicomp terminology) between the bottom sheet and the steel back plate.

[Lex]

fohat:

Here is the difference between a Model M and an M2:
https://linustechtips.com/main/topic/599781-what-is-the-differnce-between-the-ibm-model-m2-and-the-model-m/

The two YouTube videos at the bottom of the column are quite informative. There is no steel lower plate in the M2, and the only rubber mat is on top of the the three mylar sheets (the spring 'hammers' rest on the rubber mat). So I have no idea why the M2 has the reflective silver sheet.

My M2 does not have the capacitor problem mentioned in the column, because most of the keys work fine.

I'll post again when I get it all reassembled and tested. Thanks for all your guidance.


Harry

[Daniel Beardsmore]

Well you guys may not know anything about the M2, but mine has the same 3 mylar sheets that you mentioned. I used the weak alcohol mix to clean both sides of all three, althrough they were not very stained.

Does yours have the silver sheet below the 3 mylar sheets? If so, what is it's functon? It has some minor stains on it that I could not get out.

Harry

Three Mylar membranes as described is the standard configuration for a membrane keyboard. There are alternatives, such as conductive pads over a single membrane sheet (Mitsumi KPQ and KPR types), and pressure pads over two membrane layers over a PCB (such as Mitsumi KSD Type). Datanetics's original design from the 60s was more complex.

The metal backplate might be to cut down on radio frequency interference (RFI) — this was a big deal at one point in the US, and you'll see it a lot in old keyboards, often as a layer of foil. Here's a nice example of one in a PCB-based keyboard: Zenith Supersport SX. These days, metal backplates seem to be all but gone.