How to fix old Lexmark Ambra model M keyboard

[khagan]

Hi everyone,

I stumbled upon an old model M keyboard. It has PS/2 connector and some keys missing, and doesn't seem to work. I wasn't able to find screw driver for it so far; I think it says "25" on screw's face.
I am complete newb to keyboards, so I was wondering how would I go about potentially fixing it?

Here is the link to pics: https://imgur.com/a/4Pt2yKf

I found that Unicomp offers repair services; but fixing it myself would be awesome  ?

[Findecanor]

You would need a 1/4" nut driver to open it, to examine the insides.
Hopefully, some connector is only loose. A common fault (with old electronics in general) is that a capacitor has gone bad and would need to be replaced.
It is also possible that many plastic "rivets" holding the keyboard sandwich together have broken off: if they are too many, you could replace them with bolts.

I'd think that many people would have some spare keys for grey-and-white Model Ms laying around. Apparently, you'd need only one: the grey numpad-Enter key. It covers two positions.
(There are also all-white Ambra Model M keyboards, and a spare keycap for one of those would have been harder to find.)

[yui]

You will need a 5.5mm nut driver to open it (the screws on the back are 5.5mm).
After that depending on its issues unicomp sells new parts and a bolt mod might be all that is needed (seems scary, but it is pretty easy) as the plastic rivets holding it together break fairly easily with old age and If too many are broken keys do not register.
As for the caps if you are in the US finding them will be easy, if you are mostly anywhere else that can be a bit more expensive.
If you are in France(or do not mind paying part of the shipping, anywhere really) i have some that came from a unicomp 122 keys but those are gray, and some are broken.

[Findecanor]

You will need a 5.5mm nut driver to open it (the screws on the back are 5.5mm).

BTW. "1/4" hex nut" and "5.5mm hex nut" are the same thing. For some reason, Imperial is measured from corner to corner while Metric is measured from flat to flat ... and the size of the bolt shaft is something else entirely.

My nut driver has only 5.5mm engraved on it, but its packaging had 1/4" printed and not the Metric measurement.

[yui]

I did write at the same time as you, and not knowing which of imperial or metric was more useful i did keep it in my answer, i did not mean to undermine you or anything, i just find that the more information the better.

[Findecanor]

I did write at the same time as you, and not knowing which of imperial or metric was more useful i did keep it in my answer, i did not mean to undermine you or anything, i just find that the more information the better.

I understood that. It happens all the time. No offence taken.
I posted for Khagan's sake, because it is such an odd system that could be confusing, and because it is a curiosity.

[fohat.digs]

"1/4" hex nut" and "5.5mm hex nut" are the same thing.

No.

The correct size is 7/32" and 5.5mm is very slightly larger and works well as a substitute.

Be sure that the socket is "long" and "thin-walled"

[khagan]

So I was able to open case using slim plyer. here are pics https://imgur.com/a/FY8vhYC
When I touched the chip while it was plugged, I noticed it was warm so I think it is working.
How do I test if membrane circuity is working without splitting inner base? Still, no keys have been registered on my computer when plugged to PS/2 port.

[yui]

I do not remember my 122 getting warm, but then those are unicomps so might be different.
From the photos it looks like you have most of the rivets intact so if it is not the membrane no need for a bolt mod (hopefully).
Have you checked if your ps/2 ports are active (they are not by default at least on windows 8, i don't know about 10).

[khagan]

So I was able to get multimeter and checked that PS/2 connector was working. As last attempt I opened the inner base, I checked membrane circuitry for continuity and it was making connection but the exposed part of membrane that attaches to the chip board did not transfer current. So I think I need to replace membrane plastic.

[yui]

When i had the issue of PS/2 ports disabled the keyboard was getting power just windows was ignoring all that came from PS/2 (driver disabled) it still worked in the bios, failing that i guess you could check your membrane with the multi-meter. i suppose you could find schematics of the switch matrix online (here is one : https://deskthority.net/viewtopic.php?t=8149)

[khagan]

I checked membrane with multimeter and there was signal everywhere expect the tips that covered with additional layer do not send signal to multimeter. Is it possible to cut a bit of tips and expose connection lines inside the plastic? I am guessing I need something like sand paper to expose membrane circuits that are coated with plastic layer from both sides.

[pabile]

im at work and could not see photos you posted so....

anyways, did you try to plug the keyboard to the computer while it is off then turn computer on?

[khagan]

So I tried exposing plastic-sealed circuits by sanding but it actually damages both the plastic and circuit below it. Is there a way to safely expose circuits underneath plastic? Or is my last bet is to solder wire on top of it?

[yui]

I actually do not understand what you are trying to do? the traces are all exposed where they plug in the pcb and are exposed at each pad if you open the membranes (there is a sandwich of 3) and really either the membrane is completely dead (no keys making any kind of continuity) or the controller is as your keyboard does not respond it seems, if you are in the US or willing to pay for fedex, unicomp carries both (20 usd for the controller and 10 for the membrane + shipping if i remember rightly).

[khagan]

Here is the picture of membrane connector to the bus https://imgur.com/aNkcwWV
The black tips do not transfer any signal and traces leading up to them are coated in plastic so even if I cut the edge, the traces will not be exposed

[yui]

the black stuff should be carbon and be conductive, it is meant to help to get a better connection, if you have nothing there it might be that there is something that got spilled on it or that your multi-meter does not like the relatively high resistance it gives.