Key switches in vintage PC keyboard. Toxic or not?

Hi I have a vintage PC-XT keyboard, the model Keytronic KB5151 pro series. The manual of the jr model is attached, which I suppose is the same.
The manual writes:

Capacitive key switches are used with a microprocessor performing
the functions of keyswitch scanning, translating keypresses to IBM
down-up scan codes, and maintaining bidirectional serial communica-
tions with the host computer.

The problem I have is with the switches. These have a metalized film as a contact element and the two contact electrodes for each key are formed onto the PCB. The metalized film reminds me the Mylar balloons.
Now this film has become like shown in the photo. The "metal" in it seems like it has evaporated and contacts cannot be achieved. The result is that almost all of the keys do not work because contact cannot be done.

5 years ago the keyboard worked, but I then stored it into a basement which was quite cold in the winter.

I am very considered about the safety of this film material. Is that possible that it contains some kind of mercury or other toxic substance that has vaporized or deteriorated throughout the years?

I doubt it has mercury. But why don’t you just get rid of it? Do you have electronic recycling in your area?

barry said:

I doubt it has mercury. But why don’t you just get rid of it? Do you have electronic recycling in your area?

Click to expand...

Get rid of an XT keyboard? That is collective

Well, I measured these films with a continuity meter. The surface of the film that touches the pads on the PCB is non-conductive! The metal must be from the back side (the one of the plastic buttons of the keys). They seem to be capacitive indeed and the film forms the dielectric. I read about capacitive keys here https://en.wikipedia.org/wiki/Keyboard_technology
They do not say anything about toxicity...


Wikipedia states
"The movable part ends with a flat foam element about the size of an aspirin tablet, finished with aluminum foil. Opposite the switch is a PCB with the capacitor pads. When the key is pressed, the foil tightly clings to the surface of the PCB, forming a daisy chain of two capacitors between contact pads and itself separated with thin soldermask, and thus "shorting" the contact pads with an easily detectable drop of capacitive reactance between them. Usually this permits a pulse or pulse train to be sensed. Because the switch doesn't have an actual electrical contact, there is no debouncing necessary. The keys do not need to be fully pressed to be actuated, which enables some people to type faster."


So they say it is aluminum foil.
I have never seem aluminum to be "evaporated" like this

In my case there is no "thin dielectric" over the pads. The dielectric is this film on the moving side of the keys.

neazoi said:

Get rid of an XT keyboard? That is collective

Click to expand...

I doubt it. It’s a piece of non-working junk.

aluminum foil did not evaporate

it oxidized Aluminum oxide is an insulator
a few years is a cold damp basement is all it takes

they don't go bad as fast in a warm dry environment where they are used
every press of a key helps clean the contacts

I worked with IBM on these. They are capacitive and not intended to be conductive. The underlying PCB has one driven pad and one sensing pad, when the key is pressed, the disc becomes the dielectric medium and allows more signal to be passed to the sense side.

The most likely reason for failure isn't the key but contamination on the PCB itself. With care they can be dismantled and cleaned with a mild solvent or spirit alcohol. Although I've never done it, if the disc has de-metallized, I would think it could be repaired by spraying it with conductive paint or even fitting a new aluminum foil disc over the original one. Anything that increases its dielectric constant should work.

I think this type of keyboard went out of favor because of EMC problems, the length of tracks on the PCB and relatively high frequency square waves needed to ensure sufficient coupling must have created a lot of interference. The CPU was I believe an Intel 8749 (8049 with built in ROM) although it might be marked differently.

They are certainly not toxic, at least not more than any other electronic device.

Brian.

betwixt said:

I worked with IBM on these. They are capacitive and not intended to be conductive. The underlying PCB has one driven pad and one sensing pad, when the key is pressed, the disc becomes the dielectric medium and allows more signal to be passed to the sense side.

The most likely reason for failure isn't the key but contamination on the PCB itself. With care they can be dismantled and cleaned with a mild solvent or spirit alcohol. Although I've never done it, if the disc has de-metallized, I would think it could be repaired by spraying it with conductive paint or even fitting a new aluminum foil disc over the original one. Anything that increases its dielectric constant should work.

I think this type of keyboard went out of favor because of EMC problems, the length of tracks on the PCB and relatively high frequency square waves needed to ensure sufficient coupling must have created a lot of interference. The CPU was I believe an Intel 8749 (8049 with built in ROM) although it might be marked differently.

They are certainly not toxic, at least not more than any other electronic device.

Brian.

Click to expand...

In the beginning I thought the conductive area of the film was touching the PCB pads. Thank god I read about how capacitive switches work on wiki. These film disks are soldered on the "foamy" material (which is already quite unstable after the years) so they cannot be unsoldered and spayed from the inside. I will probably use some kitchen foil combined with kitchen aluminum foil or any similar thin material and solder this on top of this deteriorated soil. There is enough space to do this I think. This will save the keyboard I think.

Thanks a lot for the toxicity issue, that is a relief.

And yes EMC was an issue and it is stated on the manual as well! This keyboard has the PCB sandwitched inside a thick steel "enclosure" which is grounded. And yet they mention EMC considerations and solutions on the manual. Interesting....

- - - Updated - - -

Here are two really bad photos I took from the PCB, just for fun.