IBM Model F AT - Random key presses by just laying fingers on the keys

[r0l4n]

Hi,

I just got my hands on an original IBM Model F AT keyboard from 1985 belonging to a IBM PC AT (5170). It is in great condition but it was quite dirty when I got it, so first order of business was a good clean-up - I removed all keycaps, soaked them in warm water and soap, rub them with a cloth and dried them. I disassembled as well both front and back covers to dust it properly with a brush, as the backplate had all sorts of filth on it. Finally I put it all back together and reinstalled the keycaps.

Then I went ahead and test it. I immediately noticed random key presses, which I assumed were just poorly seated keycaps causing trouble. I indeed found half a dozen or so that we not buckling correctly, so I reseated them. All keycaps are seating and buclking properly now (tested with the diagnostic tool from the 5170). Unfortunately, the problem persist.

These random key presses occur when my fingers are touching the keys, I do not even need to press them. Just rubbing my fingers on the keys trigger key presses. I read about grounding issues, and I'm convinced that's the issue - if I reach out to the PC with one hand (effectively grounding myself) while I rub my fingers over they keys, all random key presses are gone. If I just continue rubbing my fingers on the keys but stop touching the PC, random key presses start again.

I've gone over the connections several times, used the multimeter to verify the ground of the brass backplate, the PCB, and it seems fine to me. I also looked for dry/cracked solder joints in the PCB but it all looks neat.

Any pointers as to what to check next?

Thanks in advance!

[Falkenroth]

Have you looked down the barrels to see what the foam condition is like. I own one of these myself and the foam was in terrible shape. Debris was in the barrels causing issues with the flappers. That is a common issue with Model F keyboards.

[r0l4n]

It all looks very pristine to me... I didn't even spot any dust down there. Took a few pics from barrels in different areas, not sure if it will really come through.

The amount of random keys presses is insane, dozens of characters stream in as I just rest my fingers on top of the keys, so there must be something very off somewhere...

Attached a picture of the front side of the PCB.

[Falkenroth]

I have to agree it appears to be clean in the barrels, but the foam does appear to be breaking down on that top edge in your first photo.
Just not much to fail on these robust model F boards. Hopefully someone else might know more about this issue. Otherwise I think you might have to open it up for a better inspection.

[r0l4n]

I see, so the foam not being in top shape can, on its own, provoke issues with capacitance. I'm pretty sure the foam is in a pretty terrible state after all these years, regardless of how clean and pristine anything else is.

[Green Maned Lion]

I've never owned a Model F to my memory, but just having watched videos on how they work and how they are constructed, what occurred to me reading just the headline of your thread was the potential for the barrel assembly to be wiggling slightly causing a light touch of the key to have the flipper touch the capacitive pads, and that this might be caused by inadequately thick foam padding.

[Ellipse]

I recommend opening up the inner assembly and removing all dust which may be below a flipper, so you would not see it unless the inner assembly was opened up (see the videos on the modelfkeyboards.com project web site manual for details). The project web site does sell replacement non-crumbly foam (disclosure - I am the project coordinator). I have also seen issues due to loose or oxidized connections from the controller to the cable - clean off those pins and make sure everything is secure, or replace the wire with a Soarer converter and USB cable.

[AndyJ]

You can carefully tap the upper and lower plates apart with a hammer, but you will need a large woodworker's clamp to put them back together. If you're using new foam - there are vendors on eBay - you will need about eight or ten small clamps or strong paper binders to squeeze the plates together.

A support frame made out of a piece of 1x2 or 2x2 inch lumber will help a lot for reassembly. People have put Model F keyboards together without these tools, but they make a tedious job *much* easier... and if you have to take it back apart because a flipper slipper out of position, you'll wish you had the right tools.

Even if you never use the tools again, you'll still be ahead on annoyance and frustration. And if you ever take an F keyboard apart and reassemble it again, it's just the work of a few minutes.