deskthority

Hi DT'ers

An experiment in replacing the tired foam in an IBM Model F Bigfoot.

So far I have learnt from various forum posts here on DT that there are two sorts of foam in Model F keyboards. This foam is supposed to press the barrels against the back iron plate and circuit board, and sits just behind the perforated front iron plate. I have two Bigfoots, each with one or other variety of original foam.

My first Bigfoot is older, and has Ensolite foam, a sort of PVC material that appears to be more resistant to ageing than the later variety.

The second Bigfoot has more flimsy foam, with a thin black vinyl backing. This stuff does not appear to age quite so well as Ensolite. The black vinyl feels a bit tacky, and the foam has lost most of its springiness.

I searched in vain for a fresh supply of Ensolite. I decided instead to use closed-cell polyethylene foam, three millimetres thick. I've seen this foam used in floating buoys, and it is quite resilient stuff: if it can survive years at sea, it should last for quite a while inside a keyboard. It is easy to work with too, cuts easily with sharp tools.

I bought a pair of hole punches, 12mm and two millimetre diameter, and far more foam than I really needed, and set to work. I honed the hole punches to a much keener edge, so that they cut the foam with a gentle twist of the punch. No need for hammering.

Using the original sheet of foam as a template, the holes are cut with a gentle twist of the hole punch tool.
The corners of rectangular shapes are first punched through with the 2mm tool.
All cuts complete. The slits along the top and bottom allow the fastening tabs on the front iron plate to pass through to the rear iron plate.
The original and the new foam, side by side.
Inserting the barrels. The fit was appropriately snug.
The space-bar is tricky, the flipper has to be held down in place because the space-bar must be hooked up to the stabiliser wire from inside. It's not possible to hook up the stabiliser wire from the outside, after attaching the front and back iron plates together. With the space-bar in position, the spring is compressed with a loop of floss attached to a suitably heavy spanner.
The two iron plates are pressed together with four clamps, one at each corner of the plates, with wood dowels spreading the pressure and protecting the chromate finish of the plates. These need to be left to press for a few hours, so that the foam compresses, otherwise trying to slide the plates against each other to re-engage the hooked tabs is too difficult, and could risk accidental slippage and damage.

Sorry, I didn't manage to take photos of the manoeuvre. I'll try to illustrate this with another edit later.

The conclusion: physically, the foam seems to hold the barrels in place suitably well, they don't wiggle. Electrically, I can't tell if the foam has the right properties: I didn't test with the original foam, an oversight that I shall keep in mind for future experiments. The result is a keyboard that works, but appears to have de-bouncing problems:

Bigfoot with polyethylene replacement foam:
Other Bigfoot with original Ensolite foam:
The Ensolite Bigfoot also has occasional glitches too, the back-tick is spontaneously inserted, but so infrequently that my typing errors are far more numerous, and I can live with it. The polyethylene Bigfoot has much more frequent repeated keys, which makes it very hard to use productively.

I wonder if the polyethylene is prone to static? Or is the pressure on the barrels too great? Or is the scan rate in Soarer's Controller too fast? The original cable may be a source of interference. I will try attaching the arduino to the logic board with much shorter wires, to see if that improves the spurious back-tick and bounced keys.

I prefer thinner (1/16", 3/32", or 2mm) and spongier (neoprene) foam.

Is that the stuff you sent us in our XTant kits?

Come December, I'll be back over and will give that build a go. I'd like to see how the foam feels compared to all these ancient Model Fs of mine which could all likely use a replacement too.

I think your replacement could work if you had some thinner foam?

Kinda what wcass said I guess.

I have used this foam with success
http://www.mcmaster.com/#8647k102/=xx37ry

Try wiping your PCB down with isopropyl if you haven't already. Note that any black number printing on the PCB will be wiped off by that, so avoid that area if you care about that for some reason.

Polyethylene can cause some static electricity issues, aye. Try replacing it with neoprene like Wcass said if you can, that should work fine. Easy to get, too. I'm not sure if wiping the PCB down with isopropyl would be entirely wise, but then, I don't have any experience doing so, and I'm sure people have done crazier things with success. I know I have.

Beautiful work!

My oversized-holes / hammer-and-block technique is quite crude in comparison. Your product is clean and beautiful.

3mm seems awfully thick. I have used 1.5mm (aka 1/16") art foam with consistent success, and the neoprene that I got from wcass is quite thin, and soft as well.

Personally, I like the thick stiff mats because of their feel and sound, but they can make re-assembly difficult, especially on the 122-key models.

I have recently replaced mats in an AT and a 107, and I had forgotten how easy these "narrow" ones are to work with, in comparison!

fohat wrote: Beautiful work!

My oversized-holes / hammer-and-block technique is quite crude in comparison. Your product is clean and beautiful.

3mm seems awfully thick. I have used 1.5mm (aka 1/16") art foam with consistent success, and the neoprene that I got from wcass is quite thin, and soft as well.

Personally, I like the thick stiff mats because of their feel and sound, but they can make re-assembly difficult, especially on the 122-key models.

I have recently replaced mats in an AT and a 107, and I had forgotten how easy these "narrow" ones are to work with, in comparison!

I had more problems with my XT than with my 122s , that controller gets in the way!

Thanks for the suggestions everyone.

The 3mm thickness was what I measured the original uncompressed foam to be. The polyethylene foam I used does appear to compress suitably well, but maybe it is too elastic, contributing to the key bounce? I'll take photos of the disassembly to reveal what shape the foam took.

I shall make a second attempt with neoprene as suggested. I'll buy 2mm and 3mm too, and see where it leads me.

I tried to find EPDM foam, but it doesn't seem to be as readily available as neoprene.

I think we all have our Model F foam preferences. I use 3/16" thick low density open cell polyurethane cloth backed foam, otherwise commonly known as automotive headliner. Absolute pain to cut all the holes into - but a breeze to install the plate / barrel frame assembly together when compared to 2mm or 3mm closed cell EVA foam.

Just to note: 2mm neoprene foam seems satisfactory. The same process as described above, same tools.

Hello, guys. Can I use the silicone rubber to replace the Model F foam sheet? I afraid the smell affect the health like the EPDM rubber, so I decide to buy the better material.

I don't know, but the material needs to be very thin or very compressible, otherwise you will have a great deal of trouble sliding the plates back together.

Only one way to tell. Try it out and let us know.
What thickness and what hardness are you trying?
You want something that will fill 3 mm, but compress to 1 mm.

Thanks to fohat and wcass about the nice response.^^
I have the 1m x1m x1mm EPDM rubber sheet already and not decided to use it because of the smell. If someone want to try it I can free it here.
I hope to try the expensive one 500mm x 500mm silicone rubber sheet for the Model F and beam spring. This sheet has many choices, something like 0.5mm, 1mm, 1.5mm, 2 to 6mm thick.

One more question: which foam and foil material(s) do you suggest to replace the original of BTC and Key Tronic?