Monroe 620

[Kees Bouw]

I am repairing and cleaning a Monroe 620 nixie tube calculator from the early seventies. On the keyboard, under the keys, there is some kind of foam glued on to it presumably to provide a soft landing for the keys. It is shown in one of the pictures. The foam has deteriorated much over the years and I would like to remove it. At this moment I have removed most of the foam but what remains are traces of the glue, shown in the other picture. I have only tried IPA so far, without success. I am reluctant to try acetone because that is said to dissolve most plastics. As it is the glue is a bit sticky and if you press a key far enough down it sticks to it somewhat. The keys seem to be very difficult to remove, I do not want to apply too much force because if they break a replacement is hard to find. So I will have to work around them. Does anyone have any ideas on how to remove the remaining glue without causing damage? Should I then put new foam there, possibly giving the same problem to future restorers, or just leave it as it is? The kind of plastic that the keyboard is made of is very similar to what is used inside the HP classic calculators. Any help is much appreciated.

[Maximilian Hohmann]

Hello!

I would stick on a new piece of foam. By this you avoid the hassle and uncertainty of removing the glue without damage and give the original feel back to the keyboard.
You can look for stuff that is used to repair shoes and/or make costumes, e.g. for cosplays. I have good experience with an eBay seller in Germany called "celtic_voice_leather_store" (https://www.ebay.de/str/celticvoiceleatherstore) who has all sorts of rubber and foam rubber plates in different sizes and thicknesses. So far I have only used his material for shoe repairs but it is of good and durable quality.

Regards
Max