I recently purchased a 1982 TI99/4a that has a defective keyboard. The keyboard is a Mitsumi with normal keys, but membrane type contacts.

Keyboard circuit boardWhen I press the contacts (gently) with a plastic stylus it seems to work fine, but the soft silicone core of the key itself does not seem to be able to activate the contacts. Does anybody know of a way to clean / re-activate the contacts ?

  • I did these same two things, with success. I think the eraser trick might have been the real fix since this ti99 was new-in-box and the pcb contacts looked perfect.
    – Mark Kotyk
    Commented Apr 6, 2017 at 23:37

2 Answers 2


According to jdv's answer to another question, cotton swabs and isopropyl (rubbing) alcohol will do the trick. Try to keep the alcohol away from plastics, use it sparingly and be gentle when you rub the dirt.

His answer also says that "fancy contact cleaner" will work:

For example, "CRC QD Contact Cleaner". Make sure you don't get the type used for lubricating potentiometers. We want the quick drying, stable variety that doesn't leave a residue. [...]

Bonus: Good for cleaning keyboards, too.

(In this answer, he says that "plain old contact cleaner" will do for keyboards.)

The answer also forbids using erasers, but only on electrical contacts (because it wears them down). Rubbers should be fine on membrane-type contacts, provided that they haven't become brittle and you press lightly with a soft eraser.

If cleaning the keyboard doesn't fix it, it may be that the keys don't press on the little buttons. As suggested in this comment, if this is the case, placing a thin piece of felt underneath the keyboard mylar can allow them to "push" the buttons down far enough.

  • I think this is the most sensible thing to do. I'll try it and see what happens. Commented Sep 9, 2016 at 7:37
  • 1
    You are right, and in the meantime I did try your suggestions and they worked. I cleaned both sides with 'Chemtronic Pow-R-Wash', a contact cleaner the leaves no residue, and I gently rubbed the contact carbon using an eraser. The keyboard works great again. Commented Jan 11, 2017 at 10:05

A pink pencil eraser is a reasonable contact cleaner for this sort of thing. Just scrub the black dots a bit and see if they will respond to softer pressure.

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