Sticky/Repeating Keys

About original Memotech hardware.
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Sticky/Repeating Keys

Post by AndyGarton »

All three of my MTXs have this problem - I've done a basic clean, and used the high tech solution of hammering away endlessly at the offending keys, both of which help a bit, but I still have the problem. Interestingly it's more apparent in BASIC mode than CP/M.

How do I fix it properly please?!?
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Re: Sticky/Repeating Keys

Post by Dave »


It is a common problem with these 30 year old MTX keyboards. I don't know if anyone will be able to offer a magic cure. Exercising the keys does often help a great deal - keep bashing away :-)

An electrical switch contact cleaner/lubricant might help, I was going to try this on my own keyboards, but the prices for suitable aerosol packaged cleaning fluid was VERY expensive and a bit out of my price range, you may be a bit richer of course :-)

Here is one suggested cleaner ..... ... 2a30f0df01

For the repeating keys in BASIC, you might want to try turning off auto-repeat as described on the tech tips page on my site ... chtips.htm

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Re: Sticky/Repeating Keys

Post by 1024MAK »

Part of the problem is, I suspect, the way the key switches work.
When nice and new, the contact surfaces are clean and free of tarnish and contamination.
After 30 years, the metal contacts are no longer spotless.

As the code in the programs (including the OS ROM) was designed and tested with new key switches, the code only takes account of the normal amount of contact bounce for new key switches. ALL mechanical switches, contacts etc. suffer from contact bounce. As the contacts make (or break), a small mechanical oscillation occurs so the contacts actually make and break lots of times within so many milliseconds (how long depends on the type and condition of the switch/contacts).

Now that the key switch contacts are no longer clean and free of tarnish and contamination, the "bouncing" contacts do not always make on a clean contact area. So the software sees a switch contact make, the contacts bounce, a "pause" (where the contacts are still suppose to be made, but electrically they are not) followed by the contacts making electrical contact again. This looks like another key press to the software... This can happen a number of times as you either press or release, a key, or both!

There are a number of possible solutions (but not all very practical).
  • The more you use the key switches, the more the contacts will "self-clean" (so use your MTX every day),
  • A contact cleaner liquid spray may help clean some of the dirt and contamination off the contacts (if you can get it in to the problem mechanism),
  • Renew the key switches (good luck finding compatible new ones, if anyone does find any, please post so others can buy some),
  • Rewrite the de-bounce routine in the OS ROM and also in any software that does it's own keyboard reading (keeping in mind there are at least three types of ROM chips in use and on some boards it is not easy to just plug a EPROM in),
  • Develop and fit a hardware solution that "smooths out" the bounces (but which may make the key response slower)
As you can see, there is no simple fix... :mrgreen:

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