Keyboard repair help

[Matt_H]

The "2" key and the "-" key have stopped working on the numeric pad of my 4000T keyboard. All other keys work (although I haven't tested F1-F10). Any ideas? I disassembled to the membrane and cleaned off the obvious gunk. The contact pads appear to be clean on both the membrane and the switches, but I haven't worked on them yet.

So:
1. Can someone walk me through how to clean the pads? Water? Alcohol? I've heard that it's possible to accidentally remove the conductive material when cleaning.

2. Does anyone see anything amiss in these photos of the membrane that might explain the two broken keys? (But if there was a membrane problem, I would have expected more keys to be nonfunctional.)

Thanks!

[MigaTech]

This seems to happen a lot. Probably why replacement membrane units became available.

One reason for failure is spilt drinks. Did your keyboard have any signs of this? Doesn't take much to stop those flimsy pads from working. There are video's showing how to repair these.

If other keys are working in numeric section, it is more than likely the membrane at fault.

If you try cleaning, use the softest methods first, then try the keyboard again.

The second image bottom right looks a little dodgy!

[Matt_H]

Nothing's spilled on it (the aforementioned gunk was just dust from regular use). In fact, it's been sitting unused for some time (beneath a dust cover) and was working fine the last time I used it.

Bottom corner does indeed look dodgy, but I followed that trace to the Caps Lock key, which is working fine.

I will look for some videos. Thanks!

[tommywiz]

Was there a weight of some kind pressing down on those keys?

[Matt_H]

Nope, keyboard was just sitting comfortably on a desk with its dust cover.

[bibitvi]

Originally Posted by Matt_H

1. Can someone walk me through how to clean the pads? Water? Alcohol? I've heard that it's possible to accidentally remove the conductive material when cleaning.

Thanks!

Don't use any alcohol based cleaning products or you will end up without any carbon contact areas and you can then bin the membrane.

[TommyTomorrow]

Even IPA?

[wiz12]

It could be the key plunger that is the problem. Try swapping the non-working key plunger for one of the the other key plunger that you know works and see if it makes any difference.

[pandy71]

You can try to repair membrane (if broken) use conductive glue - contact areas are made from graphite so graphite or silver conductive glue can fix issue - continuity tester (or ohmmeter) to isolate area.

[bibitvi]

Originally Posted by TommyTomorrow

Even IPA?

Graphite is greasy/oily stuff. IPA is used also for removing the grease. So yes, no IPA.

[Snoopy1234]

In the [2] key loop - under the Enter key you seem to have a fracture.

[Screechstar]

Hi. It’s worth checking that potential break and the comment about the plunger being another potential issue is really good..

I actually fixed my keyboard last night too - mine has the circuit board rather than a membrane and that had good continuity, but the carbon pad on the plunger was the issue. I tried (very gently) measuring resistance with a multimeter and it was really high compared to the others.
I’ve had success cleaning these plungers with a gentle rub on some paper to remove contaminants before. In this case I had a few spare 2nd hand plungers from eBay so stole the rubber / graphite section and now it works great.

Good luck! And I’ve seen IPA slightly wipe off the green surface off the membrane so watch out for that too.

[phx]

Originally Posted by wiz12

It could be the key plunger that is the problem. Try swapping the non-working key plunger for one of the the other key plunger that you know works and see if it makes any difference.

I don't know if "key plunger" is the correct designation (I don't even know the german word for it), but if you are talking about the soft, black, rubber-like thingy inside the keys, which make contact to the membrane, then yes, this is the most common part of failure on keyboards!

I would also try swapping them first, which is easy to do.

[wiz12]

Originally Posted by phx

I don't know if "key plunger" is the correct designation (I don't even know the german word for it), but if you are talking about the soft, black, rubber-like thingy inside the keys, which make contact to the membrane, then yes, this is the most common part of failure on keyboards!

I would also try swapping them first, which is easy to do.

Yes, thats what I'm talking about, I don't know what they are really called, perhaps conductive key stems would be a more appropriate name?

Anyways, I know from repairing my Mitsumi membrane keyboards before that these are quite common to fail and lose their conductivity over time so simply swapping them is the first thing I would try.

[Jope]

You can revive them by wiping the rubber with a sheet of paper. :-)

[Matt_H]

Fantastic, thank you for all the guidance, everyone - the tip about paper on the conductive bits especially. Will give that a try. Special thanks to Snoopy1234 for spotting a possible break!

I have some of this if the pads on the membrane or keys need some deeper resuscitation and one of these to work on any broken circuits. Anyone ever done something similar?

Originally Posted by phx

I don't know if "key plunger" is the correct designation (I don't even know the german word for it), but if you are talking about the soft, black, rubber-like thingy inside the keys, which make contact to the membrane, then yes, this is the most common part of failure on keyboards!

Is it commonly a mechanical failure (e.g., cracking) or an electrical failure (e.g., losing conductivity)? The parts look fine (just like the ones next to them) and I don't trust myself to be able to remove them without breaking anything...

[Screechstar]

Personally, I've never had any crack. It just loses conductivity.

My childhood A500 has the membrane replaced by a thin PCB and worked fine 10 months ago, but I gave it a go recently and about 10 keys won't work due to those pads losing conductivity. I'll try the paper trick first but may have to experiment with those coatings you suggested in the end.

To give it a go, you don't have to remove anything as such... with the metal backplate and membrane removed, just press the key so the thin conductive rubber part sits proud of the housing and give it a gentle rub with some paper there.

[Matt_H]

Originally Posted by Screechstar

To give it a go, you don't have to remove anything as such... with the metal backplate and membrane removed, just press the key so the thin conductive rubber part sits proud of the housing and give it a gentle rub with some paper there.

Perfect, that's what I was planning to do. Fingers crossed!

[pandy71]

carbon (graphite) can be restored with help of pencil (rather soft than hard recommended) - something in B range (like B1 is soft to B6 very soft)

[Screechstar]

And just to mention… the carbon/rubber part can be carefully taken off the plungers. It just clips in to the plastic plunger and the non-carbon rubber part is quite elastic. The reason I did swap the plunger for mine is that is the the spare plungers I had were black but the keyboard had the slightly green ones.


As a reference, when I measure resistance I get about 150ohm across the carbon pads that work for me. Even after a paper-rub, that one I removed is up at around 700ohms, so may need a more intrusive fix is needed for that, like conductive paint, pencil rub or something.