Cambridge Z88 membrane, and spurious key presses (the opposite problem)

[gbejniet]

Hi all - I wanted to reach out in case someone has detailed enough knowledge of the construction of the keyboard membrane on the Cambridge Z88, because I have a curious problem and I wonder if anyone else has seen it.

I bought a Z88 about 4 years ago and found quickly that the membrane was failing, as they are apt to do, so I quickly bought a new replacement membrane from a common source of them and fitted that with good results. I've brought it out occasionally since then, but not for a couple of years until this week.

At first it seemed fine, but then quickly I started to get the '.' key registering repeatedly, and wondered if the rubber mat was failing and sinking onto the membrane... so I detached that, and the issue remained. I disconnected the membrane and the issue stopped, no phantom .......s .

Now: I'm wondering what my options are. Bear in mind this is a replacement membrane, and I've not heard of this problem happening to anyone else. Mostly I read about keys not working/double shift not turning on etc. (I am half tempted to put my original membrane back after buffing it a bit with something, as I still have it and it did appear to work for the first 10 minutes, so go figure?)

I feel like I have isolated the issue to the new membrane. It has had very very light usage in its lifetime, and I can't see anything on the '.' pad itself that might be a short causing this problem. I do wonder if somewhere in the layers of conducting and insulating material there is a slightly worn or shorted area, although I can't think how I would start tracking that down.

So I guess my query is: would anyone know how an issue like this might arise given what these membranes are made up of? (Perhaps it will help others if they run into the same problem.)

 

[AndyO]

I've had this problem with my Z88, though it was before I replaced the membrane, not after. In my case, I replaced the membrane because the left shift, menu, index and diamond keys would stop responding after about 10 minutes use.

But, I did notice on the original membrane which had begun repeating occasional keys on the right-side, that when I removed the rubber mat and examined the membrane, there was what appeared as some form of corrosion/contaminant on the right-side, which looked suspiciously like the dried remnants of a fluid spill.

I cleaned that off with IPA, then noticed there were traces of the same on the underside of the mat in the same area, so cleaned that with IPA too. It didn't fix the issues with the left shift, menu, index and diamond keys, but the were no more uncommanded key repeats. I realised that it may not have been my IPA cleanup that had 'fixed' this, but may have been some contaminant on/in the keyboard connector which merely self cleaned when I disconnected and then reconnected.

It's probably worth spending a bit of time gently cleaning off the pads on the underside of the rubber mat - though replacement mats can also be had from the remaining UK supplier.

 

[Istarian]

Under the right conditions (warm humid environment with relatively sudden drop in temperature) condensation can form naturally on a surface and dust/dirt may assist in that process.

So there might never have been any liquid "spilled" on/into the system or keyboard.