Repairing keyboard flexible membrane

[soldar]

I didn't measure the resistance of the tracks on my keyboard but I believe relatively high resistance is normal and does not interfere with the normal functioning of the keyboard.

[GregDunn]

Yes, a typical working trace on this keyboard is between ~3-50 Ω depending on how hard you press and how far apart - so it's not anywhere close to what you'd find on a PC trace.  A friend of mine recommended this:

https://www.amazon.com/CircuitWriter-Conductive-Ink-silver-based-4g/dp/B00B88B9KI/

but as suggested above, I really hate to spend a lot of dough on something that in all likelihood will only get used once.  I tried a metallic (decorative) pen on a piece of scrap and found that it was essentially non-conductive.  Any circuit pen that I buy will cost more than the actual keyboard, but I suppose I am going to have to try something that isn't junk or else I'll just spend even more.   
 

[kizmit99]

I dunno - but the traces on my keyboard's membrane buzz out to basically 0 ohms...
This is just touching two ends of the trace with meter probes, not measuring across a pressed key...
pretty sure this is a very standard, cheap, keyboard...

[GregDunn]

My probes are small enough that I didn't want to push very hard for fear of distorting the membrane; but I could generally get the trace to measure as low as I wanted by pushing harder.  I very much doubt all membrane keyboards use identical scanning hardware, so some of them probably tolerate higher contact resistance than others.  This is the first one I've ever taken apart, though, so I have a history of exactly one to go by.   

I ordered some silver-based conductive ink so we'll see in a couple of days whether it's OK or not.

[amyk]

I very much doubt all membrane keyboards use identical scanning hardware, so some of them probably tolerate higher contact resistance than others.

It's almost certainly going to be a CMOS input on a microcontroller, so next to no current is needed.

[magic]

The silver stuff can now be bought from AliExpress in very small quantities in syringes, costs a few bucks delivered. I bought mine in Poland because I didn't want to wait, but the packaging leaves absolutely no doubt where it originated from



I tried it on a damaged IBM Model M membrane and it worked

Initially it has virtually no detectable conductance, I was barely able to get a reading of 100MΩ by bringing the probes within 1mm of each other. But after 24 hours of curing, resistance fell down to <3Ω, which is almost on par with the original traces on the membranes. I applied a bit too much and ended up needing to scratch the gaps between traces.

I also performed some testing on the keyboard itself: the original traces are 30~50Ω end-to-end per one sheet, and the controller reacts to a 4.7kΩ resistor applied directly between its PCB contacts (didn't try other values). So it looks like even a much sloppier repair with hundreds or thousands ohms of resistance would still do the job. Cheap graphite ink for TV remote keypads, perhaps?

Now gotta bolt mod the damn thing, put it back together and reinstall 102 hammers, springs and keycaps

[planenut2000]

I would bet that would work on car key fobs too!

[magic]

If there is some sort of printed membrane keypad in there that can go wrong then sure.

Note that some low power devices like TV remotes or calculators use carbon for the conductive traces and they tend to have more resistance than silver. Those devices can be repaired with conductive graphite ink, which is cheaper. Been there and done that as well.

[BILLPOD]

Good Morning Soldar,
     Adapters are made to go from PS2 to USB as well as the opposite.  Look here:  https://www.amazon.com/ps2-usb-adapter/s?k=ps2+to+usb+adapter

[magic]

Soldar is long gone from the forum and the problem with USB-keyboard to PS/2-computer adapters is that they are usually passive - the keyboard is expected to find out that it is connected to PS/2 rather than USB and deal with it. At some point in time USB keyboards (and mice) had this capability, but current ones probably don't.

edit
Just check out Amazon reviews of the products you linked: