Need help in diagnosing a keyboard issue with an Amiga 500

[insidious611]

I've gotten an Amiga 500 rather recently, and it works perfectly save for one problem. The control, caps lock, and asdfgh keys, none of these work. The keyboard otherwise works fine, no spurious keypresses, LEDs (other than the caps lock LED, which does nothing) work fine.

I took the keyboard apart and cleaned almost every part of the keyboard (keycaps, keystems, plastic tray, metal backplate, everything but the membrane which I didn't feel comfortable trying to clean). The membrane looks fine, I don't see any traces that look bad or any corrosion. After cleaning and reassembling, everything works exactly as before (same keys don't work, rest of the keys work fine).

I'm wondering if it's the keyboard controller or the membrane that's the issue, and whether there are any further steps I, as someone who's more familiar with fixing wintel boxes and feels rather out of his depth here, can do to repair the issue.

Is it possible that the issue is the CIA chip? Or does a CIA chip keyboard failure usually mean a completely dead keyboard, and not a mostly-working keyboard?

The fact it's almost exactly half of a keyboard row seems like it should mean something. I don't know what, though.

I would really appreciate anyone's help figuring out what the problem could be. If necessary I can take the keyboard back apart to take another look at the membrane and give y'all a picture, but I'm not relishing the idea because those stems were so annoying to get back into place.

My final uninformed thought regarding a cause is maybe there's a problem with the caps lock LED? Could that be causing issues with the circuit? I dunno, maybe that's a dumb idea.

 

[roy_bates]

this is more likely to be the membrane on the keyboard or maybe the plungers

it wont be a cia

if there was a issue with the caps lock led it wouldnt stop the keyboard from working

and incase your wondering, the stems dont fall out of you dont remove the keycaps,simple really


if you want to find out if its the membrane you can simply bridge over certain pads that associate with the keys and simply bridge a pad while its on and see if ti shpws on screen,if it dont work its probably the membrane if it does work its the plunger
or you can do continuity test by following the traces and see where its broken with with a meter,it could be broken at the ribbon flat connector the membrane plugs into or carbon has worn off at the connector or pads the plungers shorts across

 

[insidious611]

The thing is, it can't be the plungers because I cleaned them all seperately, so if it were them the broken keys would have changed, no? Because different plungers would have been put in different places.

I'll try what you mentioned to be sure but it is definitely sounding like the membrane is the problem then.

Thanks for helping me narrow it down.

 

[breech]

I'll try what you mentioned to be sure but it is definitely sounding like the membrane is the problem then.

It's membrane cancer.
I followed a youtube video and fixed about 10 keys with 3 patches of silver, but beware it is quite difficult due to the liquid silver stuff eating the plastics while it sets - its also carcinogenic :/
There are replacement membranes available on sellmyretro for an easy fix.