Switch type/construction details of Key Tronic KB5150?

[dramsey]

I just picked up a Key Tronic KB5150 keyboard (eBay). It's an early IBM Model F clone using a variant of the XT layout. Despite what I read in the WIki, it uses linear switches-- they feel rather like Cherry MX Blacks.

The keyboard is in spectacular condition. The top plastic case has a few scuffs, but the inside of the keyboard is pristine, completely free of the debris one normally sees in these old units, and there is no visible wear on the keys.

However, a couple of the keys-- notably the space bar and "M" key-- do not register well. The space bar especially requires a very hard strike or very firm press. Before I tear into this, does anyone have any idea of the switch mechanism?

[dramsey]

Got impatient and went ahead and took it apart. It's a capacitive foam keyboard, and I cleaned the PCB and the foam pads on the problematic keys with alcohol. Reassembled and...no difference at all. Well, there's some difference: I can't get the "M" key to register at all now.

Is there a good source for replacement foam pads? The ones in the keyboard don't look bad, but I'm pretty sure that's the problem with the space bar and "M" keys. Of course as long as I have the thing opened up I might as well replace them all, because 30-year-old foam.

It seems simple enough, if very tedious, to make your own. There's some guy on eBay selling pads for $2.85 each, and I suppose I could order two and see if they cure the problem. What do you guys think?

[Mal-2]

Got impatient and went ahead and took it apart. It's a capacitive foam keyboard, and I cleaned the PCB and the foam pads on the problematic keys with alcohol. Reassembled and...no difference at all. Well, there's some difference: I can't get the "M" key to register at all now.

Is there a good source for replacement foam pads? The ones in the keyboard don't look bad, but I'm pretty sure that's the problem with the space bar and "M" keys. Of course as long as I have the thing opened up I might as well replace them all, because 30-year-old foam.

It seems simple enough, if very tedious, to make your own. There's some guy on eBay selling pads for $2.85 each, and I suppose I could order two and see if they cure the problem. What do you guys think?

Foil and foam just sucks, but I'd at least attempt a circle of metal foil tape stuck onto the bottom of the non-working sponge (or plain foil held on by a very thin layer of glue). That key already doesn't work, how much worse are you going to make it?

[dramsey]

Foil and foam was a common solution back in the day. I worked at the long-defunct Corvus Systems, and our Concept 68K-based machines used keyboard like this (probably sourced from Key Tronic, although I don't know). So did the Apple Lisa.

Your suggestion of using metal foil tape as a quickie repair makes a lot of sense. I'll give that a try first.

[XMIT]

My understanding is that if the sponge gets too thin the foil won't get close enough to the contacts.

I'd like to experiment with replacing the foam with thin neoprene. It would change the key feel slightly but the repair would last much longer.

I've skimmed some other foam and foil repairs. I thought someone used mylar and/or anti static bag material in their repairs. Good luck.

[Compgeke]

XMIT has it right, the foam doesn't make contact anymore.

I've always been told not to use plain metal but instead mylar (such as emergency blankets and baloons) if the original foil can't be salvaged.

[dramsey]

It's become obvious that the keyboard will need to be completely rebuilt-- that is, new foam pads/Mylar disks at every key. Although most of the keys work, the foam is disintegrating on all of them. My green cheek conure-- a parakeet-sized bird-- flew over the keyboard while I was inspecting the pads and disks and several of the Mylar disks blew right off.

So if I want to salvage this, I must obtain thin shim material, foam, and Mylar (trying the scrape the degraded foam off the existing disks would be a last resort) and fabricate a hundred or so of these pads. I wouldn't have bought the keyboard had I known I'd need to do this (the seller made no claims about whether or not it actually worked), but it's in such nice cosmetic shape I'm probably going to give it a try anyway.

[Mal-2]

It's become obvious that the keyboard will need to be completely rebuilt-- that is, new foam pads/Mylar disks at every key. Although most of the keys work, the foam is disintegrating on all of them. My green cheek conure-- a parakeet-sized bird-- flew over the keyboard while I was inspecting the pads and disks and several of the Mylar disks blew right off.

Nice choice of pet. They're as smart and can be as friendly (it's a very individual thing of course) as many of the larger parrots, without the noise or the maintenance.

So if I want to salvage this, I must obtain thin shim material, foam, and Mylar (trying the scrape the degraded foam off the existing disks would be a last resort) and fabricate a hundred or so of these pads. I wouldn't have bought the keyboard had I known I'd need to do this (the seller made no claims about whether or not it actually worked), but it's in such nice cosmetic shape I'm probably going to give it a try anyway.

I've had foam and foil. I know that's what it was because I had to repair one after a soda spill incident. They never feel particularly good, even in like-new condition. I'd think twice about investing a bunch of time, effort, and probably not that much money (metalized Mylar can be had pretty cheap in the form of balloons) just because you're probably not going to end up using the resulting board anyhow.

[dramsey]

Hit some hobby and hardware stores today and had a go at making my own replacement switch, using the Mylar and shim from an existing pad.

First, 5mm art foam is far too thick-- if it were "squishy" open cell foam it would be just right, but it's not very compressible and it reduces key travel to almost nothing. I tried cutting it in half with a razor blade to get "about" 2.5mm, and that worked, but the key press still wasn't very reliable. The Mylar was fairly wrinkled by the time I got it on and I've seen differing opinions on whether this makes a difference, but the key requires a firm press to register-- I can see this dropping keystrokes like mad if I did the whole keyboard this way.

I've found an eBay seller who can crank these out on demand and am waiting a quote for a full set. I suspect this will be the least painful way to go. Yeah, it means I have leftover Mylar, glue dots, 3M 77 adhesive spray, clear plastic sheets, and art foam, but hey, you never know what else it could be good for...