Teletype 50k 227 AAH

HaaTa · 07 Nov 2016, 18:30

I dismantle a lot of keyboards, and I mean a lot of keyboards. Usually they are pretty boring, sometimes I have an interesting pcb. To me the really interesting keyboards are the ones that have switches I’ve never seen before. Even when uncovering these mysterious switches, the discoveries are kinda boring. Usually of the “yet another way to make a linear keyboard switch” variety.

HOWEVER, once in awhile, I open up a keyboard and well...leaves me utterly speechless.

THIS is one such keyboard.

I present to you the Teletype 50k 227 AAH.

DSC06800

Did I mention it pings sings!

I mean, it ticks all the boxes of a most excellent keyboard.

Clicky, check.
Tactile, check.
Capacitive sensing, check.
Magnets, CHECK.
Awesome spherical keycaps, check.
Classy 80s aesthetics, check.
Elegant and simple pcb design, check.
Foam and foil, che… er, actually, don’t mind this one.
And the list goes on…

Like many of my really interesting keyboards (photos-f62/univac-f-1355-00-t6489.html), I really don’t know where this came from. Teletype was going the route of the dinosaur in 1983, likely only used by AT&T themselves rather than being sold elsewhere.

Going from the model number 50k 227 AAH, my guess is that this keyboard was for the Teletype Model 50. However, other than a couple mentions in purchase catalogs I haven’t found any information on it *sadness*.

While not the exact patent, I did find what seems to be the base design for the switch.
https://patents.google.com/patent/US3942145A

The only functional difference is the use of foam’n’foil cap sense instead of electrical contacts.
Have I mentioned, the foam is in immaculate condition! I don’t believe I’ve ever seen mechanically used foam age this well in a keyboard.

DSC06561

The switch force curves are particularly interesting.
Long travel, 6mm.
ACTUAL bottom out dampening. You don’t see the top until around 500gf which is amazing (click on the force curves to scroll around).
The mechanism itself behaves much better in the spacebar (heavy) switch. I suspect this extra pre-load gives the switch more stability.
Due to how the magnet is held in place, the spacebar switch has a faster actuation. Release is still at the same point as it relies entirely on the magnet to reset the position.

But enough talking, time for pictures

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Flickr Album: https://flic.kr/s/aHskGcfzVi

zslane · 07 Nov 2016, 18:44

Beautiful!

A keyboard from back when Detroit still cared.

tigpha · 07 Nov 2016, 19:39

Thanks for sharing HaaTa, it's a beautiful specimen. Could this discovery indicate when "peak keyboard" occurred, in terms of refined design and manufacturing execution? The symmetry, heft and sturdy construction is remarkable even for the 1980's.

ohaimark · 07 Nov 2016, 19:56

It would be the peak if it wasn't foam & foil.

It's a foam & foil board that anyone would be proud to own, though, which is more than can be said about most of them.

alh84001 · 07 Nov 2016, 21:10

It looks absolutely amazing. And the sound. I've never actually tried F&F boards, so I am wondering how it actually feels.

And, to my model F trained eyes, it's almost unnatural to see those capacitors aligned that way.

wcass · 08 Nov 2016, 05:17

So as a hardware hacker, i'm trying to figure out how the hardware works to give that force curve.

I'm guessing that on the down-stroke, the linear force buildup is the spring compressing (plunger length decreases 2.5 mm) and the sudden force fall is the magnet breaking loose (spring compression force > magnetic force). The foil hits the pads (switch on) as the spring re-expands (plunger length increases 2.5 mm). The spring would then again compress 2.5 mm with roughly the same force curve as the first 2.5 mm. After that (key top at 5mm travel), you are just compressing the foam and the force shoots up to the limit of the foam.

On the up-stroke, the foam expands, then the spring. Magnetic force is inversely proportional to the cube of the distance, so the magnet at 2.5 mm away exerts considerably less force than when the actuator was stuck to it. The spring must therefore exert much less downward force before the magnet can pull it up. When the magnetic force > spring force (plunger length increases 4.25 mm), the foam and foil actuator will rise (switch off) this reduces the distance to magnet and the strength of the magnetic force increases very quickly thus easily compressing the spring (plunger length decreases 5 mm). The last 0.75 mm of key cap rise is the spring (and plunger length) expanding.

Does that sound correct?

HaaTa · 08 Nov 2016, 05:28

Yep. One part I might add, which Parak pointed out to me is this part.

DSC06714

This top cap is probably the secret to the very high bottom out force. As it would also act like a low travel spring if enough force is applied.

Orpheo · 08 Nov 2016, 08:45

This board is just magnificent, love everything!

ideus · 10 Nov 2016, 14:27

Wow, that case works well as an amplifier.

clickityClackity · 28 Jan 2018, 21:33

I opened the link to see I'm the high bidder... Ah, the joys of late night drunk eBay browsing. It's truly magical

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