DEC LK201 keyboard

From: Tony Duell <>
Date: Wed Oct 22 11:09:08 1997

On Wed, 22 Oct 1997, Mzthompson wrote:

> Someone gave me a box of 10 or so DEC LK201 keyboards.
> A couple of them work and the rest do not.
> Several seem to have the same problem in that the 4 status
> LED's all light up, but the keyboard does not respond.
> Has anyone encountered this problem and repaired the
> keyboard?

Uh-oh... I've seen _that_ one before...

I have never seen an LK201 with a logic fault. What tends to happen is
that a short occurs somewhere in the membrane keyboard assembly which
confuses the microcontroller at power-on. To check this :

1) Remove the feet and the 2 screws under them. Lift off the top cover.
Unplug the keyboard cable at both ends

2) Release the little clips, slide the keyboard to the right, lift it out
of the lower cover

3) Remove the heatsink clip from the 7805. Remove the screw from the LED
mounting. Move the logic board away from the keyboard unit. Unplug the 3
keyboard tails

4) Either put pieces of plastic/card under the metal clips that hold the
tails in place (to avoid shorting all the pins together) or bend up the
tabs and remove said clips altogether.

Now plug the bare logic board into a VT220 or similar. You'll probably
find that the lights do the right thing. In which case you've got a matrix

The bad news is that fixing the matrix is next-to-impossible. You _can_
attempt to dismantle it (cut off the heat-spread pins on the back, remove
the keycap mounts and the leaf springs, unfold the membrane). Putting it
back together is less easy - there isn't enough of the pegs left to spread
over a second time. I managed it once, but it was a _fiddle_.

Washing the assembled keyboard (even in an ultrasonic cleaning tank) never
worked for me either.

> Does anyone have a schematic for this keyboard that they

There are 2 versions, one (the much more common one in my experience)
based on an 8051, the other based on a 6805. I traced out the schematics
of the first one years ago, and it took about 30 minutes to do - it's just
the 8051, a few 74145's as column decoder/drivers, a line driver and
receiver for the terminal interface and the 5V PSU.

But, as I said, I've never seen a fault in that section.

> could copy and send to me? Having several DEC machines,
> it would be nice to have a few spare keyboards.
> Thanks,
> Mike Thompson

Received on Wed Oct 22 1997 - 11:09:08 BST

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