VT220 Keyboard Error 4?

[Tony Duell]

> 
> I turned on my VT220 for the first time in nearly a year. The
> screen says "VT220 Keyboard Error 4", all four green LED's (on the
> top right of the keyboard) stay lit, and nothing appears on the
> screen when I type.
> 
> Disconnecting and reconnecting the coiled-cord between the
> keyboard and the terminal causes a cursor block to appear on the
> screen, but the keyboard is still locked and the LEDs stay on.
> Power-cycling the terminal shows the same error message.
> 
> However, the boot prompts from the 11/23+ are displayed normally,
> so the receive part of the terminal at least is working.
> 
> Where should I start looking, before replacing the keyboard?

This normally means there's a short in the keyboard matrix, as if one key 
is stuck down.

What I would do is open up the keyboard casing (there are 2 screws under 
the plastic posts/feet) and remove the internals. The keyboard has 3 
'tails' that are fixed to the PCB by little metal clamp-things -- bend 
up the 4 tabs on the underside of each one and remove them. If you like 
you can remove the PCB from the keybaord frame by removing the clip on 
the top of the voltage regulator (TO220 package) and the screw through 
the LED mounting block.

Connect the PCB to the terminal with the normal cable and power up. Most 
likely the LEDs will go through the normal sequence and there'll be no 
error message, which shows the fault is in the keyboard itself. This is a 
membrane contact unit with the key housings head-staked in place and with 
leaf springs under each key. I've never managed to get one back together 
after taking it apart :-(.

I suppose you could chack if any of the keys are mechanically stuck (the 
keys themselves can be removed by carefully twising them to free the 
locking barbs), and see if any of the leaf springs look bent. But I guess 
it's most likely you'll be looking for a replacement keyboard switch 
assembly or a complete keyboard (in the latter case keep all the bits of 
the old one, just in case you ever find one with a problem on the PCB).

-tony