VT320 fault - flashing LED and no HT

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Wed Dec 24 15:24:02 2003

>
> I just unearthed a VT320 terminal at the very back corner of one of my
> store rooms.
>
> Before I go any further... should it work with a VT220 keyboard? There

Yes. The LK201 keyboard works with the VT320. It will also power up (and,
IIRC display a faillure message) with no keyboard if everything else is
working.

> was no keyboard when I found it years ago, but I do have a couple of
> VT220 keyboards lying around.
>
> Secondly - it doesn't seem to be getting any HT output to the tube. The
> green LED D509 is pulsing about once every second.

Aha....

>
> Is there a common failure mode for these things?

YES! This is a very common failure.

What happens is that the flyback transformer develops shorted turns (and
sometimes takes the line output transistor out as well). This overloads
the PSU, which owing to the design _increases_ the voltage on the 5V rail
(I am not joking!). The crowbar fires, and the PSU trips. The cycle then
repeats. If you pull the case and look at the flyback transformer, you'll
propably see a crack in the casing with gunge coming out.

-tony
Received on Wed Dec 24 2003 - 15:24:02 GMT