DEC VT 320 with Terminal Disease - help?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Feb 21 16:47:00 2003

> All,
> yep, it's me again. The 4000 VLC started and ran perfectly. I left
> it on for about 8 hours as a burn-in test. When I came back, it was
> (apparently) still running - but the VT-320 it was talking through had
> died. Arrgggh. I've done a little investigating, so hopefully I won't need
> quite so many exchanges to get this one going.
> The external symptoms of the VT320 were: no display; when power
> switch is on, the LK201 lights flash about once a second. No other sign of

A very common problem, and it's not the PSU in the VT320. Even though, as
you've correctly noticed, the PSU is 'tripping'.

> life. I turned it off, unplugged it, and opened it up. The power supply
> board at the side has an output indicator LED which also flashes about once
> a second.
> I turned it off, unplugged the power supply board from the
> mainboard, and turned it on again. In this condition (which I'll call
> "unloaded"), the output indicator LED comes on and stays on. In addition,
> the pins on the output connector go to more or less their correct voltage,
> as determined by labels on the mainboard at the other end of that connector
> harness:
>
> Labelled Ground 5V 12V 18V
> Actual Ground 6.5V 12.5V 22V

OK, so the SMPSU is doing something. Try it with a dummy load -- a
suitable light bulb -- between each of the output rails and ground (or at
least the 5V rail and the 12V rail). My guess is that the PSU voltages
will become correct.

> 1) Have I got good evidence that the fault this time is actually on the
> power supply board, and not a pull-down on the mainboard, as it was on the
> VAX 4000 VLC?

Alas not. Go back to the main board of the VT320 and look at the flyback
transdormer (the one with the thick red EHT cable to the side of the
CRT). Most likely it will be physcially cracked. Shorted turns in this
transformer is a very common problem in the VT3xx series. Sometimes it
takes the HOT (horizontal output transistor) out with it. But the PSU
starts tripping owing to excessive load.

-tony
Received on Fri Feb 21 2003 - 16:47:00 GMT