DEC VT 320 with Terminal Disease - help?

From: Kevin Handy <kth_at_srv.net>
Date: Fri Feb 21 15:23:01 2003

Mark Tapley wrote:

>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
>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
>
> The power supply has a big (10-pin) transformer inside a cage (with
>some other components) on it. The transformer seems to divide the circuitry
>in two, inasmuch as only the "Ground" traces seem to go to both sides. On
>the input (upstream, plugs into wall) side, there is a network connected to
>a UC3842N IC. Google led me to a spec sheet for that, which calls it a
>"Current-mode PWM Controller". The spec sheet also has a "typical
>application" schematic for an "Off-line Flyback Regulator" which looks to
>be at least somewhat similar to the upstream side of the power supply,
>though I have not traced the latter out well enough to be sure yet.
> There are also several large electrolytic capacitors on the input
>side of the power supply.
> Possible signs of trouble are there, in the form of scorch marks on
>the PWB around two power resistors (? They have what looks like a powder
>blue ceramic exterior, are marked 120 Ohms +/- 5% and ?? (maybe 47 Ohms),
>and are much bigger than most of the resistors on the board).
> In the "loaded" condition, the resistor with the worst scorch marks
>around it (R523, as marked on the circuit board) has 0V across it, with a
>small spike upward (as shown by the needle jerking on my analog VOM) each
>time the output LED flashes. In the "unloaded" condition, my VOM shows 15V
>across that resistor, and it gets pretty warm pretty fast.
>
>Questions for the group:
>
>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?
>
>2) Is there a good way (without use of an ESR meter, which I haven't got)
>to isolate which component is bad, or should I just replace the capacitors
>on suspicion?
>
> The power supply, at least, looks very easy to work on. Nice big
>components, circuit board is only 2 layers (front and back) and not very
>many traces on the component side, and the traces are exposed and easy to
>get to in its operating configuration.
> I'm being careful around the 120VAC (rectified to 160 DC in the
>unloaded condition).
> Thanks in advance for any help you can give me!
> - Mark
>
>
>
Look at the flyback transformer. That is the ceramic thing with the thick
wire attaching to the CRT tube. Does it have a crack around it with
hardened "ooze" coming out of it?

90% of the time the vt320's fail, it is this transformer shorting out, which
takes the horozontal transistor(?) with it. It heats up, cracking open, and
spewing out it's magic ooze. If this is the case, you can replace both of
these parts, and it will work again. DEC used some really crappy flyback
transformers in these things.

However, it would probably be cheaper just to replace the whole terminal
in that case. The transformer will probably cost at least $30.00, and the
transistor around $5.00. Then comes the fun part of trying to extract
and replace them without destroying the circuit board.
Received on Fri Feb 21 2003 - 15:23:01 GMT