DEC VT 320 with Terminal Disease - help?

From: Kevin Handy <kth_at_srv.net>
Date: Fri Feb 21 16:12:00 2003

Mark Tapley wrote:

>>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?
>>
>>
>
>Will look when I get home. I don't remember there being a wire that went
>from the PS board to the tube, other than ground straps from the cage
>assembly. There is a ceramic transformer-looking thing and the wires on it
>are rusty, which is one of my second-order questions (how to get rid of the
>rust and arrest the corrosion process).
>
>
It isn't on the power supply board, it is on the main logic board,
located under the display. Looking at the screen, it will be on the
right, about halfway back. Probably the biggest single component
on that board. A big ceramic blob, with the thick wire coming
out of the top.

The transformer is a big white thing with a high voltage cable
plugged into the side of the video tube (with a rubber gasket
to protect you from the high voltage). Be careful, because it charges
the tube to over 10,000 volts, and can give you a very nasty jolt.

You're PS is probably be Ok if it is the flyback that is shot.
I believe it is a switching power supply, and they give weird
readings when unloaded.

>>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.
>>
>>
>
>Is that transistor on the PS board as well? If so, why does the PS board
>appear to work "unloaded"?
>If that's the fault, are the scorch marks under the blue resistors normally
>there, or unrelated?
>
>
It is also on the main logic board. It is mounted near the flyback,
attached to a metal heatsink. It has a "plastic" washer to insulate it
from the heatsink, but that is often melted too.

>>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.
>>
>>
>
>Sigh. I may look on it as a learning experience, if I can find a
>replacement transformer that fits.
> - Mark
>
You have to use the correct model. I used to be able to special order
them from a local electronics shop, but haven't bothered in recent years.
Received on Fri Feb 21 2003 - 16:12:00 GMT