DEC VT 320 with Terminal Disease - help?

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Feb 21 16:56:00 2003

> >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

The flyback is on the main logic board in these terminals. The PSU is
just the 'low voltage' supplies.

> >90% of the time the vt320's fail, it is this transformer shorting out, which

I would put it at closer to 98% :-) Those transformers are really unreliable.

> >takes the horozontal transistor(?) with it. It heats up, cracking open, and

Not always. I've had VT3xx's where the transistor was not defective (but
the flyback certainly was!)

> >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"?

No, all the deflection circuit parts are on the main logic board.

> If that's the fault, are the scorch marks under the blue resistors normally
> there, or unrelated?
>
> >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.

Oh come on. Those parts are not difficult to desolder without damage. Or
at least I've never had any problems doing so.

The transistor is a standard part (I forget just what it is, but a good
TV spares place would have one). The transformer is non-trivial to find,
AFAIK DEC never supplied them as a spare part. I believe there are 3rd
party replacements for it, but I can't remember where to get them.

-tony
Received on Fri Feb 21 2003 - 16:56:00 GMT