DEC VT 320 with Terminal Disease - help?

From: Kevin Handy <kth_at_srv.net>
Date: Sat Feb 22 15:16:43 2003

Tony Duell 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
>>
>>
>
>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.
>
Many of the ones I had to repair had been left on for a long time in
that condition (...mumble...customers...), and had often burnt the
curcuit board, enough to cause the copper to come loose from the
fiberglass backing (delaminate?). Removing the components on
these, without losing a lot of the copper was a major pain.

Sometimes, though, I was lucky and it wasn't too badly fried, and
wasn't too hard to fix (just needed a good, hot iron for the flyback), but
usually they waited a couple of days before complaining that their
"computer" wasn't working, and it had a nasty smell, and smoke was
coming out of it (and should they turn it off?).

>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.
>
>
I usually replaced it anyway, since I didn't really trust it after what
it had
just gone through, and I had the thing disassembled anyway. I got the
transistors from Radio Shack (special order), but the transformer cost
me $35 to $40 from the one place I could find them.

At the time, it was cheaper than a new VT320, but I don't think that is true
any more, especially if you have to add labor to the cost.
Received on Sat Feb 22 2003 - 15:16:43 GMT