22 Feb
2003
22 Feb
'03
7:16 a.m.
Tony Duell wrote:
The flyback is on the main logic board in these terminals. The PSU is
just the 'low voltage' supplies.
No, all the deflection circuit parts are on the main logic board.
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?).
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
>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"?
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.
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.