repairing a DEC VT420

From: Mzthompson_at_aol.com <(Mzthompson_at_aol.com)>
Date: Tue Jul 17 20:35:06 2001

Gunther Schadow wrote:

> I bought a VT420 from a neighborhood garage sale ($5) that
> turned out not to work. <snip>

> Opened the box and powered on, the kathode in the tube didn't
> seem to glow. More inspection turned out that what seems like
> a ceramic capacitor was exploded or fried, all black and
> crummy, impossible for me to identify its specs. Can someone
> help me out with a schematics or databook or something?

> the blown-up capacitor is C621.

I popped the hood of a VT420 and the only markings I found on
C621 was "1J250". My years of electronics leave me with out an
opinion as to what the actual value is. As you know, that capacitor
is located inside the confines of a heatsink. That is the heatsink on
the horizontal output transistor. Without a schematic, I would guess
that capacitor is associated with the horizontal output transistor or
possibly is in the flyback transformer circuit. Experience has taught
me that is best to check the windings on the flyback, albiet hard to
do without a schematic. More times than I care to remember, I have
found a bad flyback. At that point it goes out the back door, I just
don't mess with them anymore like I used to. Granted there are
companies out there that sell replacement flybacks, but the prices
almost lead to the decision to pitch the one unit and use the money
instead to buy several others and hope for better luck.

Mike
Received on Tue Jul 17 2001 - 20:35:06 BST