WG K7400 (25K7401) trouble

[paulstevens]

First what I've done:
- Checked ESR of all caps - all are fine, so I replaced no caps (monitor is home use only so I was not expecting any obvious cap issues.)
- Replaced R811.  It measured 75K but I replaced it anyway.  (75K 1/2W)

Symptoms:
- at initial power on, picture looks fine.  Good color, good brightness, good everything.
- less than a minute later, the horizontal width shrinks an inch or so on both sides, sometimes popping back out to correct width then shrinking back.
- along with the horizontal width shrink, the colors get all washed out and grayish.  All colors are there, but the overall appearance is washed out
- along with the washed out colors, a thin horizontal line (one scanline maybe two) appears about 1/4 way down from the top of CRT.  It looks like vertical foldover but it isn't.  Vsize and Vpos can affect it but it remains unless Vsize is shrunk way down - then the line "pops" and goes away.  Colors are still washed out though.
- finally after a few more minutes, the horizontal sync goes away.

Let the monitor cool a bit, and the above will repeat.

I read a post describing similar symptoms where the original poster said B+ fluctuated with his symptoms. The resolution to his problem was never posted.

How do you connect a 60W or 100W lamp to a chassis so I can work on it without the CRT?

Any suggestions would be appreciated.

BTW, why did WG put the H-Hold pot on the deflection board and NOT on the remote control board?  There is a spot for it on the remote board but it is empty!  Instead it is inconveniently loaded on the deflection board. 

Thank you very much.

[paulstevens]

Just a follow-on to my previous post...

The May 2006 StarTech Journal has an article described "New WG 25K7401 With Faint Horizontal Bar".

Does anyone have this issue they could look up for me?

Anyone know what the problem is and what the solution is?

Thanks again.

[lilshawn]

this chassis so far has behaved its self. it's still a little off, so i'm doing some more testing. I haven't had much of a chance this weekend, being at work and all. but on monday, after being on all weekend, my image now is starting out okay, but after several hours the width decreases. I feel there might be a bad cap, but i have replaced many of them.

i'm going on vacation for a week to get out of this -20 degree cold. then i'll be right back at it.

P.S. i'm copypata this into the other thread too.

[lilshawn]

How do you connect a 60W or 100W lamp to a chassis so I can work on it without the CRT?

unsolder the right side of L100 (as viewed from the back with the flyback to the left) and bend up the component so it's straight up. attach one lead of your bulb to here. (doesn't matter which) the other to the ground (green wire of chassis power connector.) there is a small jumper wire that bridges just to the right of the power connector, i just clip an alligator clip to the board here over the wire.

use a 60 watt bulb (it's enough of a load.)

PLEAAAAASE NOTE!!! the rest of the chassis is still live and is being driven, watch out! only the B+ 120vdc is being disconnected.

you description of your issue is IDENTICAL to mine. if you measure the B+ you will find that when the picture starts to squeeze in, the voltage rolls down slowly from 120 and it goes faster and faster till it hits the wall (in my case about 102 or so) this is what is affecting the colors and whatnot, since there is not enough juice to properly drive the tube.