k7400 fluctuation of image

[lilshawn]

I have a k7400 that the seems to be trying to collapse in on itself horizontally due to what seems to be dropping B+

the horizontal slowly starts to squeeze in about 3cm (1 1/2 inches) on each side, stays that way for a few seconds (random) then pulls back out to normal. repeats with uncertainty, but does randomly. the B+ is dipping down low (102) during this time. (normal 120)

for the most part the B+ is solid. With a static crosshatch pattern, the monitor is fine for maybe 45 or 50 seconds at a time, then dips down.

one cap was swelled up (c107) replaced a few other caps that tested a little iffy with the ESR tester (C310, 117, 613) the big cap (C105) looks to be okay, it tests good. i guess i could shotgun all the caps, but then what is the issue? I do like to learn. if i can find the culprit then i can figure out why it's doing what it's doing for future reference.

off the top of your head? any ideas where to go from here?

[grantspain]

you could try to isolate the B+ from the flyback and use a light bulb as a load-then see if the voltage is fluctuating
at least you would know if it does then to look at the psu circuit
it does sound like a cap though

[lilshawn]

I lifted one side of L100 and lightbulbed it. still dumping down (~106v). same random regularity as before.

Replaced C115 since it's still part of the circuit at this time.

[grantspain]

what does the voltage read like pre the tranformer,maybe is dropping that side

[lilshawn]

what does the voltage read like pre the tranformer,maybe is dropping that side

i'll have to check onnnnn..... thursday, I have to go out of town tomorrow. was out all day today, so Ive only had about an hour to work on it.

I'll check the voltage right as it's comming out of the FET.

[lilshawn]

i went ahead and changed out a few more caps (all in the power supply section) including c105 (from a donor board.) and still had an issue.. although it took about 45mins to an hour for it to crop up... I replaced a few small caps in the vertical IC area (2-1uf 50 volt and 2-10uf 25 volt) as well as a larger 2200uf one between the HOT and flyback.

tested the voltages andit ran for about 2 hours before i left it to bake over the weekend.

we will see if it's still good.

[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.