K4600 Black Level Problem?

parkway

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Gents,

Got a call that the monitor in a Pacman I have on location looked like it needed some help. Went out to it after work and it indeed does not look good. Recapped this monitor last year (June 26, 2016) with an Ian Kellogg kit (so I know I used good caps). Reflowed all the card connectors on the cards themselves and the chassis as well when I capped it. B+ is within spec. Was working just fine after the recap and was operating with no problem from July of last year up until yesterday apparently; been going 12 to 14 hours a day for a year, so I guess I can't complain too much. Some research tells me this might be a problem with the black level pot, but figured I'd have you guys take a look before I go chasing my tail. Looks like it could be more than that maybe? Tell me what you think:

 
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So looking at the chassis again, it looks like I did replace the black level pot on the interface card when I capped it last year. Hmmm. Could this be just the tube on its very last legs?
 
Have you tried adjusting the black level pot?

If it checks out ok then go to the neck board and test resistors R419 (located directly behind the SCREEN pot) and R418. They are 330k @ 1/2 watt and 470k @ 1/2 watt respectively. You need to unsolder one lead to test them otherwise you get false readings if tested in-circuit. Sometimes R419 will go open which will play havoc with the black level.
 
Ken,

I tweaked the black level pot slightly and the image came back to normal. Would it be safe to say I have a bad pot or is something else at work here? I only moved the pot a smidge and the result is pictured below. Looks much much better as you can see.

5dbcd41114c30e43a632c9407cdbfd37.jpg
 
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