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Jun 21, 2018
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One of my MS9 monitors is gradually loosing green after being turned on. It doesn't completely go away, but I'd say the green color is reduced by 25% or so. It's very noticeable.

Basically, if I turn it on and bring up the RGB bars from the CPS2 test menu I can see the green color slowly reducing in intensity. After just a few minutes it stabilizes.

This monitor has had this problem for quite some time. I did a recap on it a year ago, but it didn't have any effect on this particular problem. I haven't given it much thought really, the easy "fix" was to crank the G CUT OFF pot up quite a bit (which means that the monitor is extremely green when cold started, but looks totally fine after 5 minutes). Actually, I'd say this is one the better monitors I have. Almost no burn, totally sharp, great convergence and no geometry problems.

Anyway, I was moving the cabs around today and thought I'd take a look at this today. I've checked all the solder joints on the chassi and neckboard and fixed a few suspicious ones, but it didn't change anything. Also changed the G CUT OFF pot to a new one, but no change.

Any ideas what to check next? :)
Agree with @BlinG . Get a rejuv to test the green gun, but I had the same problem couple of years ago.
Was issues with my remote board. A IPA clean, complete reflow and general TLC to my remote fixed it.
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Thanks. I grabbed another known working remote board (and cable) and tested but it unfortunately didn't fix the problem.

I have a couple of other monitors with weak (or non-existing) colors so I should probably get a rejuvenator and test all of them, including this one.

I've never seen a rejuv for sale here in Sweden, so I guess ebay is my only option. *sigh*
could also be your drive transistor on the neck going out from heat and age.
You could swap around the drive transistors to see if that makes any difference. Ideally need a tube tester or a known good spare chassis to swap in to rule out tube or chassis.
Swapped the Q312 (green) transistor with Q302 (red) on the neck board, but no change.

I'll probably test the tube with another chassis next, to see if it's a tube or chassis problem.
yeah so basically if the transistors are fine. What you can do is given you already know which block of the neck board is for the color in question. Check the passives for that colors circuit. So resistors diodes etc.
And also check the Spark gap, that little bulbs sometimes wich just fail due a broken foot..
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Is that what those neon bulbs are for?! I never bothered to find out but I replaced on a chassis once cause the glass envelope looked cracked
Thank you for all the suggestions, much appreciated.

I didn't have much time to look at this yesterday, so I just made some minor progress. I installed a known working chassis together with the tube, and fired it up. Colors looked awesome. I left the monitor running for 30 minutes, but green didn't degrade in any way. So it is not a tube problem, which is good. :)

Next, I'll probably start with checking the resistors etc on the green line on the neck board, or swap some components around to see if I can find the bad one. Stay tuned. :)
Some more progress (sort of).

I checked components on the neck board with my multimeter, and compared with values I got from a known working neck board. I was hoping I could find something easy to fix, but I didn't find a single thing that measured differently between the two boards. So maybe it isn't a problem with the neck board after all...

Anyway, just to be sure - I swapped the neck boards, and the problem is still there when using a different neck board. So I've isolated the issue to the chassis itself, since it's not a tube problem or any faulty components on the remote board or neck board.

So no solution yet. :(
Replacing the M51387P fixed the problem. :)

I have a non-working MS9 chassis that I've been using for parts, so I stole the M51387P from it. Now with the "new" M51387P green is back again, and doesn't degrade over time. I've only been testing it for 15 minutes, but it looks good so far. I'll probably run it for a couple of hours tonight to make sure nothing else is wrong with the chassis.

Thank you @Mukuro_W for the help!
friggin video amps lol. Those have always been on my suspect list some ever since I fixed a some sharp chassis that was missing a color. Thankfully some of these junked Wellsgardners I had used the same amp.

It helps when you have a monitor tester that lets you just inject video signals at strategic points but not everyone has that luxury