I'm stumped......weird issue

Pengo13

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I have a k4900 chassis with what I thought was an easy fix. The picture looks awesome, but the brightness is unstable + there are flashes of blue every 5 seconds or so on the picture. Thinking that it was a bad flyback, I quickly swapped it for a known good one, but the issue did not go away. It's definitely not the tube as I have plugged in another k4900 chassis and the tube works fine. Below is everything else that I've done:

-Cap kit
-changed flyback
-changed neckboard

I have another chassis, but I would like to fix this one. Has anyone come across an issue like this before? There has to be a cap on the main board outside of the regular cap kit that's defective. Anyone knows which component is resposible for this crazy issue?
 
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I have seen C505 go bad causing this same issue. It is part of the B+ regulating circuit.

Check for cold solder first, if the solder is good replace.
 
awesome....I will try your advise tonight. Thanks for replying.
 
In my case it's not the tube....another chassis works fine with the tube.
 
Does it go blue all over in those flashes? Sometimes flaky neckboard pots will be perfect in one spot, then the tiniest turn will go BLUE OVERLOAD! If your pot is right on that edge, it could be flickering between the two.

You could swap it with the green pot, then see if you get flashes of green...

Nevermind. Saw you swapped the neckboard.

Is there any B+ difference between the good one and the problem one?
 
Replacing C505 did not fix the issue. Not sure what else I can try.
 
I would inspect the 200 section mainly Q205 and Q208. Look for bridges, broken traces, legs etc. Also check the header pins for bridging, bad solder joints.
 
How long was the second chassis installed? If it is a flaky tube you may need to leave the replacement chassis installed and powered on for more than a few minutes in order to see the problem return.

You could also swap the flaky chassis into another machine to see if the problem follows.
 
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