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CRT brightness/fading issue


buttersoft

Question

This is a PC CRT question, but I thought to borrow some expertise from the AA community :)

 

I have a Smile KFC CA-2011 PC CRT rebadged by Viewmaster, and I'd like help determining if the capacitors are failing. The screen is not anywhere near bright enough unless I set the termination to high - and then it gets blurry from signal reflection along with the fact it's still not very bright.

 

However, if I displayed any one quarter of the screen white, that quarter can get fairly bright. But if i then changed the whole screen to that same white, it only stays bright for an instant and then fades to being much dimmer overall. So the smaller the overall area that needs to be bright, the brighter that area can get, but when the whole screen is meant to be bright, the overall brightness fades within half a second or so. Or doesn't fade; just never gets very bright, if no part of the preceding image was bright.

 

Does this mean the capacitors are wearing out? Given the phosphors are capable of being bright as long as the whole screen isn't, i'd say they're ok. And given the monitor is in perfect focus with the termination set to 75R, i presume the flyback is in reasonable condition.

 

I've actually pulled it apart and kinda begun cleaning it and removing the caps, so i hope they're the problem!

 

Any help greatly appreciated.

 

PS i'd also love to know if there are certain caps it's not worth replacing, etc. Do I only need to do the neckboard, or is doing the whole (huge) chassis a good idea? Would the PSU be okay to leave alone?

 

EDIT: I'm thinking it's probably not some sort of over-voltage protection for the RGB signal? The brightness setting, for a white image, scales up from really dim at zero to still bad but brighter at just under halfway. Sliding the brightness higher from that point does nothing. It would seem weird to include a circuit on a monitor that cut the brightness due to overvolting without giving the user a choice - why not just limit the brightness range to the lower 50% to begin with? Does that make sense? Maybe there's a pot on the chassis though, I'll have to have another look.

Edited by buttersoft
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