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 Post subject: Drifting purity problems on CRT, casued by bright elements
PostPosted: Fri Jun 26, 2020 7:04 am 


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So my Panasonic ct-27d20b, curved shadow mask with component made in 2000, has some pretty severe purity issues.

Basically, anywhere on the screen you have a bright object for a short period of time, the purity gets totally screwed, and it takes about a minute to return to normal.

I took some pics.

First picture shows a screen right after switching on, mostly correct, but has a purity issues in top left.

2nd and 3rd picture show how bright objects on right side will turn that side yellow

4th and 5th pictures show how bright objects on left side will turn that side magenta.

Last picture shows how bad the picture can get when the whole screen is white. Purity is basically only correct on the middle-right area of the screen

https://imgur.com/a/H6pm2c7

So what is happening here?

My two best guesses are that there is some magnetic interference between the yoke and electron gun or neck board. Or the actual shadow mask is getting magnetized by the electron beam.

Anybody seen this before?


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Fri Jun 26, 2020 11:39 am 


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Probably a silly question, but did you try an external degaussing coil? Looks almost like the shadow mask has been magnetized.


Last edited by Josh128 on Mon Jul 06, 2020 1:58 pm, edited 1 time in total.

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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Fri Jun 26, 2020 4:28 pm 


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Yeah I'm going to buy a degaussing coil, because it will be nice to have one anyway even if it doesn't fix this CRT

But what's confusing me is how this purity issue "migrates" around the screen based on where bright elements are shown. Seems like purity issues are typically more static and don't change based on what you're showing on the screen


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sat Jun 27, 2020 5:32 am 


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I think i've seen this, but not to anywhere near the level it's happening here. One of my arcade setups has purity that wanders just slightly - i took it to be because it's a consumer tube that was never meant to be laid back at 45-degrees in a lowboy, so the yoke/purity (and to a lesser extent convergence) are sitting on "just ok" with no wriggle room, and things like bright patches on the screen can mess it up slightly. But the effect is so small i'm the only one that would notice. Degaussing doesn't change this effect though.

Do you get the same result if you rotate the tube 90-degrees on the flat, to face east rather than north or whatever?


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sat Jun 27, 2020 9:44 pm 


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buttersoft wrote:
Do you get the same result if you rotate the tube 90-degrees on the flat, to face east rather than north or whatever?


Basically the same result, the bad spots just move around slightly. Like if I turn it 180 degrees, the magenta spot flips to the from the top corner to the bottom corner. And I power down before I turn it.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sun Jun 28, 2020 12:35 am 


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Does it make the distinctive degauss "thunk" sound when you turn it on? if not, maybe the degauss thermistor has gone bad, or the solder joints have cracked.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sun Jun 28, 2020 3:30 am 


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I hear a noise, but it's not the heavy thunk. I thought it just has a quieter degaussing system, but maybe what I'm hearing is just other parts of the CRT switching on. I guess I'll take a look at the thermristor.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sun Jul 05, 2020 6:13 pm 


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Damn, so I bought a degaussing coil. I degaussed about once an hour all day yesterday.... no change. The coil definitely works, the picture goes funky when I'm using it. But as I move the coil further away, the "bad spots" settle back into place.

Is this what happens with a heavily magnetized tube? Surely it would change at least a little? I'm wondering if the purity problem is caused by interference from a component running out of spec, maybe near the yoke.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sun Jul 05, 2020 6:19 pm 



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I would also have thought degaussing, cause looking at the picture that looks really severe. Usually when I saw purity issues in the past it was much more subtle than in that screenshot.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Mon Jul 06, 2020 2:03 pm 


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BazookaBen wrote:
Damn, so I bought a degaussing coil. I degaussed about once an hour all day yesterday.... no change. The coil definitely works, the picture goes funky when I'm using it. But as I move the coil further away, the "bad spots" settle back into place.

Is this what happens with a heavily magnetized tube? Surely it would change at least a little? I'm wondering if the purity problem is caused by interference from a component running out of spec, maybe near the yoke.



No, if the degaussing coil doesnt work, its probably not a magnetized tube. Ive seem youtube videos where some guy sticks a 1.5" neodymium magnet right onto the screen of a CRT and makes all kinds of colors and black-hole looking effects-- then, he simply spins the magnet in front of the tube as he pulls it away and the screen basically returns to normal. Of course, he is using the same power magnet to degauss that magnetized in the first place, but still-- the external coils are supposed to be the best.

I wonder if you have something magnetic inside the set that is "loose" and is coming near the tube? Speaker magnets, etc? could be that the yoke itself is causing the issue.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Mon Jul 06, 2020 6:42 pm 


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it might be time to mess with the beam landing/purity rings. This is from a sony service manual, so the process for the panasonic may be slightly different, but this should give you an idea of what's involved.

https://www.manualslib.com/manual/15968 ... =15#manual


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Tue Jul 07, 2020 12:07 am 


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Josh128 wrote:
I wonder if you have something magnetic inside the set that is "loose" and is coming near the tube? Speaker magnets, etc? could be that the yoke itself is causing the issue.


It's been a while since I took the back off, but I don't remember seeing anything out of place, or even any bulging caps. I'll definitely take another look tonight or tomorrow.

As for the yoke itself causing the issue, I don't see how a yoke problem would be affected by brighter beams coming from the electron beam. Unless something on the neckboard is running out of spec maybe, and causing magnetic interference?

cyborc wrote:
it might be time to mess with the beam landing/purity rings. This is from a sony service manual, so the process for the panasonic may be slightly different, but this should give you an idea of what's involved.

https://www.manualslib.com/manual/15968 ... =15#manual


When I first got this, I messed with the purity rings a little, but they were very hard to move. I assumed this meant they were still in the factory position, which should be the ideal position, right?

Furthermore, the fact that the purity changes gradually based on the intensity of the colors shown (as pictured), makes me think the purity rings aren't involved. Like, large bright objects on the left side will throw off purity on the left side, even for a minute after the bright object went away.

But I will take another look at them, and follow the protocol you linked. A similar protocol is listed in the Panasonic service menu as well

I think my last resort is going to be putting magnets all over the tube and hoping it doesn't throw off convergence too much.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Tue Jul 07, 2020 1:43 am 


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BazookaBen wrote:

As for the yoke itself causing the issue, I don't see how a yoke problem would be affected by brighter beams coming from the electron beam. Unless something on the neckboard is running out of spec maybe, and causing magnetic interference?


The yoke controls where and how the beams land, so if its pulling those beams to the locations in question, thats why those spots could be brighter or discolored. Of course, the neckboard produces the currents that flow in the yoke, so they go hand in hand.

This is all just a guess though, through elimination. If the shadow mask isnt magnetized, logic would seem to dictate that it has to be something that controls the beams or the guns themselves.


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Tue Jul 07, 2020 2:22 am 


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Josh128 wrote:
The yoke controls where and how the beams land, so if its pulling those beams to the locations in question, thats why those spots could be brighter or discolored.


But the yoke doesn't really do anything different for bright colors vs dark colors, in my understanding. It's just oscillating voltages based on the sync intervals, right?


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Tue Jul 07, 2020 2:59 am 


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BazookaBen wrote:
Josh128 wrote:
The yoke controls where and how the beams land, so if its pulling those beams to the locations in question, thats why those spots could be brighter or discolored.


But the yoke doesn't really do anything different for bright colors vs dark colors, in my understanding. It's just oscillating voltages based on the sync intervals, right?

that's correct but a systemic voltage shift can affect it too


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 Post subject: Re: Drifting purity problems on CRT, casued by bright elemen
PostPosted: Sun Jul 12, 2020 3:47 am 


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So I noticed today the yoke was pretty hot when I was adjusting it. I know electromagnets lose a little bit of their power when they get hot.

But I don't if that would explain how these purity problems happen in "hot spots" after showing bright objects in those spots. I would think any heat would disperse across the entire yoke, which would mean no "shifting" purity problems

So I'm kind of running out of ideas, I might give up soon and recycle/give away the set. I want to fix it, but from my research it seems like a very rare problem, so it could be a design flaw for this chassis, or maybe a manufacturing defect. I really don't have the technical knowledge to track down the root cause.


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