Color Purity Problems on a CRT

[maxtherabbit]

When I first got the CRT there were no purity problems - however after transporting it home and adjusting the geometry in the service menu, I noticed the right side of the screen would take on a green hue and the left side would take on a blue hue when viewing a pure white screen from the 240p test suite. Power cycling it to trigger the internal degauss seems to clear them up, but they gradually return after its been running for a while.

I don't believe the purity magnets on the neck need to be adjusted because they have not been touched since it was working correctly. The TV was not subjected to any great amounts of shock during transportation.

Is it possible that some component has failed causing the TV itself to produce the magnetic distortions? Or do I simply need to get an external degaussing coil to do a more powerful degauss?

[maxtherabbit]

after letting the TV sit for 20 minutes, I powered it on and it looked perfect

after running for about 45 seconds the purity distortions returned gradually

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

[rama]

Yeah, it could be an electromagnetic field that builds up when the unit runs.
Is there anything with large coils nearby? Power lines in the wall maybe?

Also, have you increased any currents in the service menu?
That could go over some limit of the chassis and create the issue.

[Lawfer]

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

A stepdown converter?

[maxtherabbit]

Yeah, it could be an electromagnetic field that builds up when the unit runs.
Is there anything with large coils nearby? Power lines in the wall maybe?

Also, have you increased any currents in the service menu?
That could go over some limit of the chassis and create the issue.

I've got the TV pretty much in the middle of the room. Only things remotely close to it that could have an effect are my front 3 speakers, but they are 4-5 feet away.

Only service menu items I adjusted were v size, h size and pos, parabola (pincushion), v linearity, and v centering. Most were very small adjustments

As a test, I reverted all the changes I made and I noticed it did maybe diminish the distortions slightly, but they remained.

[maxtherabbit]

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

A stepdown converter?

You mean the mains transformer in the set? It could be, I did notice it making an odd sound beyond the usual 15kHz whine

[Dochartaigh]

issues like this usually means that it took a hit at some time (even a pothole driving home, or you moving it a couple times can do it). The grill inside could even be damaged, but normally you have to adjust the yoke up a little bit to clear it up - which I've had mixed success with. Sometimes just a neodymium magnet placed near that area once it's opened up (on something metal, or tape it in place), can clear this up like 95% of the way and I've commonly done that many times before. It's probably a band-aid, but if it makes it playable and it looks a TON better, I see no harm in it.

A higher power degaussing wand can also help they say, but I have one of the big ones (which gets so hot I can only run it for a minute at once before I have to let it cool down), and it's never, ever, permanently fixed a problem like this.

[BazookaBen]

A higher power degaussing wand can also help they say, but I have one of the big ones (which gets so hot I can only run it for a minute at once before I have to let it cool down), and it's never, ever, permanently fixed a problem like this.

Well damn that's disheartening. I have one with this issue, but I adjusted the yoke enough to almost completely get rid of it at startup, and it goes away completely after about a 10 minute warmup

[maxtherabbit]

issues like this usually means that it took a hit at some time (even a pothole driving home, or you moving it a couple times can do it). The grill inside could even be damaged, but normally you have to adjust the yoke up a little bit to clear it up - which I've had mixed success with. Sometimes just a neodymium magnet placed near that area once it's opened up (on something metal, or tape it in place), can clear this up like 95% of the way and I've commonly done that many times before. It's probably a band-aid, but if it makes it playable and it looks a TON better, I see no harm in it.

A higher power degaussing wand can also help they say, but I have one of the big ones (which gets so hot I can only run it for a minute at once before I have to let it cool down), and it's never, ever, permanently fixed a problem like this.

would it change your appraisal of the situation if I told you the purity problems only manifest themselves after viewing a pure white screen for a duration? and when you change the content to something else, they gradually go away

[Lawfer]

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

A stepdown converter?

You mean the mains transformer in the set? It could be, I did notice it making an odd sound beyond the usual 15kHz whine

No a step down converter is an external unit that decrease the voltage, I don't know where you live but if you are in a place where the voltage is 220-240V and you want to use USA/Japanese hardware you would most likely need to use a step down converter to convert to 220-240V to 110-120V.

[maxtherabbit]

No a step down converter is an external unit that decrease the voltage, I don't know where you live but if you are in a place where the voltage is 220-240V and you want to use USA/Japanese hardware you would most likely need to use a step down converter to convert to 220-240V to 110-120V.

Oh. No I'm using US equipment in the US

[maxtherabbit]

couple more things I've tried:

moving the main board out of the case, away from the tube - in case any components were radiating flux - no effect

turning up and down the SCREEN VR on the flyback - no effect

also examined the yoke and it appears to be still at its factory set position with no movement or play

at this point it looks like I'm down to a potentially bad component in the deflection circuit or something is wrong with the tube itself

something else of note: this TV does not have the typical rotating magnets in the yoke for purity control, there are magnets in the yoke but they are fixed... is there some kind of electronic purity adjustment that I've never heard of?

[buttersoft]

It won't be the deflection circuits. In an undamaged tube, it's only physically possible for the red gun to fire through the red holes/slots in the mask or grille, and same for the other two colours. With the exception of blanking-level issues, colours being off is always due to the tube. Try a few small magnets around the bezel.

Philips tube? Some of those had the convergence and purity set in place at the factory. Normally those are pretty good tubes though.

[maxtherabbit]

It won't be the deflection circuits. In an undamaged tube, it's only physically possible for the red gun to fire through the red holes/slots in the mask or grille, and same for the other two colours. With the exception of blanking-level issues, colours being off is always due to the tube. Try a few small magnets around the bezel.

Philips tube? Some of those had the convergence and purity set in place at the factory. Normally those are pretty good tubes though.

do you think purchasing an external degaussing coil and using it would be worthwhile?

the TV is sharp branded, I assume they made the tube but I don't know for sure

[buttersoft]

do you think purchasing an external degaussing coil and using it would be worthwhile?

Probably not for this particular issue, but there's a slim chance. In general, if you're going to play with more CRT's, then yes, a degaussing wand is nice to have, if you have the money.

Back to the tube for a second - does it have the convergence and purity rings in place, with the little tabs sticking off them? If it does, they should free up somehow. The only tubes without adjustment that i've heard of were the Philips ones, and the neck assembly is just solid, nothing much visible.

[BazookaBen]

About the yoke, it may seem like it hasn't shifted, but even a minor shift in the yoke can really throw things out of whack. And the purity magnets arent on the yoke, they're usually like an inch or two behind it, closer to the electron gun.

[viletim]

after letting the TV sit for 20 minutes, I powered it on and it looked perfect

after running for about 45 seconds the purity distortions returned gradually

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

What if you unplug the degauss coil? Does it look the same during start up from cold?

The degauss circuit itself can magnetise the CRT's shadow mask when it fails. It's also possible that a big EMP blast (lightning) magnetised the electron guns in the CRT. Though that doesn't explain why it's good at start up...

[maxtherabbit]

after letting the TV sit for 20 minutes, I powered it on and it looked perfect

after running for about 45 seconds the purity distortions returned gradually

it's like the initial degauss is working but something is actively producing magnetic distortions while its running ???

What if you unplug the degauss coil? Does it look the same during start up from cold?

The degauss circuit itself can magnetise the CRT's shadow mask when it fails. It's also possible that a big EMP blast (lightning) magnetised the electron guns in the CRT. Though that doesn't explain why it's good at start up...

that's an interesting idea, I actually was wondering if there could be current leaking through the degaussing coil that's magnetizing the mask while it runs

I'll try that and report back

[Josh128]

Hey Max--having the IDENTICAL problem here on my new Wei-ya/Makvison monitor. Starts OK except for right corners, but color splotches creep in on white white screens gradually. VileTims
suggestion of bad degaussing circuit sounds VERY plausible-- I dont hear the "konk" sound of the degaussing pulse when powering the monitor on.

Did you ever try disconnecting the d-circuit and did it help?

[maxtherabbit]

no I kinda forgot about it to be honest, it hasn't been happening enough to bother me

[Josh128]

Did it look ~similar to what Im showing here on my white screens? Because the way you describe your issue sounds identical to what Im seeing here.

[maxtherabbit]

Yes more or less the same effect but it looks like you've got it worse

[Josh128]

http://www.repairfaq.org/sam/crtfaq.htm

"An aperture grille, used exclusively in Sony Trinitrons (and now
their clones as well), replaces the shadow mask with an array of
finely tensioned vertical wires. Along with other characteristics of
the aperture grille approach, this permits a somewhat higher possible
brightness to be achieved and is more immune to other problems like
line induced moire and purity changes due to local heating causing
distortion (doming) of the shadow mask.

From what Ive gathered, if its not degaussing related, its going to either be purity related (either normal adjustment needed or localized heating causing the issue) or possibly damage of the shadow mask. From what Ive read, the shadow mask should be quite a bit more difficult to damage due to rough handling than an aperture grille, due to the high tension needed for the AG. The shadow mask requires no such tension. On top of that, though I know its not proof it hasnt been dropped, but the box was in really good condition with no apparent damaged sides or corners to speak of when I received it.

From the way the color patch "blooms" on white screens, Im thinking Sams (absolutely awesome) CRT FAQ/Repair log may be right and this may be due to heat distortion of the shadow mask. Whether this is fixable with purity adjustments and caused by manufacturing defect or rough handling, Im not sure. I can say that the CRT is manufactured by LG, and not Wei-ya, so it "should" be of decent quality, but who knows.

Im currently having the guy on the trading station attempt to contact his sales person about the issue and any possible warranty because if it is indeed something that cant be fixed Im out fucking $650 unless the guy issues a refund or FedEx insurance pays a claim-- but Im not sure how easy its going to be to prove that this happened during shipping.

This weekend, I guess I'll try the degaussing coil thing and possible attempt to adjust purity once the screen warms up, but its hard to imagine that any changes that might fix this issue on a white screen wont fuck over the image on darker screens.

[BazookaBen]

Josh, I've been dealing with the same problem on my Panasonic TV. When I originally commented in this thread, I had just got the TV. I thought I had fixed but really I just didn't understand the problem yet.

In my case, the heat related warping of the slot mask seems to be what is going on. But what is confusing for me, is why it is happening on my TV, but there are a bunch of other similar Panasonic TV's I've seen from other posters, here and on reddit, that don't have the problem. I'm starting to think that maybe my tube has been dropped in the past, and the slot mask is more "pliable" because it might be partially detached. But I'm not 100% sure about that either, because smacking the side of the TV doesn't really change anything, which in my understanding I should be seeing some color shimmering if the slot mask was loose.

Anyway, I've mitigated the problem significantly by adding a dozen or so magnets in the back. Now the only time I get noticeable purity changes is when really a bright spot, like a full-white window in Windows, is left in one spot for while. When the whole screen is white, I hardly see any changes.

But this is a TV I got for free, so this is fine for now. And I'll probably end up swapping it another Panasonic or JVC at some point

[Josh128]

But what is confusing for me, is why it is happening on my TV, but there are a bunch of other similar Panasonic TV's I've seen from other posters, here and on reddit, that don't have the problem. I'm starting to think that maybe my tube has been dropped in the past, and the slot mask is more "pliable" because it might be partially detached.

Hey Ben-- its unclear to me as to whether the heat related "bowing" of the sm is due to being dropped or due to some kind of manufacturing defect. And more confusingly, Ive seen several similar looking problems online that degaussing did not help, but either purity ring and/or yoke adjustment DID. One of them was a WGk7000 monitor that showed a pretty wicked oval shaped patch in the blue color that would "bloom" shortly after the set was powered on. The guy eventually fixed the monitor quite nicely with ring and yoke adjustments-- but the first thing he tried was external degaussing, which did nothing. As far as tapping or knocking the screen to induce a shimmer, that supposedly only really works with Trinitron aperture grille sets due to the piano-string type setup of those vs a standard sm. I guess if the sm is barely hanging on (not sure how they are mounted/secured) tapping could show something, but I think thats very rare.


Before: https://www.youtube.com/watch?v=Gjdm-OMNOyw

After: https://www.youtube.com/watch?v=A9GO7_eYSgY&t=426s



This gives me a bit of hope that I may be able to do something here, but Im not at all confident. The wife is going to be working all day tomorrow, so I plan to hunker down and see what if anything I can make happen with this *brand new* screwed up monitor. Will post updates. Also, Im reserving my opinion on the seller who sold me this, give him a chance to see if he does anything to help make this right-- had I bought this from Betson Imperial instead of the seller here, It would have cost me about $70 more, but I would have certainly had some kind of guarantee against DOA. This was indeed a brand new set, the guy seems to be a stand up guy, but if I end up losing $650 and end up with a boat anchor, I'd certainly advise others NOT to buy one without some kind of warranty against DOA.

[maxtherabbit]

My gut says it's a manufacturing defect not impact damage, as least with respect to our slot mask sets. I've never dropped mine

[Josh128]

Im thinking the same thing, but cant be sure, because the guy I bought it from did not test before shipping as the units were brand new in box. He only opened them to check for physical damage and insert some additional cushion/packing material. After doing some more testing, Im almost 100% positive the problem is the warping mask when heated by bright colors-- I performed the "tap" test on the unit after the large patches of discoloration bloomed on a bright screen, and lo and behold, when I knock on the screen (think doorknock with knuckles) in the areas affected, those areas exhibit a slight visible "vibration/wobble" , while non-affected areas dont. Shadow masks shouldnt do that at all, but if a section bows out / comes out of the same plane as the rest of the mask, its essentially "floating" and it makes perfect sense to see wobble a tiny bit if tapped on.

I played a bit with convergence and purity rings last night, marking their positions first, to no avail. The only thing that worked, and worked fleetingly, was passing a neo magnet over the affected areas, I was able to "erase" the discoloration and return white to white somewhat, apparently by magnetizing the warped areas in such a way that pulled the beams in properly or something, but within 5 minutes the patches were back again.

Im so sick with this. I really hope Im not out all this cash with nothing to show for it at all. Still waiting on hearing back from the seller, who contacted his sales rep. He bought a pallet of units straight from Huai-i electronics in Taiwan.

[BazookaBen]

Savon Pat, a guy that refurbishes CRT's in California, has stopped using UPS and Fedex completely. He only ships CRT's on a pallet via freight now, because he said they almost always were damaged in transit with the UPS and Fedex.

[Josh128]

Sad thing is, I was told the same by Betson Imperial, the authorized US distributor for these units. I "saved" about $70 by going with the seller here and choosing FedEx Ground.

[Josh128]

Took a few pics when I could on scenes before the blooming crept in. As soon as bright scenes happen you have about 20-30 seconds before the ugly color blotches begin blooming. Despite what many say about the Wei-ya, this thing has an incredible picture when working properly, its a damned shamed this unit is messed up. After seeing the picture this crippled unit puts out, I most certainly want to put a non-defective one in my cab. It looks every bit as good in 15KHz as my Wells Gardner K7000, except for worse convergence in the corners, but thats to be expected with the flat vs curved tubes I suppose.

Both 15 KHz and 25 KHz look amazing, 31 KHz is great too, but I dont have any pics. The color in particular impressed me a lot, many of the photos dont do it justice. Very nice, and uniformity isnt bad for such a flat CRT. Not perfect by any means, but a bit better than my FD Trinitron Wega.

15KHz Mode (All these taken with cellophane still on screen!):







25KHz Mode (Partially removed cellophane):