CRT colour patch, degaussing doesn't fix

Thread Starter

Domarius

Joined Sep 1, 2013
28
I've put together a home made arcade cabinet with PC and emulators, so I'm using an old CRT Panasonic TV I picked up off Gumtree.

It's got a purple patch in the bottom left, and I've even bought a degaussing wand – the standard "walk backwards making circular motions" doesn't fix it. Someone recommended a "flicking" technique where you flick the button very quickly to produce random interesting results, and I got something not perfect, but better, but after a few days it reset itself back to having the purple patch at the bottom.

It's internal degaussing doesn't fix it either.

Yes I should try and replace, but they're getting scarce when I've checked the last few times, and it's hard to find one with this set of features (S-Video, and Component input, and not too big but not too small) I like this one, so I'd like to explore repairing... as you can imagine, repair stores don't really do this sort of thing anymore.

Info from back of TV:
Panasonic Colour TV
Model: TX-68PS12A
Chassis: MX10A
240V ~ 50Hz 165W

Fully aware there's enough voltage to kill you, around the front of the screen, I've read a lot about this – though I'm very much a noob. But I'm getting desperate.

Here's a photo of the purple patch in the bottom left:

It's purple while the screen is blue but it's even more pronounced when you have a red scene and the patch is yellow instead. Here it is on Donkey Kong when the girders should be red but they're yellow:


What kind of problem could we be talking about here, and what would be involved in fixing it?
 

ElectricSpidey

Joined Dec 2, 2017
770
If you degauss and it clears up a little but then returns, I would suspect a nearby magnetic field.

Unshielded speaker?

Unless there is a problem with the CRT itself.

CRTs can get magnetized from sitting close to something magnetic when in storage…and that may never be fixed.
 
Last edited:

RichardO

Joined May 4, 2013
2,268
Since you say it goes away and then comes back, I agree with @ElectricSpidey. I suppose it could be a fault in the internal degausssing coil circuit. There is a thermistor that is supposed to slowly turn off the coil after the TV is turned on. The thermistor might be bad. Try unplugging the degausing coil and then manually degaussing. I would expect this would effect how quickly the spot returns.

If you can't fix it by degaussing then it _might_ be a convergence problem. Unfortunately, doing a convergence requires a test pattern generator and some skill in using it.
 

Dodgydave

Joined Jun 22, 2012
8,571
Looks like it's a Pil type tube, there are a set of purity rings on the neck of the tube, as well as static convergence magnets,.

Magenta is caused by a lack of Green (R+B), the tube could have been dropped and disloged the purity magnet.
 

MaxHeadRoom

Joined Jul 18, 2013
19,400
I have seen it happen in this area at the bottom when the room vacuuming is being done and the motor has been switched off when just below the TV set.
Usually a degaussing coil gets rid of it though.
Alot of the tools such as cross hatch generator and degausing coil are hard to find now since the advent of LED/LCD screens.
Max.
 

ian field

Joined Oct 27, 2012
6,539
I've put together a home made arcade cabinet with PC and emulators, so I'm using an old CRT Panasonic TV I picked up off Gumtree.

It's got a purple patch in the bottom left, and I've even bought a degaussing wand – the standard "walk backwards making circular motions" doesn't fix it. Someone recommended a "flicking" technique where you flick the button very quickly to produce random interesting results, and I got something not perfect, but better, but after a few days it reset itself back to having the purple patch at the bottom.

It's internal degaussing doesn't fix it either.

Yes I should try and replace, but they're getting scarce when I've checked the last few times, and it's hard to find one with this set of features (S-Video, and Component input, and not too big but not too small) I like this one, so I'd like to explore repairing... as you can imagine, repair stores don't really do this sort of thing anymore.

Info from back of TV:
Panasonic Colour TV
Model: TX-68PS12A
Chassis: MX10A
240V ~ 50Hz 165W

Fully aware there's enough voltage to kill you, around the front of the screen, I've read a lot about this – though I'm very much a noob. But I'm getting desperate.

Here's a photo of the purple patch in the bottom left:

It's purple while the screen is blue but it's even more pronounced when you have a red scene and the patch is yellow instead. Here it is on Donkey Kong when the girders should be red but they're yellow:


What kind of problem could we be talking about here, and what would be involved in fixing it?
Sounds like a purity adjustment if degaussing doesn't fix it. If you degaussed the back of the CRT - you're probably screwed. The old delta gun tubes had lots of purity adjustments (tabbed magnetic rings on the neck like shift magnets on a mono tube) but were a fair bit of work to set up. The inline tubes that came after were still evolving right to the end - bonded on scan yokes and ever decreasing adjustment options.

A warped shadow mask is a possibility, often caused by an intensity hot spot heating the metal and causing irremediable distortion. a loose shadow mask can also happen - once I got a TV in with a rainbow in the top RH corner. stand the set upside down and the picture was perfect. You seriously don't want to know what happened next...........
 

Thread Starter

Domarius

Joined Sep 1, 2013
28
Thanks heaps everyone. Well the disappointing thing is it looked fine when I got it and during testing while I was building the arcade cabinet. It just seems to have appeared there over time.

This sounds like an external influence, but I can absolutely assure you there is no external magnetic interference, there's nothing much down there, the only speakers near it are one foot above and they are (good quality sounding!) computer desktop speakers, so hardly a threat, especially at that distance, plus the blotch is at the bottom.

Below it are some old consoles and their power adaptors (Sega Master System, Super Nintendo, Nintendo 64) on a shelf but they are about 10-15cm away, separated by a 16mm MDF shelf, and I don't expect this is out of the ordinary for a TV in a standard home theatre situation.

Nevertheless, I've picked up another good looking TV and will swap it out, that will prove once and for all it's not an external magnet issue.

Try unplugging the degausing coil and then manually degaussing. I would expect this would effect how quickly the spot returns.
Thank you - First I guess I need to work out which plug belongs to the internal degaussing coil. But just so I understand clearly; the internal degaussing coil is something that comes on intermittently (eg. during power on) and you're saying it might be doing it wrong, so if I unplug it and use the degaussing wand and leave it, we will see if it comes back by itself over time, and if it doesn't, maybe it's the internal degaussing coil directly introducing the problem itself?

Looks like it's a Pil type tube, there are a set of purity rings on the neck of the tube, as well as static convergence magnets,.

Magenta is caused by a lack of Green (R+B), the tube could have been dropped and disloged the purity magnet.
Thank you for this information. Well if it got dislodged, it didn't take much. I felt I've been relatively very careful with it, so if it did get disloged, it was from a relatively minor bump, eg. from one corner of the TV coming down the small distance of me pulling my fingers out from underneath. Which may be possible if it's old I guess?

Sounds like a purity adjustment if degaussing doesn't fix it. If you degaussed the back of the CRT - you're probably screwed. The old delta gun tubes had lots of purity adjustments (tabbed magnetic rings on the neck like shift magnets on a mono tube) but were a fair bit of work to set up. The inline tubes that came after were still evolving right to the end - bonded on scan yokes and ever decreasing adjustment options.

A warped shadow mask is a possibility, often caused by an intensity hot spot heating the metal and causing irremediable distortion. a loose shadow mask can also happen - once I got a TV in with a rainbow in the top RH corner. stand the set upside down and the picture was perfect. You seriously don't want to know what happened next...........
Well I definitely didn't degauss the back of it, I can assure you!

Well... a lot of this sounds irreparable by a tinkerer like me. But that's what I posted to find out, so I really appreciate everyone's advice! I'm still open to ideas, since I will keep it as a back up if the new one doesn't work (I can see myself hoarding these things until OLEDs become a viable option...)
 

ian field

Joined Oct 27, 2012
6,539
Thanks heaps everyone. Well the disappointing thing is it looked fine when I got it and during testing while I was building the arcade cabinet. It just seems to have appeared there over time.

This sounds like an external influence, but I can absolutely assure you there is no external magnetic interference, there's nothing much down there, the only speakers near it are one foot above and they are (good quality sounding!) computer desktop speakers, so hardly a threat, especially at that distance, plus the blotch is at the bottom.

Below it are some old consoles and their power adaptors (Sega Master System, Super Nintendo, Nintendo 64) on a shelf but they are about 10-15cm away, separated by a 16mm MDF shelf, and I don't expect this is out of the ordinary for a TV in a standard home theatre situation.

Nevertheless, I've picked up another good looking TV and will swap it out, that will prove once and for all it's not an external magnet issue.

Thank you - First I guess I need to work out which plug belongs to the internal degaussing coil. But just so I understand clearly; the internal degaussing coil is something that comes on intermittently (eg. during power on) and you're saying it might be doing it wrong, so if I unplug it and use the degaussing wand and leave it, we will see if it comes back by itself over time, and if it doesn't, maybe it's the internal degaussing coil directly introducing the problem itself?


Thank you for this information. Well if it got dislodged, it didn't take much. I felt I've been relatively very careful with it, so if it did get disloged, it was from a relatively minor bump, eg. from one corner of the TV coming down the small distance of me pulling my fingers out from underneath. Which may be possible if it's old I guess?


Well I definitely didn't degauss the back of it, I can assure you!

Well... a lot of this sounds irreparable by a tinkerer like me. But that's what I posted to find out, so I really appreciate everyone's advice! I'm still open to ideas, since I will keep it as a back up if the new one doesn't work (I can see myself hoarding these things until OLEDs become a viable option...)
The PTC degauss thermistors usually have around 6 minutes recovery time. I would expect a relay switched PTC in a console, but there are 2 manual types - PTC/PTC one of the elements is across the mains in and cuts off the other by heating it. The other is PTC/NTC - the NTC is in series with the AC feed and heats the other as a result of current draw - there is potential for a loud bang if you fit the wrong type.
 

Thread Starter

Domarius

Joined Sep 1, 2013
28
The PTC degauss thermistors usually have around 6 minutes recovery time. I would expect a relay switched PTC in a console, but there are 2 manual types - PTC/PTC one of the elements is across the mains in and cuts off the other by heating it. The other is PTC/NTC - the NTC is in series with the AC feed and heats the other as a result of current draw - there is potential for a loud bang if you fit the wrong type.
Thanks Ian. From what I'm reading, if it's a "purity" problem and involving those magnetic rings at the back, I have no hope. I suppose what you're suggesting is replacing the PTC just to see what happens, to rule out that problem, so I will keep that in mind and come back to this if the new TV doesn't work out. Having trouble making sure the emulator programs don't use a video mode outside of 480i, limited success but possibly more reasonable to attempt than replacing components in the other TV.

A warped shadow mask is a possibility, often caused by an intensity hot spot heating the metal and causing irremediable distortion. a loose shadow mask can also happen - once I got a TV in with a rainbow in the top RH corner. stand the set upside down and the picture was perfect. You seriously don't want to know what happened next...........
Well now I'm curious...
 

ian field

Joined Oct 27, 2012
6,539
Thanks Ian. From what I'm reading, if it's a "purity" problem and involving those magnetic rings at the back, I have no hope. I suppose what you're suggesting is replacing the PTC just to see what happens, to rule out that problem, so I will keep that in mind and come back to this if the new TV doesn't work out. Having trouble making sure the emulator programs don't use a video mode outside of 480i, limited success but possibly more reasonable to attempt than replacing components in the other TV.


Well now I'm curious...
Refitted the tube upside down, spun the yoke round to get the picture right way up and wedged a lino tile under the anode cap to stop it cracking over.
 

ian field

Joined Oct 27, 2012
6,539
Nice, McGyver would be proud :)
Also been known to fix holed pistons on motorcycles with short countersunk bolts.

Did a motorcycle cut & shut once to get a front brake that was fit for purpose - the handling was a bit too interesting so I donated it to a bunch of kids to rag across the fields.
 

Thread Starter

Domarius

Joined Sep 1, 2013
28
Also been known to fix holed pistons on motorcycles with short countersunk bolts.

Did a motorcycle cut & shut once to get a front brake that was fit for purpose - the handling was a bit too interesting so I donated it to a bunch of kids to rag across the fields.
lol I hope they're ok!
 

KL7AJ

Joined Nov 4, 2008
2,224
If you degauss and it clears up a little but then returns, I would suspect a nearby magnetic field.

Unshielded speaker?

Unless there is a problem with the CRT itself.

CRTs can get magnetized from sitting close to something magnetic when in storage…and that may never be fixed.
Yep....that would be my first guess.
 

SgtWookie

Joined Jul 17, 2007
22,210
Perhaps the screws you used to attach the TV shelf were magnetized? The built-in auto degausser should take care of that after a few on/off cycles, but it sounds like your set's degausser isn't working, as you only notice a difference when you use an external degausser.
 

ian field

Joined Oct 27, 2012
6,539
Perhaps the screws you used to attach the TV shelf were magnetized? The built-in auto degausser should take care of that after a few on/off cycles, but it sounds like your set's degausser isn't working, as you only notice a difference when you use an external degausser.
Neodymium screws are quite rare.

Hi-Fi speakers are high on the list of suspects. The ones in the set should have adequate screening.
 
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