• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Discolored corners in crt tv: What is the reason? How to solve?

Tanmay_Karmakar

New Member
Though I have no idea about this type of color rendering problem, I have checked the rgb panel and it as well as every components in it seems to be working fine. I need a solution to repair, kindly help me out.
 

Attachments

Ramussons

Active Member
Such type of problems are quite common to CRT TV displays because of Earth Magnetic field. TV's normally have a Degaussing coil around the front face connected through a Thermistor. Whenever the TV is cold powered on (not from standby), the degaussing coil does its work of nuetralising the effects of the earth Mag field. Check the thermistor. Coils rarely fail.

Also make sure that there are no Magnets around the TV. These include Normal (unshielded) speakers, Magentic toys, etc.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, it's an antique CRT set - and it's a purity error.

As already said, any nearby magnetic fields can cause it - speakers were a common source.

CRT TV's have a deguass circuit that demagnetises the screen when you turn the set on from cold, these can fail - and require the posistor replacing.

Applying an external deguassing field might be required to cure it, and these used to be a common part of n engineers tool kit.

Last option is the purity needs setting up, but you MUST eliminate all other options first - a common cause on Sony Trinitron sets was the scan coils actually sliding back on the CRT neck (part of the setting up is sliding the coils backwards and forwards, then locking them in the optimum place).
 

unclejed613

Well-Known Member
Most Helpful Member
they used to make handheld degaussing coils to eliminate residual magnetism on the shadow mask and internal steel parts on the TV... they are about 18" diameter hoops with a power cord and switch and many turns of wire make up the hoop. the technique is to turn on the coil and move it close to the screen, with the plane of the coil parallel to the CRT face, and move it in a circular motion (best to do this with a colored screen, like red so you can see if you're demagnetizing the crt properly). then continuing the circular motion back away from the screen until the effect from the coil is no longer very visible. then turn the plane of the coil perpendicular to the screen and shut off the coil.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
they used to make handheld degaussing coils to eliminate residual magnetism on the shadow mask and internal steel parts on the TV... they are about 18" diameter hoops with a power cord and switch and many turns of wire make up the hoop.
In my experience those were the best type, but fairly rare (we only had one like that, and were unable to source any more) - most you could buy were rod shaped, a bit like a small rolling pin. I would have thought the chnaces of finding them now is pretty remote, as CRT is long obsolete.
 

unclejed613

Well-Known Member
Most Helpful Member
iirc, there were some TVs an computer monitors where you were supposed to do the degaussing and purity adjustment with the crt facing north or south (or was it e-w?)to minimize the effects of the earth's magnetic field. gegaussing hoops were common in the USA, the only rod shaped ones i ever saw were small ones made for tape head degaussing. i suppose one of those could be used for a crt in a pinch, but they only cover a small area at a time
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
iirc, there were some TVs an computer monitors where you were supposed to do the degaussing and purity adjustment with the crt facing north or south to minimize the effects of the earth's magnetic field. gegaussing hoops were common in the USA, the only rod shaped ones i ever saw were small ones made for tape head degaussing. i suppose one of those could be used for a crt in a pinch, but they only cover a small area at a time
Not much point setting up the purity facing north or south, and then moving it to where you want :D

Best to set it up where it's going to be used, if possible - but otherwise facing North or South is often suggested as the way to go. But it makes absolutely sod all difference :D

Tape head degaussers are tiny, they have to be to reach to the tape heads - crt ones are fairly large and completely different - here's one, presumably American as it has an American style plug?.


I've never seen a square section one before, they were usually round, as I said, like a rolling pin.

I must say, I can't find a single image of one I've ever seen :D

Most of course have long since been binned.
 

unclejed613

Well-Known Member
Most Helpful Member
all of the ones i have seen were circular... one can be made with #22 enameled wire with 100 or so turns (maybe more, the final wire bundle should be about 3/4" (1.9cm) thick and a hoop of 18" (45cm). add a power cord with an inline switch and the whole wire bundle wrapped in electrical tape. that's for 120V coils... for 240V, double the amount of wire (maybe this explains why ring shaped ones were common here, and linear ones more popular across the pond)...

GC Electronics still makes one, and they have 220 in stock for $97.00 each
 

ramondo

Member
In a pinch you can use a larger soldering gun or ac transformer held near the face of tube,just to verify the effect. If you make a DIY degaussing coil MAKE SURE YOU USE a momentary power switch, It wiil generate some heat if left on unattended.
 

unclejed613

Well-Known Member
Most Helpful Member
It wiil generate some heat if left on unattended.
it will also generate heat if you keep it close to most CRTs for too long, as they often have a steel mounting strap that acts like s shorted turn
 

mab2

Member
the degaussing coil of the crt is driven on startup by a component that was commonly called a degaussing 'posistor'; if you google it for that make of tv you may still find it as a spare part - otherwise if you can salvage one from another tv of the same make there's a good chance it will be an identical component.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
the degaussing coil of the crt is driven on startup by a component that was commonly called a degaussing 'posistor'; if you google it for that make of tv you may still find it as a spare part - otherwise if you can salvage one from another tv of the same make there's a good chance it will be an identical component.
As I recall we used to keep three different (and incompatible) types, two were three pin, and the later one was only two pin. Those three fitted pretty all TV's.

If you unclip the plastic top of the posistor, you can often see cracks or damage to the two 'discs' inside, so they are obviously faulty. If you can't see any damage, they are most likely OK.
 

wkrug

Active Member
When degaussing has no effect, the hole mask inside the CRT could be out of position.
Then the CRT is'nt reperable, because You can't open it.
I had this once long time ago at a Sony TV.

I guess the tube has got a mechanical shock in this case to get this failure.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
When degaussing has no effect, the hole mask inside the CRT could be out of position.
Then the CRT is'nt reperable, because You can't open it.
I had this once long time ago at a Sony TV.

I guess the tube has got a mechanical shock in this case to get this failure.
That sounds a HIGHLY unlikely occurrence?, and Trintron tubes don't even have a shadow mask (which I presume you mean by 'hole mask'?), they have an array of vertical wires instead, supported by a small number (two or three) horizontal wires.

As I mentioned early, Trinitrons are very prone to scan coil movement?, did you go through the alignment procedures?. Degaussing doesn't help unless the CRT is correctly setup.

As for shadow mask damage, I have seen that following lightning strikes, where I presume the massive pulse has deformed the shadow mask - which you don't find out until after you're already repaired dozens of faults!.
 

wkrug

Active Member
I've take a look into my TV screen error Book.

It say's:
1. Degaussing unit failure. Degaussing Coil out of position ( That was testet here ? )
2. Faulty Mask by massive mechanical shock

Fact is the Electronic Ray doesn't fit to the luminiscent pixels at some positions of the screen.
Tanmay could show a grid or a test picture on the screen to see other faults too ( E/W or N/S correction ).
 

Nigel Goodwin

Super Moderator
Most Helpful Member
I've take a look into my TV screen error Book.

It say's:
1. Degaussing unit failure. Degaussing Coil out of position ( That was testet here ? )
2. Faulty Mask by massive mechanical shock

Fact is the Electronic Ray doesn't fit to the luminiscent pixels at some positions of the screen.
Tanmay could show a grid or a test picture on the screen to see other faults too ( E/W or N/S correction ).
I suggest your book is a load of crap :D

Neither of those are in any way likely, and massive mechanical shock is more likely to smash the glass than dislodge the shadow mask.

I don't see any relevance whatsoever to pincushion distortion?.
 

Tanmay_Karmakar

New Member
Thanks everyone I have got some idea.. I looked for solution in youTube... And tried degaussing with a drill which helped a little... I've found that the ptc for degaussing coil shows a value of around 1.57 kOhm in circuit in no supply.. is it in good condition?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
What make and model is your TV?.

You can't check a positor very easily with a meter, and certainly not just a simple single reading.

As I mentioned earlier, faulty ones almost always have cracks or chips on the discs inside.
 

EE World Online Articles

Loading
Top