LG TV Model 21SA3RL Magnetized... ?

[DkinG]

Hello...

I have a 21" CRT TV that works very well... But it has a very large circle that is magnetized so i can't see the correct color in the screen... I know that some TV's like this has a circuit that solves this issue. I've been looking for the Service manual and i can't find it. I didn't open the TV because I want to do the best effort to solve this problem before doing that.

So all the sugestions are welcome and if you could provide me the service manual, it could be great.

Greatings...

David Mix

[wraper]

All CRT TVs have demagnetization loop. It is just a coil around CRT close to the screen. When TV is powered up AC current flows through it. Coil is powered directly from mains through NTC PTC thermistor or PTC + relay. When PTC heats up quickly, current decreases drastically and do not affect CRT operation after that.

[DkinG]

All CRT TVs have demagnetization loop. It is just a coil around CRT close to the screen. When TV is powered up AC current flows through it. Coil is powered directly from mains through NTC thermistor or NTC + relay. When NTC heats up quickly, current decreases drastically and do not affect CRT operation after that.

So if this TV has that Demagnetization loop and it still Magnetized, it means that something is wrong... Maybe I should open it... 

[wraper]

Probably NTC PTC blow up and demagnetization do not work. However if CRT is magnetized high enough, like someone played with magnet in front of the screen, internal demagnetization loop will not be beefy enough. If that the case, you could use external coil powered from AC to fix problematic region. Another possibility is that there is something wrong with beam alignment magnets located on CRT neck. Don't recommend to touch them however, might be PITA to adjust them if alignment disturbed. Oh, memories from the past 

[DkinG]

After this circuit makes its job, it should be locked or something like that right?... because if the NTC get cold it should start working again... So I think it could be a problem too... That's what i expect If all the other things are normal...

Cheers!!...

David Mix

[wraper]

It works only for a short moment after power up. If there is no relay, it works only when using physical switch on the front of TV or disconnecting from mains. And PTC remains hot all the time when on standby. To cool down it will require at least 10 minutes powered off, maybe more. If there is a relay, demagnetization should work every time when switching on from the remote. As of schematic, probably this is it: http://elektrotanya.com/lg_21fu6tl-tlg-rlg-t4-z4_chassis_cw81b_sch.pdf/download.html  At least seems that it should be the same chassis after a bit of googling.

[wraper]

If this is right schematic, here it is on picture. As you can see, relay is optional. So probably you need to use physical switch. BTW it is PTC, not NTC, brainfuck, 4 at night here ha ha.
Edit: there is an error on the schematic, T802 is drawn wrong.

[helius]

the degauss oscillator circuit on a color CRT turns on for a couple of seconds when the monitor is switched on. on some units it can also be triggered manually using a button.
it is audible as a HMMMMM... sound with decreasing volume. it also is easily visible by a wavering of the entire image, like ripples in a pond. the magnitude of the oscillation is damped, so that it fades away before switching off at the end of its cycle.
degaussing is used to remove residual magnetic fields from a magnetic surface, and must always be decreased in power like this to leave the surface in a demagnetized state.
however, the degauss system is not able to fix all cases of magnetization, just those that arise in the course of normal operation (for example, the magnetic field that is induced when the CRT is turned relative to the Earth's field). If a strong magnet is placed too near to the CRT, it can leave the surface magnetized beyond the capacity of the degauss system.
another common cause of the problem is damage or tampering to the purity magnets around the CRT's neck.

[DkinG]

Thanks!!!.... It will be helpfull... So i have to check the PTC thermistor (i think its labeled TH801 / TH802) and The relay, if it has one... Thank u wraper, maybe in a few hours I can check that and see what does it have.

I'll tell you whaen I Check it.

Greetings and have a Good Night There... Here its 11:15 PM now...

Cheers!

David Mix

PD: Thank u Helius for your explanation, now i can understand much better how degauss circuit works... I hope it works with a relay...

[SeanB]

TH801/2 will be a black or white plastic cased component right near the mains power input. The 3 leads are easy to unsolder, and when it is out shake it. A faulty one will typically rattle. Please note that you must make a note of the orientation of the writing on the top, and place the new one in the same direction, as some are symmetrical and can be put in backwards. If you put it in backwards it may not work well.

[DkinG]

Hello SeanB... Do you mean this device, from the pic or something like that?...

Greatings!

Edit: f this is the one i should find, how do i test it? Should I have to measure this device?

[wraper]

It should not rattle inside. IIRC measured resistance should be somewhere around tens of ohms from central to both another pins. As this is 3 pin device (there is 2 pin option on the schematic), seems that your TV do not have the relay, therefore degauss coil will activate only by switching mechanical power switch on tv and waiting at least 10 minutes between switch off and switch on again.

[RLBennett]

I always hated doing the convergence alignment on tv sets. Always screwed it up looking at the mirror and turning the wrong side of the convergence board pots and inductors. If you have an old school soldering gun, you can try that too. Every shop I worked at had a "De-Lousing" coil. Ahhh, the good 'ol bad ol' days![emoji3]
Good luck!


Sent from my iPad using Tapatalk

[RLBennett]

Also, after you turn on the set, do not reorient the set. I replaced a few varistors or thermistors in the de-lousing circuit. Have fun and be careful poking around. When it bites you, it is not pleasant. At least I never thought so.


Sent from my iPad using Tapatalk

[DkinG]

Thanks for your replyes...

I didn't open the TV jet because i need a space and time to do that, but i hope to do it as soon as possible... thanks for your explanations.

David Mix

[GreyWoolfe]

I always hated doing the convergence alignment on tv sets. Always screwed it up looking at the mirror and turning the wrong side of the convergence board pots and inductors. If you have an old school soldering gun, you can try that too. Every shop I worked at had a "De-Lousing" coil. Ahhh, the good 'ol bad ol' days![emoji3]
Good luck!


Sent from my iPad using Tapatalk

Years ago, I had an uncle that taught me to degauss using an old school soldering gun.  He taught me to only get it close enough to work and keep it moving in circular patterns.  His words were-if you hold it too close and in 1 spot, you could rip the shadow mask up.  I've degaussed a few tv sets that way.

[DkinG]

hahahhahah.... thats a good idea...

[justanothercanuck]

Years ago, I had an uncle that taught me to degauss using an old school soldering gun.  He taught me to only get it close enough to work and keep it moving in circular patterns.  His words were-if you hold it too close and in 1 spot, you could rip the shadow mask up.  I've degaussed a few tv sets that way.

I've heard of this method, but I've never tried it myself.  What I did (and this is probably more dangerous, but meh), was take a long piece of 14/2 wire, make a "square" shaped coil out of it, attach a mains cable to one end, marrette the other end together, and tape the whole thing so it retains its shape.  Works the same way, plug it in and slowly swirl it in circles in front of the screen until the crap goes away.  Only thing is, if you leave it plugged in, bad things can happen. 

[helius]

If you use a "manual" hand-held degaussing coil, the same rules apply:
you must bring the coil far away from the CRT before turning it off, or some residual can be induced on the screen by the magnetic field collapse.

[DkinG]

Hello Again!!.....

Finally, i openned the TV. Everything has a good look. So i started to identify the devices for the Degauss circuit. And as someone says the thermistor must have a tens Ohms but when I measure the thermistor of this TV I have the following Measurements;

(The device is labeled as: MZ73 4.5RN270V)
From Pin #1 to Pin #2: 1246 Ohms
From Pin #2 to Pin #3: 1848 Ohms
From Pin #1 to Pin #3: 3380 Ohms

All measurements were made with a DMM Fluke 87 III.

I need to know if all the devaices has the same value, because i have some of them that I took from other TV's.
Greatings

David Mix

[SeanB]

Same part number same number of pins same device. Normally resistance will be in the 30R range when cold.

[wraper]

Should be 27R +/-20% when cold, therefore dead unless you measured it hot.

[DkinG]

That means the device is dead....  because in cold state i have the measurements listed in my last comment.

Thanks.

David Mix

[N2IXK]

If you use a "manual" hand-held degaussing coil, the same rules apply:
you must bring the coil far away from the CRT before turning it off, or some residual can be induced on the screen by the magnetic field collapse.

Yes. And you should rotate the coil so it is perpendicular to the screen plane after backing the coil away before cutting power for the same reason, minimizing the magnetic field from the collapse.  Back when I worked in TV repair, the customers would often give you REALLY strange looks when you were degaussing a set in the home.  If you had no clue what was going on, it would resemble some kind of strange voodoo ritual dance or something, waving this "magic hoop" around the set and making the picture freak out.

As mentioned, an old transformer type soldering gun works as a degaussing coil in a pinch, as does a handheld bulk tape eraser.

[DkinG]

Now im going to change the device, So ill be posting the results...

Greatings!!