My CRT Television had blue patches on screen when turned ON, what could be possible reason?

After moving speaker magnet(BIG) around the screen, it was possible to pull the blue patches from everywhere, and push it a corner of screen?

How was this possible?

  • 1
    \$\begingroup\$ Does your TV make a sound like "WANNnnnn" when you turn it on? If so, that's caused by the large current in the CRT degaussing coil. The sound happens because it accidentally vibrates the steel frame and the iron slot- mask inside the CRT. The coil is wrapped around the widest border of the CRT tube. It demagnetizes the slot-mask, to remove any color splotch. \$\endgroup\$ – wbeaty Oct 25 '17 at 18:14
  • \$\begingroup\$ @wbeaty , i do on -off -on -off repeatedly and fastly till the sound comes ..or else it does not power on. \$\endgroup\$ – Amruth A Oct 26 '17 at 4:24

The magnet was the cause, at least after you "pushed" the blue spots around.

There is a very fine grid behind the glass of a CRT.

If this becomes magentized, it will disturb the electron beams that are used to create the image on screen.

Depending on which end of the magnet you used, the magnetized areas either pull or push the electron beams off course. In either case, the electron beams don't go where they belong, and you get wrong colors.

Modern CRT TVs had built in degaussing coils to neutralize magnetized areas. Yours either didn't have them, or they weren't working right.

TV technicians have large coils that generate a changing magnetic field using alternating current. These magnetize and demagnetize the whole surface repeatedly at 50 or 60 Hz(power line frequency.) Bringing such a coil close to the CRT evens out the magnetization. Slowly moving the coil away from the CRT reduces the magnetization. When you get far enough away, the magnetization is pretty much neutralized, and the picture should again be clear.

Image of a CRT screen that needs degaussing:

enter image description here


Usually demagnetizing the tube should solve the problem, but color CRTs are usually demagnetized when switching them on. Either your demagnetization coil or circuitry has a problem, or the color mask is magnetized beyond its capacity or the color adjustments of your CRT are messed up.

The instructions for color adjustment tend to be in the service instructions (in olden times, included in a satchel within the TV) and involve fiddling with a number of magnets mounted on the tube and with adjustable cardboard disks (also including magnetic material) around its neck.

But I'd likely start with a big demagnetization coil at the front of the screen. Switch it on and back off slowly.


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