CRT or its control going down?

  Diemmess 16:01 04 Jan 2010

Have had a Panasonic TV for probably far longer that it seems! Model TX-21AP1

Noticed recently a yellow tinge affecting a circular area on the right side of the screen.
This seems to be noticable only when the screen is largely white and still. In fact if the image changes it seems to wipe the yellow away in a few seconds.

More recently, I think a similar patch of Cyan is occurring on the left third of the screen.

It seems a fault which can only become worse, though how quickly remains to be seen.

If this rings a bell with TV experts out there, is this something which in these days is more costly to correct than to replace the whole thing?

  oresome 16:23 04 Jan 2010

I suspect the shadow mask within the the CRT has become magnetised and the auto degaussing has stopped working.

Most likely cause is that the temperature dependant resistor has gone open circuit that feeds the degaussing coils.

  Diemmess 17:16 04 Jan 2010

I assume that degaussing is automatic in such a box?
If taken to the shop that sold it, there will be a smug smile on the owner's face and a comment of "Time you had a new DVB and HD set Sir?"

Once upon a time I had a monitor with a button for this, but I never needed to do anything!

  wiz-king 18:09 04 Jan 2010

Yes it is automatic, but only works when the mains is turned off for a few minutes. If you leave it on standby it wont degauss. You should hear a 'thump'or short hum on turn on if it is working correctly.

  Arnie 18:28 04 Jan 2010

oresome is probably correct. It can also be caused by the shadow mask screen bending slightly as the screen warms up. This will cause some misconvergence of the beam target area regardless of a correctly working degaussing system.

I have known your same problem to occur even when the tv circuitry is ok.
The culprit was a radiator too near to the tv.
Loudspeakers will also cause the same problem when they are not shielded, or in close proximity to the set.

If it is a degaussing circuit problem, a quickly made degausser can be made by using an old ferrite core ring from a large loudspeaker, or an old magnetron from a microwave oven.

Tie some thick cord to the ferrite core and grip the end of the cord in the chuck of a battery powered electric drill.
Switch on the drill and gradually move the core about 150mm from the screen moving laterally and vertically around the screen. Do not contact the screen since this can make the situation worse.

With the drill still turning pull far away from the screen and switch off the drill.
This will have to be repeated from time to time.

What you are doing here, is creating a rotating magnetic field which will randomly align the magnetic build up on the shadow mask screen in the same way as a commercial degausser wand.

I have also used a bulk magnetic tape demagnetiser with equally good results.
If you are unsure of the procedure then it is best ignored.

  Diemmess 19:45 04 Jan 2010

Thanks for those interesting suggestions

You all agree in principle, so I'll call on a friend who will probably have the means to try and cure.
He certainly has the knowledge and experience, and while I can reproduce the problem on demand I feel it is so unobtrusive for normal purposes, that I can afford to wait and see if it becomes worse.

Switch over in our area is only a couple of months away, so I will have more excuse to pressure my manager into buying new should it become worse.

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