Weird happening with CRT TV...

DublinWriter

So there I was today, re-setting up my AV system when I put my Denon Centre channel speaker on top of my 28" JVC CRT widescreen temporarily while I repositioned some of my components in the rack.

I thought there wouldn't be any problems as the speaker was rated as shielded, but after half an hour, I took the speaker unit off the top of the CRT and bingo, a great big red blob appeared on the top left of the screen.

Crap, I thought, I started mentally preparing to buy a new TV. I was watching something or other for about 2 hours, and the blob was ever-present.

I go out for about 6 hours, switch on the TV, expecting to see the blob, but hallelujah! It's gone!

Now from bitter experience I know that once you take anything magnetic to the screen of a CRT that you'll permanently damage your TV.

But this appears to have totally fixed itself, without me doing anything like a degauss of the screen.

Answers on a postcard anyone? I'm baffled!

Calibos

I go out for about 6 hours, switch on the TV

TV's degauss every time you switch on. thats why it was gone when you came back

DublinWriter

Calibos wrote:

TV's degauss every time you switch on. thats why it was gone when you came back

I thought so too, and I switched it on and off an few times after it appeared to no avail.

ZENER

The degausser unit uses a temperature dependent resistor. At switch on from cold (6 hours is good) the resistor has a low resistance so a large alternating current flows in the coils of the degausser. As this current flows the temperature of the resistor increases thus increasing it's resistance slowly reducing the current in the coils until it fades out to nothing. The resistor stays hot while the TV is on.

Some sets use a relay to disconnect the circuit after its been used. These sets tend to have a manual degauss option somewhere on the remote or or menu system for the user to apply at will even with the set in use - my Sony 28" did this..

The reason for the current decreasing to nothing slowly is that if the magnetism produced were to stop abruptly the tube would be magnetised to the pole and level the coils were at before interruption of the current.

. . . I'll get me anorak now . . .

ZEN

DublinWriter

ZENER wrote:

. . . I'll get me anorak now . . .

Well, at least I don't have to get a new tele!

Thanks for the info. See you at TrekCon 06!