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Old 16th Oct 2019, 9:03 am   #43
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 72
Default Re: Cretinous idea - The combined volume pot and mains SW.

Originally Posted by G6Tanuki View Post
The TV "combine brightness and on/off to avoid spot-burn" thing seems like a good idea, but I'm wondering - once we got tellies with push-button on/off switches in the 60s and then the 1970s arrived and TVs had remote-controls with separate brightness and on/off functions, was there ever any attempt at 'spot-blanking' applied ?

I certainly don't remember seeing a central switch-off-spot on any push-button-on/off or remote-controlled TV when switching off using the remote.

Was the whole 'switch-off-spot' thing a bit of a red-herring? I can understand it being an issue with old-fashioned power-supplies using a mains-transformer with an EHT winding and significant smoothing-capacitance so there was a bit of stored energy, but surely not an issue with flyback-derived EHT and 'coated' tubes with only a few hundred pF of capacitance to store EHT-energy?
It was never that big a problem .

The electron beam, for that short while, didn't really do much harm.

What did cause the classic burnt "spot in the middle of the screen" were the negative ions, which were also, unfortunately, emitted, but not deflected by the magnetic deflection, so continued on their merry way to hit the screen "dead centre", all the while the CRT was in use.

They travel slower than electrons, but are far more massive, & are much more prone to damage the phosphor.

Earlier CRTs had "ion traps" where the electron gun was initially pointed off centre, then the beam was deflected back into the right path by a fixed magnet.
the ions were not so deflected, & hit the side of the tube.

This was a complex system, & was later made obsolete by "Aluminised" screens, which were sufficiently "ion burn" resistant to do without.
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