• TV
  • MOVIES
  • MUSIC
  • SHOWBIZ
  • SOAPS
  • GAMING
  • TECH
  • FORUMS
  • Follow
    • Follow
    • facebook
    • twitter
    • google+
    • instagram
    • youtube
Hearst Corporation
  • TV
  • MOVIES
  • MUSIC
  • SHOWBIZ
  • SOAPS
  • GAMING
  • TECH
  • FORUMS
Forums
  • Register
  • Login
  • Forums
  • Gadgets
  • TV and Home Entertainment Technology
White patch on tv screen (?)
darkpaw
21-06-2007
Just noticed when I switch off my portable CRT telly (Sharp; about 10 years old), there is a white area remaining on the middle of the screen glowing in the dark, which gradually disappears after about 5 minutes.

It is slightly spooky but I am sure there is a scientific reason for it. What causes it and is it a fault?
RobAnt
22-06-2007
Sounds like an old valve driven telly - if you have Joost, they imitate it when you close the program down.

It's not a fault - just a remnant of a bygone age.
Chromatron
22-06-2007
When a CRT is powered off the cathode continues to emit electrons to some extent as it cools down. The accelerating voltage on the final anode will be retained sufficiently to produce a weak unfocussed electron beam with just enough energy to excite the phosphors on the screen and give a pale blob visible in the dark.
Nigel Goodwin
22-06-2007
Originally Posted by Chromatron:
“When a CRT is powered off the cathode continues to emit electrons to some extent as it cools down. The accelerating voltage on the final anode will be retained sufficiently to produce a weak unfocussed electron beam with just enough energy to excite the phosphors on the screen and give a pale blob visible in the dark.”

Nice explanation!

Because of this (nicely explained) effect, it's normal for sets to have a circuit that 'discharges' the CRT when you turn the set off - it simply drives the tube hard on, which stops these effects.

There have been a number of sets over the years which have required modifications to cure it, in particularly bad cases it's a bright spot, and this can burn the tube.
Chromatron
22-06-2007
Thanks for the compliment!

My ancient Philips G11, bought in July 1978 and still in daily use until last November, exhibited this effect to some extent, but it was only visible for a minute or so in almost total darkness.
darkpaw
23-06-2007
Cheers for the explanation guys.

I can sleep soundly now knowing there is nothing that needs calling in the Most Haunted team
VIEW DESKTOP SITE TOP

JOIN US HERE

  • Facebook
  • Twitter

Hearst Corporation

Hearst Corporation

DIGITAL SPY, PART OF THE HEARST UK ENTERTAINMENT NETWORK

© 2015 Hearst Magazines UK is the trading name of the National Magazine Company Ltd, 72 Broadwick Street, London, W1F 9EP. Registered in England 112955. All rights reserved.

  • Terms & Conditions
  • Privacy Policy
  • Cookie Policy
  • Complaints
  • Site Map