Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!samsung!brutus.cs.uiuc.edu!apple!xanadu!michael
From: michael@xanadu.com (Michael McClary)
Newsgroups: sci.electronics
Subject: Re: funny phosphorus illumination after power to tv has been turned off
Message-ID: <1990Feb2.134136.10841@xanadu.com>
Date: 2 Feb 90 13:41:36 GMT
References: <6805@lindy.Stanford.EDU> <17660040@hpfcdj.HP.COM> <6893@lindy.Stanford.EDU> <32965@cci632.UUCP> <1990Jan8.044720.12788@sj.ate.slb.com> <6967@lindy.Stanford.EDU>
Reply-To: michael@xanadu.UUCP (Michael McClary)
Organization: Xanadu Operating Company, Palo Alto, CA
Lines: 29

In article <6967@lindy.Stanford.EDU> sorka@ucscb.UCSC.EDU (Alan Waterman) writes:
>
>But not in the same continual pattern. And besides, it does it from one house
>to the next when the tv is moved.

Picture tube illumination after powerdown is a standard phenomenon.
There is a lot of energy stored in the charge on the second anode,
and the cathode(s) stay hot enough to emit for quite a while.

Once the power and sweep stop, and the voltage on the control grid
and focus anode go out of spec, a spot will appear (slowly moving
and changing shape as the fields continue to change) and stay visible
until the cathodes finally cool or the second anode is sufficiently
discharged.  15 minutes is not unreasonable.  (On color displays
you get three separate spots, because the beam convergence fields
have also gone goofy.)  If you've let the set stabilize before
you shut it down, the spot(s) will always follow the same track.

With the sweep off, the beam current can be enough to burn the
phosphor after many shutdowns.  Because of this, most TV and
monitor designs either don't let the control-grid(s) go to turn-on
bias after shutdown or are arranged to turn the beam(s) full-on as
the power fails, to discharge the second anode in a single shutdown
flash while the sweep decays.  Obviously, your monitor either lacks
discharge-flash circuitry or has a defect.

Note that, even if a monitor DOES do a discharge-flash, the second
anode can still have enough charge to throw you across the room
after the flash is done.