Re: dot re-appearing on an lv2000 (g05)

On Thu, 7 Mar 2002 17:39:24 -0000, you wrote:

>I did notice when i switch it off, the beam flicks around a bit then returns
>to center and fades out to nothing....
>
>but then..... the beam comes back about 5 seconds later, it fades in to
>almost full power and sits there while everythign is switched off, after
>about 15 seconds or so it starts to go dim and then dissapears...
>
>from what i know of the circuit and from reading the theory in the manual i
>know it should have something to do with the spot killer ;) [i'm clever, me]

        According to the theory of operation, the spot-killer circuit is
supposed to behave like this when the monitor is turned off:

         "On power down, Z is cut off as Q503 and Q504 are not conducting,
         while D506 blocks discharge path of C504 long enough to hold
         cathode voltage until heater filament cools down."

        So I'd check D506 and C504 next - it's possible that D506 isn't, in
fact, blocking C504's discharge path (it could be leaky or shorted?), or
that C504 itself is leaky and not charging/discharging properly. This
would, if I'm reading the schematics and Theory Of Operations correctly,
allow the cathode-to-Grid-1 voltage differential to become small enough for
the electron beam to get past Grid 1 before the filament has cooled
sufficiently to prevent electron emission from the cathode. (These
monitors, IIRC, don't have any circuitry to self-discharge the tube when
the monitor is powered down, so that 15Kv "pull" is still present at the
front of the tube, ready to yank those electrons straight into the phospors
if they can get off the cathode and past the grids.)
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"For the children" - the phrase politicians use to justify a course of action
so irrational it cannot be justified in any other way.
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solarfox@DON'TMESSWITHtexas.net (Gary Akins jr.)
http://lonestar.texas.net/~solarfox
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Received on Thu Mar 7 15:07:02 2002