Path: utzoo!attcan!uunet!lll-winken!netsys!cucstud!wb8foz
From: wb8foz@cucstud.UUCP (David Lesher)
Newsgroups: comp.sys.ibm.pc
Subject: Re: How did this program burn out two monitors?
Keywords: better living through electricity
Message-ID: <247@cucstud.UUCP>
Date: 25 May 88 23:20:29 GMT
References: <10244@agate.BERKELEY.EDU> <17460@glacier.STANFORD.EDU> <507@cmx.npac.syr.edu>
Reply-To: wb8foz@cucstud.UUCP (David Lesher,Guest)
Organization: Columbia Union College; Takoma Park, MD 20912
Lines: 32

those above and others wrote:
>>       This is an old known bug.  You can burn out the IBM monochrome
>>monitor by stopping the horizontal sweep while keeping everything else
>>running, and the Hercules card gives you enough control to do this
>>under software control.  
>Now I'm wondering why the monitor doesn't burn out when it is
>turned-on before the pc (i.e. before there is a sweep signal at all).
Well you see, it can and will.
Did you ever wonder why IBEAM had that strange power plug on the
mono monitor??
If you take away the horiz. SYNC (not sweep, which is a
internal monitor function in a 'dumb' monitor) the unit should
'free run', the way your boob tube does between {active}
channels. Well, as I heard the story, with the sweep circuit in the
5150 {or whatever # it had}, the horiz. oscillator stalled, the
moment sync disappeared.
Instant Smoke
Anybody else would be tempted to fix the oscillator, but IBEAM
instead made the monitor plug ONLY into the PeeCee, and made
the outlet on it switched with CPU power.
But then the CGA monitor drew too much current for the switch
in the PeeCee, so it had to have a regular cord. (Its design
was TV based, I suspect, so it had to free run)
Why do TV sets/monitors smoke when you kill the horiz drive, you may
well ask. In a circuit trick dating back to the earliest
designs, the retrace time (the big burst of deflection current
to pull the beam back from the right side of the tube to the
left {whilst the electron beam is 'blanked'}) is used [via
another winding] to supply the ~10kv+ high voltage. That
voltage is what the picture tube needs to pull the electrons
up to splat on the face of the tube, and light up in disgust
at their fates (;-}). So -- lots of watts flow through that
horiz circuit, and if it get constipated, stand back.