Path: utzoo!attcan!uunet!husc6!bbn!rochester!cornell!batcomputer!itsgw!nyser!cmx!billo
From: billo@cmx.npac.syr.edu (Bill O)
Newsgroups: comp.sys.ibm.pc
Subject: Re: How did this program burn out two monitors?
Message-ID: <507@cmx.npac.syr.edu>
Date: 25 May 88 00:04:26 GMT
References: <10244@agate.BERKELEY.EDU> <17460@glacier.STANFORD.EDU>
Reply-To: billo@cmx.npac.syr.edu (Bill O'Farrell)
Organization: Northeast Parallel Architectures Center, Syracuse NY
Lines: 32

In article <17460@glacier.STANFORD.EDU> jbn@glacier.UUCP (John B. Nagle) writes
>       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.  The video chip lets you select the horizontal
>and vertical sweep rates independently, and zero rates are possible.

Well, looks like I was close.  I had said:

>I believe that software can select either 50 or 60 Hertz scan rates.
>I have definitely heard that putting the wrong scan rate into a
>monitor can cause it to burn-out

And went on to suggest that a multi-synch monitor might be immune.
Now I'm not sure if it is -- can it survive the horizontal sweep rate
being reduced all the way to zero?  If not, then what is the benefit
of a mult-synch monitor at all? (They sure are expensive.)

J.B.N goes on...
>... the horizontal sweep is used as the oscillator for a switching
>power supply, as is typical in TV circuits, and with the sweep rate at
>0, DC flows through a coil with high inductance but low resistance,
>producing an excessive current that burns out the coil.

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).
Perhaps with the sweep rate set at zero, the voltage on the synch line
is kept high (positive or negative?)  -- as if it were at the
beginning of a sweep -- instead of zero. That voltage would then
put dc current through the coil. Hmmm.. Am I close?

Bill O'Farrell, Northeast Parallel Architectures Center at Syracuse University
(billo@cmx.npac.syr.edu)