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From: grog@lemis.de (Greg Lehey)
Organisation: LEMIS, Schellnhausen 2, 36325 Feldatal, Germany
Phone: +49-6637-919123
Fax:   +49-6637-919122
Reply-To: grog@lemis.de (Greg Lehey)
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	id JAA13440; Wed, 10 Jan 1996 09:16:18 +0100
Message-Id: <199601100816.JAA13440@allegro.lemis.de>
Subject: Re: Dual Monitor Systems...
To: joerg_wunsch@uriah.heep.sax.de
Date: Wed, 10 Jan 1996 09:16:18 +0100 (MET)
Cc: hackers@freebsd.org (FreeBSD Hackers)
In-Reply-To: <199601092137.WAA00405@uriah.heep.sax.de> from "J Wunsch" at Jan 9, 96 10:37:42 pm
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J Wunsch writes:
>
> As Greg Lehey wrote:
>
>> You've obviously never burnt out a monitor :-(
>>
>> Running monitors out of spec *does* damage monitors.
>
> The usual cause for burning a monitor is _over_driving its horizontal
> frequency.  In a dumb fixed-frequency monitor, the CRT accelerator
> voltage supply is simply done by the horizontal output stage (``line
> transformator'', like in any TV set), and overdriving its spec'ed
> frequency causes this stage to produce way too much high voltage, and
> finally too much power dissipation in the driving transistors, hence
> overheating either the transistor(s), or melting the isolation out of
> the transformator.

The line transformer is a tuned circuit which runs most efficiently at
the tuned frequency.  Deviate to either side and you'll have losses
which ultimately dissipate as heat.  How much heat is generated
depends on the deviation (it's a Gaussian curve) and the rest of the
circuitry, which under these circumstances may show a difference
between too high and too low frequencies.

> _Under_driving the frequency has the ill side-effect of reducing the
> high voltage, which is far less dangerous.  If a monitor is operated
> in this state for a really long period of time (weeks), this might
> also cause the cathode of the CRT to become `deaf' (reduced
> emittability), but that's certainly not a problem if it's running a
> few hours in this state.

It's not the voltage that's the problem, it's the heat dissipation in
the line transformer.

> Running a fixed-frequency monitor (f[H] usually between 60 and 90 kHz)
> at VGA speed (31.5 kHz) is plain underdriving.
>
> (``multi-sync'' monitors have a separate accelerator voltage power
> supply, or some other sort of self-control, and usually protect
> theirselves against overdriving.)

Unfortunately not all of them.  You can burn out multisyncs, too.

Greg