Death by Poking

From: John Lawson <jpl15_at_panix.com>
Date: Wed Jan 17 22:08:30 2001

  On the subject of early 'fragile' monitors, kill-able by the wrong
refresh rate: I was always under the impression that, being by neccessity
rather cheaply engineered and produced devices... the designers used many
of the long-standing "tricks" from decades of consumer TV practice to "get
by with less". Anyone who has worked at all with (especially 60s-era) TV
sets will know what this means.

  The beam deflection circuits in most monitors have magnetic deflection
coils around the neck of the CRT (called the deflection 'yoke) which carry
the vertical and horizontal sweep frequencies and thereby move the
internal electron beam around on the face of the CRT screen. Since the
early monitors were designed to run at very limited sweep rates (or even
at one single sweep or refresh rate) the engineers could use a factor
called "resonance" to get by with less wire in the coils and simpler
electronics to drive them

  Basically, the yoke coil is calculated to 'ring' at a certain frequency,
and of course this is the horizontal refresh rate. The circuit is designed
so that at that freq, the yoke is in resonance, and therefore uses less
current to get the job done; hence less wire and lighter, cheaper parts.
The circuit is made to be resonant over just a narrow range of frequencies
(called the 'Q' of the circuit) and if the driving signal strays very far
from this range either way, the circuit is no longer in a condition of
resonance, it begins to draw large amounts of power trying to do the same
work, and, in a lot of the 'cheap' monitors, the whole thing actually
overheats and burns up while you are looking at the jagged lines and
trying to figure out what to do next.

  Any kind of setting (hardware or software) which could alter the
horizontal rate without reagrd to the type of CRT device it's driving is
liable to this kind of smoke-producing behavior. Newer multi-sync CRTS
have spoiled us...

   ALSO: Long ago in the Big Iron days... the were some machines in the
Philco line whose power supplies in some configs were marginally
inadequate. It was possible to load the machine (with programs and data)
and trip (or burn) the power units... but this was an isolated case.
Program damage to most older computer systems was mostly caused by making
peripherals do things they shouldn't... usually in cases of the device
handler software being abused or tweaked by The Unwary and causing
resultant mechanical damage.

  Cheerz

John
Received on Wed Jan 17 2001 - 22:08:30 GMT

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