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From: brad@kontron.UUCP (Brad Yearwood)
Newsgroups: net.micro.pc
Subject: Re: software that cooks PC monitors
Message-ID: <123@kontron.UUCP>
Date: Thu, 25-Apr-85 12:30:26 EDT
Article-I.D.: kontron.123
Posted: Thu Apr 25 12:30:26 1985
Date-Received: Sun, 28-Apr-85 09:07:55 EDT
Distribution: net
Organization: Kontron Electronics, Irvine, CA
Lines: 33

Yes, it is possible for software to fry hardware.
I have seen a Compaq monitor destroyed by software run
amok.  The same thing should be possible on an IBM monochrome.

If the 6845 CRT controller is grossly misprogrammed, either
deliberately or by a wild jump into some part of the BIOS ROM,
it seems possible that the horizontal or vertical drive outputs
will be continually asserted.  (See the cautionary note on page
1-115 of the IBM XT Technical Reference manual.)

The horizontal drive circuit in the IBM monochrome monitor (and
apparently also in the Compaq) is DC-coupled.  A relatively small
transistor is used to drive the primary of a transformer.  If the
horizontal drive is continually asserted, a DC current of about
300ma. will be passing through this tiny transistor, which amounts
to about 4 watts with the 15V supply used in the monitor.  (There
is a 47ohm 5W resistor in the circuit.  This is fine for limiting
peak current in normal operation, but...)

Typical maximum continuous power dissipation figures for transistors
in small plastic packages are in the 300-650mw range.  (The fried
transistor I replaced in the Compaq was in a small plastic package.)
Given a chain consisting of a 5W resistor, a transformer primary, and
a tiny transistor, the weakest link will usually be the transistor.
The funny smell is the plastic package burning.  Damage to the
transformer is not out of the question.  Fortunately, the Compaq's
transformer survived.

The components are entirely adequate for the normal mode of operation.
The transistor in question handles the peak current only for a narrow
duty cycle, and is properly specified for that application.  Unfortunately,
the direct coupled input affords no protection against a continuously
asserted horizontal drive.