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From: hart@cp1.UUCP (Rod Hart)
Newsgroups: net.micro.pc
Subject: Re: Re: software that cooks PC monitors
Message-ID: <1044@cp1.UUCP>
Date: Sun, 28-Apr-85 12:32:28 EDT
Article-I.D.: cp1.1044
Posted: Sun Apr 28 12:32:28 1985
Date-Received: Tue, 30-Apr-85 03:42:05 EDT
References: <123@kontron.UUCP>
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Organization: C & P Tel. Co., Baltimore, Md.
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> 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.

Is there a possibility of this happening with the AT&T 6300?
-- 


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  Minicomputer Technical Support District 
  Chesapeake & Potomac Tel. Co.
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