Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!seismo!rutgers!caip!clyde!cbatt!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: net.micro.pc
Subject: Re: Hard disk shut-down (really terminal fires)
Message-ID: <302@neoucom.UUCP>
Date: Thu, 23-Oct-86 09:26:58 EDT
Article-I.D.: neoucom.302
Posted: Thu Oct 23 09:26:58 1986
Date-Received: Fri, 24-Oct-86 00:59:21 EDT
Organization: Northeastern Ohio Universities College of Medicine
Lines: 62
Keywords: computer on, but monitor off please

Hi,

	After seeing the pletohora of articles about leaving one's
computer turned on, I'm finally goaded to put in my two cents'
worth.

	I agree that leaving the HDU powered up for long stretches
of time is probably a good idea.  In honesty, probably turning the
computer off over night while one is away from the office is
probably not going to be too much for the system and HDU to bear.
The designers are indeed aware that this is normal behavior.

	Turning the computer off every time one gets up from the
desk for a cup of coffee is probably a bad idea.  Anyway, an IBM pc
with 640K of memory takes an excruciatingly long time to power up,
never mind the wear and tear!!

	Running "shipdisk" or whatever the HDU head parking program
on your version of MS/PC DOS is called just before turning the
power off is a good idea.  My dad has a PC on a floor stand where
he works.  He lost the HDU when the cleaning crew slammed into the
PC with a vacuum cleaner (with the system off).  With the power on,
ironically, the system probably would have survived the impact, due
to the air cushion between the head and platter.

	The newer 3.5 inch based hardcards have a head unloading
spring that automatically pulls the heads up when the power is off.
They'll survive all but the most violent impact.

	Now for the real meat of this letter.  Its a darn good idea
to turn the monitor off (ie. no 110v going in, not just the
intensity down) while everyone is out of the office.  We have 50 or
60 terminals and monitors of various types around here, and have
had several spontaneously catch fire in the CRT assembly.
Fortunately, somebody has always been around to do something to put
out the smoldering monitors.  If it happened at 2:00 AM, we might
have had a real mess.

	The problem is usually related to the high-voltage power
supplies in the monitors.  I've seen two failure modes.  First, a
lot of dust is attracted to the high voltage anode lead of the CRT
due to the intense field.  Inevitably, the dust causes leakage
currents which then cause the flyback drive to incease and cause
overheating in the horizontal drive transistor and associated
circuitry.

	Method two failure relates to Televideo terminals.  The
horizontal output transistor is AC coupled to the flyback
transformer by a nonpolarized capacitor which is under-rated.
About 1 amp of current flows in the cap. which causes it to get
very hot, even in normal operation.  Eventally the cap shorts when
the electrolyte dries out.  Poof.  I've replaced the HV power board
in several televideos; now they're using a bigger cap, but the new
caps still go bad after a while.

Bill

Bill Mayhew
Division of Basic Medical Sciences
Northeastern Ohio Universities' College of Medicine
Rootstown, OH  44272  USA    phone: 216-325-2511
(wtm@neucom.UUCP  or  ...!cbatt!neoucom!wtm , etc.)