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From: daver@ECE.ORST.EDU (Dave Rabinowitz)
Newsgroups: comp.sys.handhelds
Subject: Re: Burned-out infrared printer
Keywords: HP infrared thermal printer, smoke, fire, warranty
Message-ID: <1990Dec19.015322.16037@usenet@scion.CS.ORST.EDU>
Date: 19 Dec 90 01:53:22 GMT
References: <4491@altos86.Altos.COM>
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Organization: Oregon State University -- Electrical & Computer Engineering
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In article <4491@altos86.Altos.COM> steve@altos.com (Steve Scherf) writes:
>and print and never have to replace a ribbon or ink cartridge. Then one
>day (when it was a mere two weeks old) I turned it on, and POOF! A little
>column of smoke rose from the area of the printing head. When it attempted
>to print what I had sent to it, it just made a bunch of black streaks on the
>paper. The spot on the paper where the head had been at rest had a hole burned
>through, and the rubber platen behind it had melted. That head must have
>gotten really hot to do that. The bubbled and warped plastic on the head
>itself was testimony to that.

The head normally operates on a very low duty cycle (it's pulsed on for a very
short time and then remains off until the next column).  If a dot is left on
for more than a few seconds it will self-destruct (it gets really hot).  The
CPU runs directly off the batteries and if the battery voltage dropped low
enough to lock up the CPU while a dot was on the dot would be dead, so there
is a low-voltage detector which resets the CPU (clearing the dot outputs) 
before the voltage gets low enough to cause a problem.  In your case it sounds
like there was a short in one of the inputs to the head driver (a separate
current driver chip plus a transistor for the eighth dot - the driver chip was
designed for the 7-row non-graphics printer version) was shorted to a value
which caused it to be permanently on.  Since it didn't happen until after you
had the printer and had used it successfully it might have been caused by a
scrap of metal which somehow got into the unit.