HP Deskjet 1200C Power Supply Repair Story
(From: Marc Geyskens mgeysken@innet.be).)
A while back, I got this broken down printer, most
of the time the power supply wouldn't kick in after pressing the
standby switch. After checking elementary things, I got stuck on
the PCB that holds the standby switch, on it reside passive components,
a few LM431 and a UN3854 all of the components where ok except for
the LM and UN chips. Since I had no schematic and no sheet on the
UN3854 I turned to the chip directory which led, via a few stops,
to the Unitrode site and yes, a data sheet for the UN3851. Plugged
my scope (power supply on isolation transformer) onto the ucc line
for the UNxxxx, noticed that the voltage was almost 0 raising slowly
to 12 V and then fast to 20V after that time the PS worked normal.
So I traced the supply back to a FET 2SK537 on the power supply
mainboard which is part of a protection, its gate pulled up by a
zener ZD1 and a resistor of 680k from 400V and pulled down by a
transistor in case of the transformer's output voltage gets to high.
Well, it was the 680k resistor, infinitive resistance yet not a
scratch on it.
Thanks to the Chipdir site, and the Unitrode people.
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