Printer and Photocopier Troubleshooting and Repair Collection : HP DeskJet 1200C power supply repair sto..  
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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.