More switchmode power supply grief - Cisco IGS router

[Phil Blundell]

On Thu, Feb 03, 2022 at 06:06:10PM +0000, Peter Coghlan via cctalk wrote:
> Today I finally managed to check it out.  The ceramic F4A mains input fuse
> beside the power switch on the back panel had blown.  When I opened it up,
> I found a POWER-ONE MAP80-4000 power supply.  The main chopper transistor
> labelled Q1 on the PCB is almost a dead short.  It is a large plastic
> packaged FET mounted on a piece of aluminium which is in turn screwed to
> the case for heatsinking.  Unfortunately, there are no markings on it so
> I have no idea what to replace it with :-(
> 
> As Q1 is shorted across all three terminals, whatever drives it may be
> damaged too :-(

Does that PSU have a PWM controller IC, or is it built entirely from discretes?
If there is an IC driving the chopper transistor then you may be able to get
some clues about the likely characteristics of the transistor from the IC
datasheet.  Is it definitely a FET?  Some, particularly older, designs used
bipolar transistors there.

As you say there is a fairly high likelihood that other components on the
primary side will have blown up as well so you might be looking at a fairly
extensive repair.  Are there any other obscure, unmarked devices or is this
the only one?

p.