More switchmode power supply grief - Cisco IGS router

[Peter Coghlan]

Phil Blundell wrote:
>
> 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.
>

It's all discretes.  There are only about five or six TO-92 thingies in there
which are likely to be small signal transistors plus a few TO-220 style items
mounted on the heatsink that could be drivers and/or output rectifiers.  Come
to think of it, I suppose a FET is unlikely to need a driver on a heatsink?

>
> Is it definitely a FET?  Some, particularly older, designs used
> bipolar transistors there.
>

Even though there are no markings visible on the transistor, it's marked
S,D,G on the PCB!

If it was a bipolar, I'd probably chance a BU208A from the TV spares box.
What could go wrong :-)

>
> 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?

I've been a bit reluctant to handle the thing much to examine it closely,
between dust from the fan, stray electrolyte and heatsink compound where the
internal heatsinking used to be screwed to the case, it's very messy :-(

Except for the two main reservoir capacitors which seem to be sound, the
other electrolytics all seem to be Nichicon units and they all seem to have
leaked :-( I think lots of components are going to need to be desoldered
from the board to clean it.  That's going to get smelly :-( At least access
is very straightforward compared to a H7821 and way better than a H7816 :-)
No live heatsinks in this thing!

The bridge rectifier doesn't seem to be shorted.  The other heatsinked
components and small signal components seem to have markings on them so
it might not be too difficult to find replacements for those where needed,
provided I don't manage to swipe the numbers off them when cleaning the
up the leaked electrolyte...

Oh, there's a couple of RIFA capacitors with cracked cases in there too
but they're not input filters so no magic smoke escaped yet...

Regards,
Peter Coghlan.

>
> p.
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