"Fred Bloggs" wrote in message
...
In article , Roger R
writes

The mains input fuse on the board has blown but nothing is burnt.

I'm hoping someone can advise on the likely faulty component.

Agreed with Arfa. If the mains fuse is silvered/blackened on the inside
of the glass (hard to tell from the photo), it's a short in the primary
side, most likely one of D1/2/3/4. D2[obscured] looks like it's been
farted about with, have you had someone look at this supply already?

There's no primary-side switching transistor, which is unusual. Maybe
it's inside IC1, but your picture isn't clear enough for me to read the
markings on IC1 to look up the spec.

Not unusual these days Fred. There are many 8 pin (usually actually only 7
pins but a 'standard' 8DIP package) 'switchers-on-a-chip' now. They are used
extensively in LCD TV smps to implement the PFC front-end or standby
supplies. The TNY266 is a typical example of this sort of chip. Modern FETs
have such low resistance switched drain-source channels, that very little
power is dissipated in the device itself. These supplies can achieve
efficiencies in excess of 90%. So as long as you can make sure that there is
enough insulation resistance around the package pins to which the switching
FET is internally connected - and that's why they have the 'missing' pin 7 -
then using them in a switcher man enough to drive those LEDs, is something
that they can easily cope with.

At the moment, I'm still erring towards the problem being one of the
front-end reccies though, as *usually*, although not always, s/c failure of
those chips is very evident in that all of the magic smoke will have escaped
from the gaping hole in the top ... :-)

Arfa

Fred Bloggs