Hi all, I know there are some very experienced / SMPSU folk on here so
I'll run this by you if I may.

The PSU on the MendelMax 3D printer I built with / for a friend over 5
years ago (then bought off him) died the other day. ;-(

I noticed it had stopped towards the end of a 90 minute print and the
printer was off (very unusual).

I power cycled it and it appeared to come up again, but when I told it
to preheat (extruder and bed) it nearly immediately powered down.

Disconnecting the heated bed, the printer would both power up and pre
heat the extruder but the software then halted the printer because it
noticed the bed wasn't heating. ;-)

So I dug out an old linear 13.5 x 12A PSU I had from my Packet radio
station and used that to power the heated bed and the printer then ran
ok and I was able to re-print the job.

When I powered it down it didn't seem to go off as normal and after
taking it out and removing the top, I noticed one bulging cap (one of
4 x 1000uF, 16V) on the output / smoothing stage, so replaced all 4.
Alas (and sorta as expected), it didn't really help so I'm not sure
what to do (and if it's worth doing) now (knowing how little I know
about SMPSUs).

However, I am fairly familiar with working on stuff that has mains and
have been a component level field support tech for some of my career
(Tele/datacomms/Microfilm and built all my own disco lighting) so
don't mind giving stuff a go, if anyone is willing to point me in the
right direction and thinks it would be worthwhile.

This is the unit out of it's frame.

Front
https://ibb.co/5McshGF

Rear
https://ibb.co/w6Wq6hj

I think it had a small fan, stuck to the side of the big transformer
but it got noisy so we took it out and the PSU only used to run warm.

That said, with everything on I think it was pulling current close to
it's max rating so we can't complain that it's lasted that this long
(heated bed ~10A (measured 11.5 on 13.5V)), extruder 3.5A, 5 x stepper
motors at 1A each and a few fans and the Arduino Mega and LCD display
etc).

After changing the caps, I ran it up without a load and it was a
'flaky' 12V (onboard LED on but the voltage was waving about a bit)
and when I fitted a 5A load, it wouldn't start up at all.
(crowbarred?).

I've measured the bridge (diode check on my DMM) and that seems ok. No
strange noises, I did sense that 'hot electronics' smell when going
back to the printer and it looks like the windings of the output choke
thing are darkened but nothing obviously burned or blown.

I assume the 3 legged device on the LV side is a switching transistor
/ FET of some sort and the 5 legged thing on the HV side, some sort of
SMPSU controller device / IC? I can't tell what they are without
desoldering them (but can do so etc).

R7 looks like it's been warm but measured at around 4K7 I think.

As an aside, a quick Google for LJYD-0540 suggests (and it's name g)
it, or the PCB was designed to be a 5V 40A PSU and so has been built
into a 12V 20A model somewhere along the line?

Cheers, T i m