On Sunday, May 16, 2021 at 5:38:56 PM UTC-7, T i m wrote:
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. ;-( ...

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

I assume the 3 legged device on the LV side is a switching transistor

TNo, that has to be the low-voltage rectifier (if it really IS rated for 20A, it has to
be heatsinked); if it's leaky it would be a quasi-short-circuit, and that will deliver
AC to the capacitors you replaced, which would kill 'em soonish.

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test. Sometimes there's two diodes, center
pin common, in this form factor.