On Mon, 17 May 2021 15:43:12 -0700 (PDT), whit3rd
wrote:

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);

The two TO(xxx?) packaged devices under the board were bolted to the
square U section case with insulators. eg, the whole ally plate
chassis and perforated ally 'lid' would be a heatsink.

OOI, as it seems this PCB was originally designed to be 5V / 40A, so
is it likely they would have used the same component there for 12V /
20A (I know it has to dissipate the same number of watts but wondered
if it might be able to handle 20A easier if it was originally specced
to take 40A)?

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.

Ah. ;-(

That's the likely culprit, probably will need to be desoldered to find the part
number, and will be easy-ish to test.

Understood.

Sometimes there's two diodes, center
pin common, in this form factor.

Yes, I think I've seen such on the various SMPSU diagrams I've been
checking. ;-)

As advised elsewhere, I think I'll pull and test (and possibly replace
*anyway*) the two small electrolytics first and then move to the
rectifier.

FWIW, I do read around 10V DC on the output but it's fluctuating. I
could put my PC digiscope on it (after checking the output for AC on
my DMM)?