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

snip

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.

Coming back to this before buying / replacing the switcher, I tested
it out of circuit on my DMM (diode test, all ways etc) and it seemed
to check out ok?

Whilst rummaging though my spares last night I came across a similar
looking device (with the two diodes facing the centre pin marked on
the case) so assuming the spec checks out similar (at least 12V etc),
should I try swapping that in and see what happens, in case the other
is breaking down in use?

FWIW I measure about 10V DC on the output (no load) and only about .5V
on the AC range (ripple / noise)?

Cheers, T i m