Hi, sorry, been suffering from a heatwave here and other issues (including my 30v 40A power supply dying!).

Glad to hear you're all right!

BTW - I actually have a 100A DC switching power supply, and I recently asserted a load of around 75A on it (which is rare and not something I usually do) and saw it shutdown pretty much the same way this small power supply did and also not come back to life until it rests for like 10 minutes. I did notice it got hot but the fan never started running. Makes me wonder if whatever the problem is that we're searching for, it may actually be more common than people may realize!

Be careful attaching the scope - run it on batteries only. Make sure your connections are sound before you switch the PSU on and never try and look at the primary side wave forms. And do not connect to a PC or laptop unless that's running on batteries too.

It's a battery-operated hand-held, the only power connection is USB for charging and data transfer to PC. So it is perfectly isolated and would never be used connected to wall power or PC. Not sure what the concern is about checking primary side, I've used it recently to look at 120VAC wave form on the mains and from modified and pure sine wave inverters just fine. It's supposed to handle up to 800V on the 10X probe setting.

As a first test, lets look at the waveform across pins 5 (signal) and 3 (ground) of the transformer when idling unloaded, when operating normally at a small load,and when overloaded...

Ok, sounds like we may be back in business on this project! I'll post results on that shortly..

 

Not sure what the concern is about checking primary side, I've used it recently to look at 120VAC wave form on the mains and from modified and pure sine wave inverters just fine. It's supposed to handle up to 800V on the 10X probe setting.

Just for your safety - transients in a SMPS on the primary side coil off a rectified AC mains can exceed 600+v. For me, unless it says Cat 1 or Cat 2 1000v and that's traceable (not some Chinese manufacturer's wishful thinking) I'd be wary, but if you're comfortable with it...