[Radio Wrangler]

Now why would they have a high voltage linear regulator in an SMPS?

The only thing that comes to mind is for a start-up supply.

Quite often in SMPS a dropper resistor/zener is used and the high value droppers have a habit of going high, leading to a supply that fails to start. Apple supplies were well known for this and it makes for an easy repair.

Have you identified which control chip it uses, Michael? Data on that and its application-note circuits is a good starting point for unravelling what's going on. You may have just seen that regulator and thought as the largest device on the board, it had to be the main switch. In a good, efficient, design there will bee very little heat in the switch device(s) so they could be fairly small.

With SMPS, in the past, I've identified their main power rail and used an external bench supply to operate the control chip and checked for oscillation and gate drive to the switch device(s) all this with no mains applied. A second bench supply can then be used to put a lower voltage , say 12-30v onto the reservoir capacitors. This is enough to see if the switch device is alive and to get rectifiers turning on a bit on the secondaries for outputs over 10v

In this way you can try power without risk of destruction or personal danger. Bench supply current limits protect hardware.

I've been involved in SMPS design, including some very weird ones, and in trying out something new, you don't even have the comfort of knowing it once worked. The low voltage supply trick pays its way. I had a 350v bench supply for when I wanted to get things going properly and see it going into proper regulation. Only when something worked on current limited supplies would I go on to applying mains.

David