The 6237b saga has concluded.
Service engineer error. When I disassembled the unit to swap out the RIFA for a Kemet
I managed to squeeze one of the wires from the power switch to chassis as I screwed the side panel back on.
When I first inserted the mains plug after this bodge, I shot the fuse.
[insert wait for a 500mA fast 1 1/4" x 1/4" fuse] Then, second time around, I must have inserted the plug the other way, so the fuse held, but the 3-phase RCCB for the workshop and computer power said CLACK! and the UPS refused, again, to work. (another project on the pile, that. Perhaps should start with a more sensitive RCCB for the bench..)
Finally, I did what I should have done from the start, and measured resistance to ground for the primary side circuit. Of course it was a dead short on one leg, but only with the mains switch on. Found it by staring at things, and realised what I'd done. Disassembly, unsolder (yes, the Chinesium sucker gun still is amazeballs, especially considering the price.) and some clear heat shrink on the poor wire.
In the end, as soon as I'd fixed the short, and bolted things together, the supply does what -
hp- / Harrison supplies do, works as designed.
And, the repair queue decreased by one.
I also found that unless you engage the LPF, my TTi counter will over-guesstimate the "sine" output from my FeelTech function generator with 20%. Probably not the best sine in the world.
Edit: Since the FeelTech is old and ungrounded, it might be a ground offset issue.