if it's anything like the K7400 or K7500 I'm willing to bet it's high B+ forcing it into HV shutdown. one day I'll memorize this, but I think the test point is the same on the U5000, I just can't remember if you go from TP202 or TP203 lol. I think it was the one closest to the B+ adjust pot.
now under normal circumstances, which I learned the hard way, do NOT fuck with the B+ pot, re-cap the chassis first, and if there's no change, then you have to break that epoxy off (without breaking the pot) to adjust it. you want 117V I believe.
that is of course if it's a B+ issue. you could alternatively also inspect the flyback for cracks, see if it took a poo. my information could potentially be wrong, cause I've never had to do this on a U5000 before, only the other 2.