[McMurdo]

Yes, check the DC volts, it won't be much higher than it says in the manual and certainly not in excess of your 600V range!!

If you're in any way unsure of what you're doing then don't put your fingers inside with the mains plug in the wall and double check you've removed it before touching anything.

The mk4 in particular were used in the pottery industry on spray dryers which essentially were machines that generated a lot of fine mineral dust. It was not uncommon to see the bottom part of the casing and the entire circuit board submerged in this grey powder where the EHT had been acting as a sort of air purifier and hoovering up cupfuls of the stuff. Always got 'em going again! Nice easy Friday afternoon job.

If the main PSU is not working then check the small electrolytics around the SMPSU chip and in particular the base drive capacitor. Then check the high value resistors in the same circuit as per previous advice including R12. The set HT pot can also fail so worth replacing, it can go noisy and cause jitter or potentially a SMPSU blowup.