I am pretty sure that 500mA will not be enough to start the scope, so you might see it folding back, specially during startup, and then this could be misinterpreted as there being a problem in the second stage of the scope power supply. I can't remember exactly which external power supply I used back then to run this test but it was close, probably 42V if not dead on 43. I think it was a 48V supply that I managed to dial down... or maybe it was the Heathkit IP-27 which can deliver up to 50V and 1.5A.
In any case if you got only 40V give it a go, but if the scope doesn't power up in max 1-2 seconds (keep an eye on the green front panel power LED) watch for any abnormalities in the scope (heat, smoke, etc) and be prepared to shut down everything very quickly, ideally with just the flick of a switched AC outlet.
But definitively do this test first. It would have saved me a lot of grief had I done the same in my scope.
Let us know how it goes.