After further review and firing up my own 503 for comparison, here are some
1. V659 should be emitting a steady glow while it's regulating. Yours is
flickering, but that doesn't necessarily mean the tube is faulty - it could
mean that V659 is not seeing a steady positive DC voltage and is thus going
in and out of regulation cyclically.
2. V692, the leaded (not socketed) tube soldered in at the bottom of the
ceramic strips near where the mains cord comes in, should just have a quiet
orange glow. Yours is going to town and appears to be arcing over within
the tube. This tube is a rectifier tube responsible for generating the -3kV
acceleration voltage for the CRT. That same -3kV is a bias voltage on the
heater of the CRT, and at least one other person on this list has already
commented that the CRT heater winding on T601 was shorted on their scope. I
think it is very likely that you have the same problem.
3. Your V620 (6DQ6) sure looks like it's running hot. I think I can see a
cherry red glow in there aside from the orange glow of the heater just
under the cap. Red is bad. I wouldn't at all be surprised if it was due to
the -3kV leaking back onto the 190Vrms winding at T601.
Once you get this figured out, you should take a close look at all the
silicon diodes to make sure they haven't been overstressed. I feel like
D652 (feeding V659) may already be shorted.