Chuck,
I had the same fault as in Joel his 515A in one of my 502As.
It seems you checked filament voltages delivered by T601 and concluded that these were good. But the CRT winding fault is not a short in the winding itself. The problem is that the winding leaks to ground (or whatever) and heavily loads the CRT cathode circuit. When easy to reach a check would be to disconnect both CRT heater leads from the transformer.
Albert
