Repairing a 455 with no beam


Hi all...
Here's a summary of today's project. I thought I'd post it here in hopes that it helps someone else.
Visual clues:
C721 center shriveled up
R523 smoked
DS563 and DS564 do not light up
One filament wire from T550 broken at PCB
Basic measurements found abnormal:
+5, -5, +32 V all zero
F736 blown
Steps taken:
Replaced C721 although it tests okay at low voltage
Resoldered both filament wires to PCB
Removed R523 and checked nearby components
C522 shorted.
VR524 DMM diode test 0.45 V one direction, 0.87 V the other.
Conclusion: Current through C522 probably took out R523 and VR524 and blew F736
Remedy: replaced all three of those components and F736
HV verification:
DS563 and DS564 flash at turn-off, otherwise dark
-2kV test point gives 1980V under 4 megohm load
Unplugged CRT HV lead. At socket, 7.6 kV under 13 megohm load
Beam appears...everything works!
Note: neon lamps slowly lose their gas. 40 years seems to be enough. So if they're dark, it doesn't mean you don't have high voltage. Be careful!