The Philco 49-502/702 and 50-701/702 7" televisions have a major problem in the high voltage system. Even after replacing all old parts the HV system will only operate for a short period of time and then the voltage will gradually drop until the screen is not watchable. A few people have tried manufacturing a replacement HV transformer, which I have tried, but even this seems not to have been the solution.
 

 I went a different route and designed a modern solid state replacement. This unit is small enough to plug into the 6V6 socket in the high voltage cage on the chassis. It will put out nearly 7,000 volts, although only 5,000 volts is required to operate the 7JP4 in the set. I've built two of these units for sets that were in for restoration. Both were built on perfboard using point-point wiring, the board was cut down to fit inside an octal tube base and secured with epoxy.

 The heart of the unit is a small  encapsulated DC-DC HV module that requires only 12vdc to operate. The voltage is taken from the filament connections on the 6V6 socket, and the HV connection is made at the 1B3 socket.

The HV module that I originally used is an E101 from EMCO High Voltage Corp., capable of 10KV @250µa. I was able to find a few surplus units for about $35 but none have been available since. The new cost for this module is $145, a bit steep compared to the value of the Philco set. I found two other units that are somewhat reasonable, the Q series from EMCO  and  the Uxx series from Matsusada.

Both units have a linear output depending on the input voltage, 6KV output with 12VDC in, 3KV with 6VDC in. The unit I designed uses a full wave voltage doubler to convert the 6.3VAC filament voltage, from the 6V6 socket, to about 15-17VDC that feeds a LM317T adjustable voltage regulator. I'm posting the schematic in PDF format for anyone who wishes to build their own supply.