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If the tubes are good, then I would use an ohm meter to trace around the heater string from one ac input to the other end of the string, following the path shown in the schematic. Could be a bad power switch, a broken wire, bad tube socket, dirty heater pins on a tube or whatever that is causing the open circuit.
This test can be done with the radio out of the cabinet and, of course unplugged.
jr
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