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If it has a selenium rectifier...replace with silicon diode such as 1N4007 with a 100 ohm 10 watt resistor between the anode of the diode and the power line to pad down the voltage to the correct level and help absorb the turn on surge. The problem could be low voltage due to bad selenium rectifier. The old selenium must be electrically disconnected from the circuit. Also use a 1/2 amp fuse right where the power line comes in.
If there is some hum the electrolytics are starting to go bad...replace them and any wax paper caps or black caps with color bands which are plastic encased paper. Measure your power supply voltages and make sure they are OK once you repair the power supply.
Resistors are usually OK and most tubes are OK too...the FM mixer/oscillator tube such as the 12AT7 or 12DT8 is the most suspect to be bad. Don't go looking for tube problems though till you make sure your power supply is ok.
Some of these Zeniths run the dial lamp off a dropping resistor directly from the AC line...in which case it is a 28 volt bulb I believe. The older ones use a neon bulb indicator.
Also please observe pertinent safety precautions...the chassis is connected directly to the power line and there can be a shock hazard from charged caps, etc...you may want to do a little more research on these hazards before you begin as it can be dangerous if you don't take the proper precautions.
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