To restore an old radio, the first step in to do a visual inspection
above and below the chassis to look for signs of damage and to see if
all of the parts are there. A continuity check of the power transformer
confirms that the transformer is sitll good. If so, the electrical restoration
can begin. Almost all of the capacitors will need to be replaced. The electrolytic
capacitors provide filtering of the higher voltage section and if shorted can burn
up the transformer. These get replaced with new ones with the appropriate voltage
rating. Next to be replaced are the old paper and foil capacitors. These are used to
couple the stages of the radio and isolate DC voltages from AC signals.
Then the resistors are checked to see if their values have changed over time
Old carbon composition resistors can drift up in resistance to several times
their original value. If the measure more than 20 percent off they will
be replaced with new ones. Then all of the tubes are tested and replaced if weak.
Tube sockets, the volume and tone controls, and the band switch (if applicable) are are cleaned.
A new power cord is installed and any deteriorated wiring is replaced. Then the radio is ready
to be turned on at this point. Once on, the performance of the set can be evaluated and if needed,
troubleshooting and adjustments can be performed. Finally an alignment of the RF section is done
using a signal generator to optimize reception. Dial bulbs and dial cords or belts are replaced if needed
while the chassis is out. Once everything works right, the chassis can be reinstalled into the cabinet.
Here's a view of the Zenith chassis after restoration