In the majority of cases it will be found that this condition is caused by an intermittent open in the .01 mfd. audio coupling condenser. Examination of the small wires which make contact between the internal parts of the condenser and the terminals on the bakelite housing will usually show that this wire is solid. The condenser should be replaced with the later type having the stranded wire instead of the solid wire. (The condenser to wire lead connections were the culprit. Using a new style capacitor here will suffice.)
In addition to this fault, it will sometimes be found that the bakelite terminal strips on the side of the tuning condenser are loose. Tightening the screws which hold these strips will correct the condition entirely.
From the Philco Serviceman – March, 1935.