In most cases this condition can be corrected by changing the type 15 oscillator tube. In some cases it is necessary to lower the cathode resistor No. 12 in the wiring diagram of Service Bulletin No. 166 from 6,000 ohms to 4,000 ohms.
In a few cases extremely thin or cracked mica has been found in the high-frequency oscillator compensating condenser No. 9 in the wiring diagram of Service Bulletin No. 166. The trouble can be readily corrected by replacing the mica and readjusting the high-frequency condenser in the usual manner.
From the Philco Serviceman – Month ?, 1935.