[Findm-Keepm]

When the thermistor is in the set, it starts out at ~ 120 ohms, and DROPs in resistance as it warms up, to about 2 ohms or so hot. Shorting it out puts no voltage in the degaussing coils, so your degaussing is disabled until it is replaced.

From the RCA patent:

The thermistor is a device which when cool has a relatively high resistance but when hot has a relatively low resistance. The degaussing magnet coils are energized by the voltage drop across the thermistor in a circuit which includes a varistor (voltage dependent resistor). The varistor is a device which has a relatively low resistance when a high voltage is impressed across it and a relatively high resistance when the impressed voltage is low. It will be observed that the resistive properties of the thermistor and the varistor are of a reciprocal nature.

The energization of the degaussing coils is as follows, assuming that the thermistor is cold, i.e. at room temperature. immediately upon closure of the power switch the current flow through the thermistor to the power supply produces a relatively large voltage drop across the thermistor. At this time the resistance of the varistor is low so that alternating current flows through the degaussing coils at considerable amplitude. As a consequence, a relatively strong alternating field is established in the screen region of the picture tube. As the thermistor warms in response to current flow it's resistance decreases thereby decreasing the voltage drop across it. As a result of the decreased voltage the resistance of the varistor increases thereby decreasing the current flow through the degaussing magnet coils. As a consequence, the strength of the alternating field produced in the picture tube decreases. The thermistor ultimately heats sufficiently that its resistance and the voltage drop across it are very small so that the resultant voltage applied to the varistor is so small that the current through it and the degaussing coils is so small as to be effectively zero.

For BPC sets, the thermistor is about 16-25 ohms cold, too low to energize the coils in your set effectively. Get an FR191, FR291, or FR922 thermistor for your set. All three have relevant RCA part numbers with the last digits of the RCA part number matching the 191, 291, and 922......betcha a lot of folks wondered how they got numbered.....now you know:

107191 = FR191
107291 = FR291
RCA drawing number 942922 and all suffixes = FR922, or FR291, both are heavy duty 120 ohm thermistors.....

Cheers,