The 478A Thermistor Mount is an RF power sensor head that is compatible with the 431 and 432 series and the N432A RF power meters. Its frequency range is 10 MHz to 10 GHz, and its power range is -30 dBm to 10 dBm. Maximum power is not to exceed 30 mW (+15 dBm) average power or 5 W*microsecond energy. It has a Type N male input connector. Introduced in 1961, it is still produced and sold by Keysight today, as part number 478A-H75/H76.

If a sensor has been over powered then it might read power but not be able to be zeroed There is a possible fix and one should consult the early service manual but basically within the power sensor are 2 screws and by monitoring the Zero level as we want the Zero level to read Zero on the Meter scale ( left hand Zero Not 0 Dbm ) adjust each screw individually by 1/8 of a turn as in adjust screw 1 by 1/8 of a turn then the same for screw 2 provided you see the meter needle going down scale keep going you only have about 2 turns to play with before the sensor is toast / NBG if All is ok turn the Zero pot on the 431/432 half a turn of the end stop and adjust for Zero this will give you a small amount of zero rang of adjustment The sensor will now work but the Temperature compensation will be degraded slightly