Thermal cutoff

Thermal cutoff switches are common in all Tektronix oscilloscopes starting in the mid-1950's. If the chassis gets above a certain temperature, typically about 133 Fahrenheit (56 Celsius), the switch opens. Typically this interrupts the mains power coming into the scope and the scope will cool down. Thermal cutoff switches used in Tek scopes are self-resetting. After the scope cools down sufficiently, the thermal switch will close, allowing power into the instrument.