Harris Gates series airflow sensor assembly
Airflow sensor with 2nd sensor for right side blower
In what was clearly an afterthought in the Gates series of AM transmitters, Harris used an innovative way to detect blower fan failure using no moving devices such as vanes. This circuit worked by running two thermistors into a voltage comparator, one of which was attached to a resistor generating heat. As long as there was airflow over the heated thermistor’s heatsink, the comparator wouldn’t trip, but if the airflow stopped, it would shut off the 250V main PA power supply and sounded an alarm. These airflow sensors, of necessity having to live in the often dirty airstream entering the rear of the unit, tended to fail and be bypassed, depriving the transmitter of its only overheating protection. We stepped into this gap by basically cloning the original, except for a replacement miniature relay, as the original was NLA. Drops in, works great.