After long operation (~1h) my hot air refused to heat. I've checked heater- OK (cold resistance approx. 70 ohm).
I decided to draw schematic from the PCB to understand what is going on.
The failure was open R4 instead of 100R. It dissipates about
160mW 300mW.
This resistor biases 7V5 Zener which is the only regulated voltage in this device (!).
My suggestion is to increase the resistance to 220R but this change may affect temperature calibration.
So after this mod I used thermocouple thermometer to calibrate the device using front panel accessible 'CAL' potentiometer.