ioch has opened our ZD939 hot air SMD rework station after it started reporting too low temperature and heating up uncontrollably. 

After opening the instrument, he couldn’t decide if a solder blob across the input capacitor is a design decision or just a “quality mark” of the maker.

Later ioch poked around the op-amp responsible for thermocouple conditioning and decided to change it, which fixed the problem.

After that it was just a matter of calibration. Unfortunately, the station measures the temperature of the heater, not of the air itself - the actual temperature of the hot air is ~50°C lower than the reported one. That said, the station still does the work now!