What is (was) this component at ZD102 (top left, next to powered serial connector)
TA7805F can handle up to 35V input voltage. I would remove the diode or use a continuity tester to see if the diode is parallel to the 12V power supply.

Take care, the diode might be shorted so better remove-it from the board. 

If it was a Zener it should be with cathode to 12V and anode to ground and a breakdown voltage higher than than maximum voltage of the power supply but lower than the maximum input voltage for any component powered from 12V line. 

It seems that the board is double sided so it should be easy to follow the 12V line to see where it goes. I suspect that it goes only to TA7805F and some filtering capacitors and you can read that rated voltage.

If the power source is reliable you can put just a simple diode keeping only the reverse voltage protection which I suspect that it is the source of the fault.

The later revisions of the board have a diode in series with the 12V line which leads me to the conclusion that reverse voltage is a common source of failure for this device.