What about D213 and D216? You can simply measure the voltage at the cathode of D213, and the anode of D216 (or at the Base of the associated transistor, if that's easier to get to). Should be around 10V.
If the diodes are open, the output voltage would be just about what you are seeing (around 16 to 18V).
It pretty much HAS TO BE either the transistors, the Zener diodes, or an external short across the transistors (both of them).
Pete
saipan59 (Pete)