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The unbanded end of CR6 is the wrong location to measure AC. You need to measure AC across the unbanded ends of CR5 and CR7.

RectifierBoard-54.jpg

Good catch!
The game you linked was Xenon so that's the one I looked up. It appears Flash Gordon (which came out next after Xenon) also has those diodes marked the wrong way around in the schematic.

Remove the 4A fuse at F3, do you still measure any voltage at TP3?

Funny thing is that Fathom also has the correction in the schematic, but every game schematic after it reverted to the wrong diode numbering even the last game on this platform Cybernaut which came out in '85 and its rectifier board schematic was redrawn.

There's got to be a short circuit on the board and judging by the voltage I'm guessing with half the phase of the solenoid voltage.
Since the output of that bridge circuit at TP3 only goes to pin 10 of the J3 connector, with the machine off set your multi-meter to continuity mode, put one meter lead on TP3 and touch the other meter lead all over the rectifier board until you measure continuity to something other than CR6 and CR7.

A close visual inspection should also reveal the short. Post some clear high res pics of the board if you need some more eyes to look at it.

Have you got an oscilloscope? It will tell you the whole story.

If not, at this point I would hook up a load and remeasure the TP3 voltage. Use a 12V lamp something around 12W, 1 amp as a load connected between the GND and TP3 test points.

Yes, a #906 incandescent globe should be fine.

Try a different multi-meter if you have one. Make sure the rectifier board is being properly grounded too.