Troubleshooting update.
Observing that 3.3V linear supply was not enabled, and that the linear supply needs to come on before anything else happens, I tested the supply board alone by connecting the battery supply to the '+12' pin and 'GND' pins on the 2X4 socket on the board, leaving the enable open the 3.3V output appears on its pin. The enable signal taken to GND turns the 3.3V regulator OFF.
Next, I plugged the 3.3V board in via a 2X4 socket with long pins so that I could bend up the enable pin to leave it open, then pressing the control panel button turns on the front section of the other supply board to send the battery voltage to the 3.3V linear regulator. Still did not work. Then I noticed that the input current from the battery jumped to about 500+ mA as long as the button is pressed. Ohmmeter indicates approx 0.6 ohm between Vdd and GND. So it looks like the orignal failure was some damaged component on the Vdd bus caused by a turn-off transient.
I examined a number of components with a sensitive DVM directly across the pads of each component, hoping to find one that showed a larger voltage indicating that it was the location of the short. First tried across supply chokes L201 and L202 (no voltage rise with supply ON). Then several other of the many bypass caps, IC directly connected to Vdd. Not really successful. Nevertheless, I removed C501, then IC501, IC502 but Vdd still shorted.
