Start by taking your meter and setting the selector to Voltage (DC). Then place the black lead of the meter firmly under the braided wire in the backbox (Ground) or use alligator clips to make a secure connection to the braided wire. You now have ground on the black wire.
Now go to the back of the dmd display, and measure the voltages on the pins where the cable from J604 on the display driver board is attached.
Place the red lead on pin 1 (Orange), and write down the value measured (Should be very close to -125 V or -112V)
Place the red lead on pin 2 (Blue), and write down the value measured (Should be very close to -113 V or -100 V)
Place the red lead on pin 6 (Gray), and write down the value measured (Should be very close to +5 V)
Place the red lead on pin 7 (Gray/Yellow), and write down the value measured (Should very be close to +12 V)
Place the red lead on pin 8 (Brown), and write down the value measured (Should be very close to +62 V)
NOTE: If -125 is measured on pin 1, Pin 2 needs to measure about -113. But if -112 is measured on Pin 1, Pin 2 needs to measure about -100. The important thing here is that the difference between Pin 1 and 2 is 12VDC.
I bet one or more of the voltages are way off or missing. But report back with your measurements... It could very easily be one of the "big white" resistors on the display driver board.
Display voltages (resized).png