in DC voltage mode -- there's no beeping =)

Check page 84 of your manual to see the voltages on the schematics for the power supply board.

To get ideas on what to check in what order -- study this:

http://techniek.flipperwinkel.nl/wms11/index2.htm

Also - you can search for "test points" to quickly see what values to test for.

F3 on which board? power or aux power?

Search "site:pinside.com" + "system 11C" + "F3" or "site:pinside.com" + "Diner + F3" to get a sense of the range of ideas on what to check.

also, you can substitute Diner with other 11c's

Progress! Looks like you may have things isolated to the master display board.

https://www.pinwiki.com/wiki/images/d/d2/D-12232.jpg
https://pinside.com/pinball/forum/topic/system-11c-display-issue-single-digit-broken

shot in the dark:
With the power off -- check for a short on U19 UDN7180

get them in front of you.
https://www.ipdb.org/machine.cgi?id=681
-mof

http://techniek.flipperwinkel.nl/wms11/index3.htm

internet -- how do you check for chip shorts on the backside of the master display board?

Seems tricky.

http://73.21.92.229/sys11/index3.htm
"Testing the UDN7180/UDN6118 Chips and Score Glass for Shorts."

good luck ! DMM set to diode test... 7 * 2 tests per chip... hunt for a difference!

Was this machine in a known state or unknown to start? I don't recall whether we expect everything to be great and something recently changed, or is this a recent pickup, and an unknown.

Continuity check the ribbon cable wire by wire. That's low hanging fruit to isolate. If the DMM points are not small enough, wrap a dead diode leg (clip the legs off) around the end of each probe tip.

-mof