I unplugged the HV cable for the 'Upper CH2 Field' planes (bottom-most connector on card 14). I shorted the central conductor to ground on that end of the HV cable. Then I looked for a finite resistance between a signal pin on U13 and the chamber ground. All pins showed open circuit (as they should).
So, either our conclusion that a field wire had bonded to a signal wire
on U13 is incorrect or the problem fixed itself. I will try plugging in
a new A/D card when I get the chance.