For people reading this thread in the archives, here's what I found as the cause
of the dim display on my Sony STR-AV780 receiver. (This probably also applies
to the STR-AV790 and STR-AV880 receivers.)

All the DC voltages on the fluorescent display appeared normal. Then I checked
the AC voltage across pins 2 and 75 of the display while it was dim. It was 3.1
volts. The correct value is not marked on the schematic so I waited until the
display was at full brightness. Now the voltmeter read 4.6 volts! I was able
to correct the dim display by tugging on the cable attached to connector CNJ33
at the main board. Then I found that a little push on the 1.5 ohm resistor R818
which is in series with the power supply circuit to the display also brought
back full brightness.

I resoldered all the connections in the area of the resistor. It seems to have
fixed the problem. I'll watch it for a few days before I put the cover back on.