Hi Steve,
We progress.
To innocent A&R, put a '1' on display ; that digit does not transmit the signal for segment 'a (only 'c' -disp0- and 'b' + 'dp' on disp1).
On the scope there must be no signal on disp2 pin 9.
Steve wrote:But, flexing the main PCB ever so slightly in the area near the disp2 output (lower right-hand corner of board) causes the display glitch to come and go at will.
Do that with the scope probing A&R pin9 : it should "disambiguate" the problem. If the image on the scope is ok (no 6v pulse), then it is a mechanical problem.
The problem could be a bad (old) solder (if your equipped, pump them all and then resolder the chip).
I can reproduce this 'glitch' easily bringing a 'spike' positive voltage on line disp2.
I can't say more remotely : if you have a video cam handy, you can try to show me your 'flexing action' :
- do you touch the A&R chip,
- or just the PCB? and where?
Look this pic, the trace disp2 takes a relay on component side : check continuity.
Check also the hole in the pcb for disp2 (no green traces??) ; clean the path with vinegar+water solution on an old toothbrush.
Check the continuity from A&R pin 9 to anode driver with a ohmmeter or better with a scope.
And displace the display pcb probe, in direction of the connector, while you do your 'flexing' until you'll find the spot that causes the discontinuity OR the short circuit with a voltage.
But the same problem on 2 different display PCB would be curious.
However I've seen things stranger.
Concentrate on the link : the disp2 hole in the main PCB and the connector pin.
Very interesting case.
Your close to the solution,
Take care,
Jacques.