Thanks for your attention. I had found those 2 schematics. Unless some detail is not shown, the resistance between the sleeve/shield contacts of the scope input jacks (J1 and J2) should be low/zero. Same for the GNDA header pins 28 and 30, all 4 of those points should be common.
I find no connection between the J1 and J2 shields/gndA and "anything".
I've made the jack connection 3 ways; via the shield/sleeve on an inserted cable, with a probe in the jack, via probes on the solder pin at the board.
(The shield of a plugged in cord is common to the solder pin of the same connector so know the cord and jacks are fine.)
Also no connection between either shield and either pins 28 or 30.
I do find a connection directly between pins 28 and 29... indicating that they are properly soldered to the common "GNDA plane/trace"
A bright light inspection of the board seems to reveal a sub-surface "GNDA" plane under the jacks.
The solder connections (at the surface of the board) of the J1 and J2 GndA pins look fine... this suggests a "via" failure on both the jacks.
That seems unlikely to be a completely isolated occurrence. Has this been previously noted?
Do I have a new run? (DB9E1A0, REV C, PB200-418)
It might be possible to repair by heating and "reflowing" the solder on the shield pins... drawing it deeper into the board.
If not, I think perhaps the sub-surface GndA trace/plane might be exposed and the missing connections added.
Please comment on the best repair approach as I'd rather repair than return (if practical).
Too many words, see the image.