does your desk have VUs? if so, the solo isolate switches are most likely in that 3x3 block on the 651 that has the spectra features on bargraph consoles.
i'm not at the studio, so i don't know which ribbon goes to those switches. usually, another d-sub clamps onto that ribbon (on the cable itself), and this hooks up to the wires on the upper bus cards.
which begs the question - are there any cables soldered to the underside of your upper bus cards?? if not, you don't have the solo isolate mod.
the only other things i could think of: dodgy upper bus card connectors (not sure, but i think it's S3 or S4), or a faulty trace on the card itself. did you look for bent pins on those connectors?
just check the solo bus up there for continuity, and i'm sure you'll find the failure.