Steve, this will not help you for SI but will confirm transducer wiring can be repair. Several years ago I had an HB 597 or 598 on the bow and the cable got nicked badly. This unit was 2D/DI (no SI). I successfully spliced the cable, soldered all wiring, shielded them, and put it all back with the proper protective sleeve, etc. It worked just as good before getting nicked.
So I don't know if your rework is the cause or your crystals are bad. I am just confirming reworking the cables can be done.
LIVE PURPLE LOVE GOLD