There is a group called "TekScopes" that has more traffic, you might want to post there as well.
I owned a TDS744A that came to me with analog input issues. Mine did have a blown input module, but there is another issue that you should consider. There are metal shields over the ceramic boards at the analog inputs. This shield is soldered at several points around the board and it provides the ground connections. I had a channel with a relay that was not switching and it turned out to be a cracked solder at one of those points.
Also the relays on those boards are through-hole parts that are "standing" on the board with small solder points, so if you tear it down that far it's certainly worth checking those as well.
When soldering to those ceramic boards be careful with heat. Use just enough to get it to solder and then get out. The ceramic boards are delicate, and the thick-film metallization can leach into the solder. There is probably a specific solder that is recommended (maybe someone can comment on that) but in my case I just used SN63 solder.
Good luck - keep us updated.
