...my suspicion is that somewhere I'm loosing signals on the main board.
That may very well be. Could also be an intermittent. I would pay extra attention that logic signals are within acceptable TTL range: <=0.8V low, >=2.0V high. CONT DATA is particularly suspect because it is shared amongst a bunch of chips and one of them could be killing it.
U140 cascaded to U150 forms a 16-bit shift register. Both chips control the relays, but if U140 isn't receiving any data, U150 won't clock in any data either. So, it's not a big surprise that none of the relays are working.
I'd love to know what U140 pin 2 is doing. That's CONT DATA. And the shift register clock on U140 pin 8, ATTN CLK.
If those look ok, you could check the outputs of U140 to see if they're changing, such as pins 3 and 4 when you change to and from AC Coupling. They won't have a persistent state since other values get shifted through U140, but you should at least see something. (The attenuator relays are latching relays, so they aren't powered except for changing state.)
Definitely check the power supply directly on the chip pins in the suspect areas, like U140 and U150.
Good luck! Let us know what you find.