Problem solved !! One of the modules on the second bus board was not seated correctly - I'd tested the "defective " modules on the bus that was actually source of the issue
So all is good - -apart from a VERY low level coming out of my 1996 A-188. Just under 1V. There was another thread about this 3-4 years ago I think. I'll check that before posting again
Thanks all for commenting on my stupidity :-)