thank you Dan/Mark. nearly there...
There was no reference voltage . I've therefore replaced U752 (LM741). This has restored the sweep on the interface board, AV, AU (holdoff) and can now see the sweep on TP450
I still have the 7B92A on the vertical slot and cannot see any sweep
I found that Q902 and Q912 were leaky and have replaced both on the output board.
I can now see the sweep for the delayed time base but not for the main timebase (ie horizontal line).
I can see the sweep at TP450 so not sure where things go wrong on the output board
It is not that easy to troubleshoot the output board with the horizontal logic board on top of the interface board