[SiriusHardware]

On consideration, and following my own advice of 'always try everything you can before spending more money', let's do that-

-Reinstate UB3 pin 16 connection
-Remove UD6 through UD9, remembering where each one goes
-Power up and see if the output of UB3 pin 16 is distressed / is not distressed.

If it is not distressed, replace each of UD6 to UD9, checking the output of UB3 pin 16 after each reinsertion.

If it starts off not distressed and becomes distressed when you have one or more of UD6 to UD9 in, try to work out if it is just one of those ICs which causes the effect or if it is related to the number of ICs you have inserted.

Edit, Ah yes, I missed where BA10 and several other lines circle round and go up to the top left of sheet 1.