I do have spare resistors if required.
Going back over the schematics I cannot see anything else (other than U4,5&6) which may be causing the problem. Then in turn it could be any other IC hanging off the RIOTs.
Process of elimination - remove all the ICs involved and install them one by one until the signal remains high.
Each hour you spend on the repair will be an hour closer to the solution and you will learn as you go along.
