Well done on solving the problem.
I notice looking at the schematic that there are a large number of 100R grid stopper resistors on the circuitry. Unfortunately Telequipment use a specific type of these in some of their scope production runs of this era. The ones in your scope might in fact be a different design or from a different supplier and be just fine. I had multiple resistors go open circuit in a D52 scope of a similar vintage. The problem is described in the second half of this brief article: (the first half of the article is another topic)
http://www.worldphaco.com/uploads/EL...AIN_FRAMES.pdf
So if you see any of these sorts of resistors in your scope, its a good idea to replace all of them.