Moving on quickly from a shaky start, I've checked the output voltages from the mains transformer (no valves or bulbs) and they are broadly in line with those shown on the Manufacturers Service Sheet No. 116:-
U50 Anodes: 281V
U50 Filament: 5.2V
Scale Lamps: 6.9V
Anywhere else I should check at this time?
I'm going to replace the smoothing caps and other electrolytics if only to try and understand the work that has already been carried out. On the Trader Sheet these will include C14 (25uF), C17 (4uF), C24 (16uF) and C25 (20uF). These either measure very high or no reading whatsoever?
I appreciate that it's not good practice to connect new components across the terminals of a duff capacitor but is it ok to use one terminal only of the C24 electrolytic as an anchor point?
I still haven't worked out what the electrolytic capacitor to the left of the output transformer is doing: -
Normally with caps I would replace then test one at a time but as we had an internal short between one of the anodes and the filament in the UL50 rectifier valve I'm not taking any chances! Unless of course anyone can suggest where the problem may have been - my money is on the smoothing caps?