What I meant in the Feb19th post when I wrote it was that the voltage between collector and emitter would be too low for the transistors to conduct the required curent. This was a mistake in my logic since in this circuit when the collector voltage (nominally +5V) drops the emitter voltage also drops until the transistor can conduct the current. Frankly a lot of my attempts to understand the interplay of the power supplies were wrong and are probably best ignored. At least I have learned a little even if I have confused things for you. Apologies.
Tracking down the 'short' circuit on +50V should not be too difficult, the first step is to isolate the various boards by unplugging the power in and power out ribbon cables, note the orientation but they should be marked with an arrow on the connector and on the PCB. In most cases the short circuit can be measured with the power off just using a multimeter to measure resistance between +50V and ground on each board. On the circuit schematics you are then looking for components connected between +50V and ground either directly or with very small value series resistors. Electrolytic capacitors are prime suspects, but you might find mechanical damage or a foreign object. If you have a suspect component then unsolder one lead and test the component again. If it is difficult to unsolder one end then you can at your discretion snip one lead, test, and remake the cut lead with some solder.