Thanks Chris, your input is much appreciated.
I have looked at the transformer and there are clear signs of oozing varnish and molten solder drops where the +50v supply lead out wires enter the secondary windings. It is inside the windings so not easily accessible and the whole thing is dipped in varnish so it would get messy. This ooze was there when I initially did a visual on it before powering up so it has been heading for failure for some time. When it powered up successfully I thought I had got away with it
I have thought about adding a separate transformer as I have done that before when heater transformers have failed. Unfortunately there is no space inside the Cossor, it is very well stuffed! I did measure the voltage between taps m and n before it went down and it was 56.3v loaded, so I probably need 60 to 70v unloaded. There is another tapping off the transformer, points o and p on the circuit A schematic, that goes to the -50v supply. That measured 56.0v loaded so it is the same and still works so I will measure that unloaded and see what the voltage is.
The -50v supply is fused at 1 amp and the +50v at 500mA. As the +50v is lower rated I was tempted to connect both the +50v supply and the -50v supply inputs to the o and p tapping but I don't want to overload it and blow another winding. It can't get much worse because the transformer is already broken. But, I am puzzled why they would use two tappings with the same voltage when one with a higher rating would do. Is there any reason why they would go to the expense of splitting them in two? Better regulation perhaps?
To get the scope going again, at least for testing, I will run the +50v supply from a bench power supply so I can monitor the current and consider connecting both supplies to the one tapping.
Does anyone know if there any way of assessing the current capability of a tapping?
Cheers
Mel