After a few hours on this I am still having a problem with R8 blowing open.
What I have done so far in investigating this:-
I have lifted R83 (the 1000R resistor in the regulator feedback line going to Q15)
I connected a PSU to the lifted end, so I can feed a controled voltage to Q15 base through the resistor.
I connected a voltmeter to the pad where R83 was lifted to see what feedback voltage I get. I have disconnected the plug from the cathode, and the lead to the EHT is disconnected. I also disconnected the focus POT, so nothing directly connected to the -2950HT.
This results in me being able to manually regulate the voltage and measure feedback voltage. In theory regulation will occure when these two voltages match.
I am also monitoring Q1 base for oscillation with an oscilloscope.
Powered it up, and at -0.3V to Q15 the PSU oscillates and I get a stable output of about 1800V. Feedback voltage is about +50mV as HT is very low.
However after about 30 seconds, R18 blows open again!
I have removed C6 (0.1uF (its an error on the drawing showing 0.01uF)) and the same thing happens.
My diagnosis is there is a resistance to ground, but I can not find it. An ohm meter shows infinate resistance, and a 1kV megger shows infinate resistance (after disconnecting a winding of the transformer)
I am running out of ideas!
Still can not work out CR7 (400V 1 amp diode) (strapped between 300V AC and -2950V DC) which effectively shorts out the two rails!