Quote:
R306 removed:
Input V of pin 2: 0V
Input V of pin 3: -1.9V
Output V of pin 6: -5.2V. Varies with HV adjustment.
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That seems to back up Bills observation that T301 is leaky. The only way you should get a negative voltage at the non-inverted input to the 741 IC is if T301 is turned on. In that case there will be a current flow to ground providing a ground reference to the rectifier. The opposite side of the rectifier then becomes negative at around -1kv output relative to ground. With R306 disconnected, T301 should be turned off. Instead it appears to be leaking slightly more current allowing the voltage on the -1k rail head further into the negative which in turn shifts the voltage at the wiper of R318 slightly further into the negative.
WME_Bill is right. The 200v rating of the BD157 is evidently not sufficient to survive the possible transient voltages in the circuit and so failed under stress. The +163v noted on the circuit at the collector of T301 is the nominal running voltage, but the simulator confirms a much higher transient does occur for a fraction of a second at power up. The MJE13003 sounds like a good option.
BTW, RS sell a KSE13003 which appears to have similar characteristics.