[WME_bill]

HM307
Hang on chaps. Bright-up circuit.
Pin 5 collector of Opto-coupler should be same as emitter of T352, not the collector. The collector voltage from your measurements is 2v positive (less negative), which suggests the transistor is hard on. The emitter will be somewhere between 0.6v and 2v positive of the 867V rail.

W/D comments that the voltage at pin 4 of IC304 moves when you switch the blanking input by changing from Ext X to Sweep on, not triggered. But T353 appears dormant. Have you changed it?
My reasoning is that Ext X will give full brightness. Sweep on, not triggered wil give sweep blanked. That means Nornal trigger, not Auto. no signal input, or Trigger Level set way off.

The sweep blanking signal from the timebase into the Opto-coupler is at 0v during the sweep and +3.4V when blanked and TB waiting to be triggered.

Measuring small voltage differences when floating at high levels is neither comfortable, nor likely to be very accurate, unless you have a 5/6 digit voltmeter, to sensibly read 0.6v on 1000v scale, which your measurements of yesterday afternoon imply.
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