If the set do have stb on, then i will say you should first check the B+ cap and measure the voltage.It should be around 100v and above but not more than 160v.If the B+ voltage is there then go the next step by checking on the horizontal drive circuit to see if there is a signal to the HOT base.If there is a signal at the base of horizontal output transistor then must be working by then unless there might be something wrong just around the flybacck circuit and the x-ray is kicking in to provent more damage to the set.But if there is no signal to the HOT base,start by checking the base driver circuitry there might be a n open resistor.
Francis Kuuderb, September 2017
