Thanks Roger and Mark,
The trace dims or disappears at slow sweep rates and improves as the sweep is switched to faster speeds. I tried it with a faster signal than the internal calibrator and it did the same but at higher speeds. It starts to behave normally at sweep speeds that show a couple of cycles across the screen, but at slower speeds there's nothing. I checked those test points on the Z axis board with another scope and they looked OK. Test point 1778 did not show any pulses,just a DC voltage that varies with the intensity control settings. Since the focus & A and B intensity controls are working I'm not able to interpret that further. The behaviour is the same with either timebase. I'm suspecting the intensity grid DC restorer but as it's buried in the bowels of the power supply I only want to look into it as a last resort. Ditto for R1659 and R1684 and the other high value resistors as suggested by Mark. If I do have to get into the HV board I'll replace the lot.
It's unfortunate that the schematics for the 7904 (non A) don't show typical voltages or waveforms.
More news as it breaks!
Morris
