Hi Chris,
The scope electronics are all good. There was a problem with the EHT regulator, the previous owner had put the power transistor in back to front i.e. swapped emitter and collector. One of the voltage quadrupler caps was loose and because there was an insulating fluid in the EHT box the cap connections were intermittent. I repaired the EHT board and refilled the EHT assembly with a high quality clear epoxy as the original setup was prone to leaks. After I did this all the tube voltages were OK and within spec from the tube data sheet. The EHT could be set exactly and was very stable. I have a pintek EHV probe good to 39 kV to do these measurements. The problem with the tube is that the two spots are oval with the long axis in the horizontal direction. I have made a couple of other posts which provide photos of this and other details. The two spots also appear flat on the top/bottom of the spot closest to the screen center. It's almost like something has happened to the beam splitter plate so that it is casting a shadow on the spots and diffracting them. The problem is you cannot see the gun assembly apart from the cathode and grid as the tube neck is coated with aquadag.
I realize it is very unusual for tubes like these to develop faults, but with the time-base turned off and with no vertical input the spots cannot be focussed to a round shape. And as I've said all the voltages appear OK. I thought the deflection plate mean voltages might be off, but these were spot on, (sorry the pun). I checked the plate voltages in differential and there was no ripple on them at all under the no- timebase/vertical signal condition. I checked the timebase with it on and it was excellent, very linear and noise free.
When I saw the problem initially and with the EHT way off due to the regulator, I was confident that it would be the tube power supplies, but alas I cannot fault them. All of the tube controls, brightness, focus, geometry and astigmatism as well as the vertical and horizontal positions work as expected with the exception that the spots cannot be focussed into fine circles.
I would really like to get another tube of the same type/model as I am sure that this will fix the problem.
Best Regards
Rob