Here is an update:
I checked the tube socket and it is clean as a whistle! The U550 in this scope failed a while back in that, the tripler section started blowing the fuse. Since this is not available anymore, I created my own U550. I did it in a way that the tripler is removable from the rest of it. That contraption of mine worked for almost a year or more till it blew the fuse last week. It only blows if the anode is connected. Assuming that my tripler has gone bad, for whatever it is worth, I rigged another 3 diode/3 cap tripler and made sure the 10kV is there and then connected the anode. The fuse blew again. It tells me that my original tripler is still good. The next thing was to make sure the rest of the U550 is not gone bad. So, I hooked up the original TEK U550 (without the EHT connection) and hooked up the external tripler. Again the 10kV is good but no sooner the anode is plugged in, the fuse blows. This made me think the tube might have gone bad and is drawing too much current. Luckily for me, I have another spare tube. So, I hooked that up. Well, it is the same result! So, now I am lost. It is clearly too much current in the anode circuit but what is causing it? I read through the HV section in the service manual and can't say it is pointing me to anything in particular. I have checked the transistors in the HV regulator and HV oscillator etc. But did not find any issue. There still could be a diode that breaks down on load while showing good on the meter. The only major item I cannot check by replacing is the HV transformer as I don't have one. But if that was bad, wouldn't it blow the fuse even if the anode is not connected? Also, it does generate the 10kV. Unless, it is shorting under load, what else can go bad?
