Well that was a short lived success but I know a root cause now.
After doing some debugging and pulling the vertical again, I have confirmed that the second multiplier is now dead as well. Not only that but when it went it took the multiplier winding of the HT transformer out too. Turns out this winding is more fragile than the fuse is. Poking around it appears that the HT oscillator feedback resistor from the high voltage side of things (the thick film array one) is shot causing the voltage to shoot up to the overvoltage protection. It has basically been running wedged up there until the multiplier fails. Really I suspect it is a manufacturing flaw that was possible to cal out with the grid bias as there's no sign of use otherwise. Not even any dirt in the fan.
It's dead. RIP. Now 100% BER. So it's now an organ harvesting operation.
Just a quick postscript. I am now officially giving up on restoring old Tektronix scopes. I'm thoroughly done. Many have passed through my hands now and I am not going to do any more. I need a new challenge. Here's the tally though. I think I did good considering everything I bought was broken or for scrap or was supposedly working but wasn't or wasn't shown as working...
Restored (20 scopes)
- 3x 475
- 2x 475A
- 5x 465
- 3x 465B
- 2x 453
- 1x 2235A
- 3x 2225
- 1x 7603
Scrapped (5 scopes)
- 1x 465B
- 1x 465
- 1x 475
- 1x 453
- 1x 2235
I'm not including the 5x Philips, 2x Hameg, 2x HP, 1x Thurlby, 2x LG, 1x Hitachi, 6x Telequipment (3x D83, 2xD61, 1xS61) ones on top of that as well
Phew
...