I hope it's not the CRT and IMO more likely the HV grid biasing IME. Old drifted carbon comp resistors is where I looked in the past and replaced heaps of them especially those in M 
values.
Short sweep can be something similar although checking sweep timing is worth a look at too.
At least it's a runner so shouldn't be too difficult to get right. 
Yeah a runner as you say, just some mild electrical debugging, the usual !
Got carried away again, spent the afternoon on the thing, so didn't get anything done in the house, again..
Anyway found the cause of the weird distorted/sloped trace. I was on the right track with the CRT. Thank you dear 575 for teaching me this lesson, it's already proving useful. So I checked the connector at the CRT neck, zero oxidation, so not that. Deoxit-ed it anyway while I was in there, can't hurt. Then I thought OK let's Deoxit the 4 pins/terminal for the deflection plates.
Guess what ? One of them was disconnected ! Wire was dangling a few millimeters from the pin ! It was the right horizontal deflection plate. It's a miracle I could have any kind of sweep on the screen ! Capacitive coupling saved the day maybe ? Anyhow, I reconnected that plate and problem solved !
No more inexplicable distortion of the trace.
Better : it ALSO solved another issue : the severely reduced horizontal span of the trace, like 50% of the screen width. Solved too. Makes sense of course... so the horizontal amp was not to blame in the end !
Piccie below to prove
So now I was left mostly with vertical issues. DC balance all over the shop mostly. Except that the condition of the scope was somehow deteriorting at the speed of light, in real time, I could see it happening as I was playing with the thing. Symptoms would appear and worsen in the course of just a few minutes !
I was now suddenly having an EXTREMELY noisy trace (was displaying the calibrator output all along), and it would flicker and sometimes go out completely for long seconds, come back out of the blue... it looked like massively corroded/dirty contacts, but I knew for a fact there was no corrosion in that scope, never mind to the extend that would be required to cause such dramatic effects. Plus, of course, just last night when I first tested it, it was perfectly fine in this regard !!
So I tried playing with the attenuator knob, as well as its secondary red knob, but clearly wiggling them had no effect on my problems... I sprayed some deoxit anyway and exercised the knobs, to no avail, as I thought.
Then I started noticing that the trace would shift upward, up and up and up... soon the vertical position knob would be out of range and would not be enough to bring trace back onto the screen. Within minutes it was so far off the screen that the only way to see a little something, was to crank up the amplifier sensitivity, to 0.1V/DIV while also cranking up the calibrator output voltage, to 100V. This way I could get the bottom part of the square wave to "enter" the screen real estate...
Well I already know from restoring my first 317 that this massive offset problem was a tired 6AU6 in the V amplifier, causing an imbalance between the upper and low triode of a given stage. So I looked at the four 6AU6 that make up the amp, looking for their glow. They were al glowing. Moments later trace disappeared for good from the screen... looked at the 6AU6's again and oh, what do you know, two of them just died, they were not glowing at all any more !
I pulled a couple 6AU6 from another Tek scope, condition unknown but well you gotta try something eh.
Problem fixed ! Trace back on screen + no more dramatic noise. It also cured the DC balance issue I was having ! Now the trace stays in place no matter what the position of the attenuator knob !
However... now I was having the opposite problem : trace would shift DOWN ward.. albeit not as bad. Trace would end up almost at the edge of edge of the graticule, but the Vertical position knob still had enough range to bring the trace in the center.
So I looked at the 4 6AU6's again... noticed that the two I just implanted, were glowing a lot brighter than the two I had left alone. So I guess a difference in emission/gain hence an imbalance again.
But who cares, the point was to troubleshoot my problems, that's done. I now know the problems were indeed the 6AU6. So if I were to restore this scope I would just have to order 4 new 6AU6 and that's it.
The some more problems arrived.. I told you it was deteriorating in real time !
I would notice something strange : with the input grounded, the trace would flicker, and even sometimes come and go... BUT if I display some signal, calibrator output again, then trace was stable and flickering.. hmmm...
After a moment, the grounded input trace would disappear for good...
Trigger mod was set to AUTO all along, so even with no signal present I should still have a trace on the screen...
So eventually I noticed that AUTO mode was not working... and that's why I could have a trace on the screen ONLY if I had a signal present, because then it would be able to trigger on that signal rather relying on AUTO.
Even weirder : later I noticed that the scope COULD display a trace with no input, IF you select trigger on the FALLING edge rather than the RISING edge ! Somehow ! Using the rising/falling edge knob, I could 100% reliably get the trace to come and go at will !
I don't think that there is a problem with trigger sensitivity, it just does not match the symptoms. It would not cause the trigger to work only on a specific edge, and also it can trigger just fine on the calibrator signal. I tested it, it can trigger reliably for a signal amplitude of only 2 minor divisions peak to peak, which is perfectly good in my book.
So I don't quite know...
Anyway, it's the only a minor issue, can't be that hard to fix if I so wished one day.
Scope is basically fixed.. putting covers back on and into storage, so I have zero storage spot left for it so it will have to stay in a corner of the bench... I guess if removed the brakes discs and pads that have been sitting on the bench for a month, I could put the 317 there...
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