Hi chaps,
Thanks a lot for your help !
George it's incredible that you used that scope back in the day when it was brand new ! Reading you feels like I am in a time machine !

I hope you enjoy this thread then, and am now extra motivated to post about it... and will make sure to try and revive my other 502 for extra content !

So I finally "fixed" it ! I will cut the story short but in the end yeah, Factory was right, the only "problem" was the stability adjust trimmer that needed a tweak !

I apologize if you feel that I might have wasted your time

... but as for me I don't consider it a waste of time as it allowed me to dig much deeper into that subject than I have ever done before.
I learned things, which is an important part of working / fixing these old scopes which were alien to me just a handful of years ago when I bought my first tube scope, or tube anything.
This knowledge will serve me reaaaal soon might I add : 2 or 3 years ago I presented on TEA my second 502A. Started work on it and gave up after many, many hours of head scratching, trying to understand why the freaking scope would not trigger when everything seemed alright.... the prospect of having at long last a solution to this issue has me overjoyed, and I shall therefore revisit this scope real soon !

So, to my defense, here is why I didn't play with that trimmer sooner :
1) It's a trimmer, it's got no knob, it's recessed and requires a screwdriver.... and the general rule when working on these old scopes or old anything, is to not mess with trimmers unless you know what you are doing, because it's likely you are going to throw a calibrated scope completely out of cal and create a lot of pain for yourself later.
2) The manual explained what this trimmer was for : make sure that the sweep generator free-runs ONLY when asked to do so, i.e. when the trigger control is set fully CW to the "recurrent" position, and NOT at any other time.
And well, the scope was doing just that as is ! Scope was free-running just fine when set to recurrent mode, and would never free-run under other circumstances.
So, I thought that whatever it was set to, it was most likely set properly since it was doing its job !

Here is a picture showing the issue fixed.... we have a nice stable trace, well two of them, of the calibrator output. It bings me joy just looking at that screen

.. means that scope it now working fine and now we are down to fine tuning, checking for calibration of everything, and chasing offsets / gain issues / tired tubes in amps.
So that's what we are going to do then...
Let's start with :
- The horizontal amp : I measured the frequency of the calibrator, it's pretty much 1kHz, clocked at 1,0139Hz , however as you an see the trace on the graticule occupies only 4.4 DIV , so 8.8ms, that is 1,136Hz so about 10% too much.
- Still in the horizontal amp, as stated earlier : the position control is heavily biased toward the right side. A tired tube somewhere in the amp probably.
- Vertical amp : the calibrator is set to 10Vpp but the trace as you can see (2V/DIV) is more like 4.2 DIV so 8.4Volts not 10V... the calibrator could be out of adjustment, I will check/adjust that first, but I doubt it's out by THAT much, so the vertical amp(s) most likely need to be checked for gain and/or tired tubes.
- The trace is not perfectly aligned with the graticule, tube needs to be rotated just a hair CCW.
- The intensity balance trimmer, I tried to adjust it but failed... I mean it does have an effect but I failed to adjust it properly due to the very poor mechanical implementation of it, so to speak.... --> when you shove a screwdriver threw the hole in the metal shield of the CRT circuitry, the trimmer runs away from you !
It's not supported at all, it doesn't stay in place !

I will remove the shield and see what's what, but I think the problem is likely that the trimmer has all of its 3 pins on the same side of its package, and all 3 must be soldered on the same ceramic strip. So if you push on it with the screwdriver, the trimmer just bent downward

To be more clear... it makes this trimmer absolutely unusable. I think Tek eventually realised this, because I noticed that on the later 502A, they relocated that trimmer to the front panel, where it "just works".
So I first need to find a "fix" to this problem, find a way, a safe (with regards to the HV present) and durable and reliable way of holding that trimmer in place, so that I can at last adjust it properly.
OK. So before I do that, now that the scope is basically working, I think it's now time to give the inside of the scope a good, good clean, to revive its former glory, make it more pleasurable to work inside, and also give all the contacts / switches a good dose of contact cleaner, as well as, as George said, clean the tube sockets and exercise all the tubes in their sockets. This will fix many small issues from the get go. This way if I am seeing an issue, I know it might be an actual issue that needs diagnosing, rather than just some dirty contact somewhere, a red herring that would waste lots of time needlessly.
Will try and do that this week-end...
So, looking back.... in the end, this attic find that looked so terrible... needed in fact nothing but a new rectifier tube and a tweak of a trimmer on the front panel to get going again !! Not bad at all eh ! Could have been WAY worse !
Hmm... this restoration is going really well, motivation is at a record high !
