OK worked on it most of the day, to calibrate it. Studied the manual, followed the procedure in order :
Power suppliesWell you gotta start with that eh.
Started of course with the reference -150V supply, especially since ISTR it had an issue, whereby I had to turn the trimmer fully to get to -150V. So I thought I would have to do some troubleshooting and search for drifted resistors somewhere. Well either it fixed itself, or I got mixed up with the other 502, but today that rail was just fine. The trimmer could produce an excursion of between -140V to -160V, so 10V either side of the target, perfect. Was easy to work with too, not touchy at all, and not noisy. I had no trouble adjusting it just right.
All other supplies followed suit as they should, no worries there :
-6.3V = -6.26V
+100 = 99.8V
+350V = 348.5V
Spec is +/- 3% so needless to say it's excellent...
Ripple : much better than I thought. I scoped every one of them. At first I could not make sense of it... it looked like a 1kHz square wave... sounds familiar doesn't it ! I turned the calibrator output off, which improved things quite a bit !

Somehow the square wave was polluting the supplies big time.
Result :
-6.3V = 30mVpp Manual has no spec for that. It's only used for the heaters of the 4 pre-amp tubes. However maybe this could pollute the input signal when using the most sensitive setting, 200uV/DIV ?
- 150V = 5 or 6mVpp, spec is 5mV so quite good, and doubly so since it's the reference supply.
+100V = 5mVpp, spec is 5mV so happy again.
+350V = 20mVpp, spec is 10mVpp, oh well, still pretty reasonable.
So it's certainly good enough for a calibration... I don't exactly run a modern cal lab, so it's good enough for me...
I mean there is no blatant, huge issue, like a 100mV or 1V ripple or something, you have to agree...
Caps are getting tired of course, but far from dead...
Calibrator outputIt's got a trimmer to adjust the reference voltage to 100V. I measured it at 100.3V, 0.3% error after 65 years of service... can't complain eh.
That's way more accurate than it needs to be for its purpose. However I played with the trimmer just in case, and it was not touchy at all, nor noisy. I was able to effortlessly adjust it much closer to 100V. It's now ridiculously accurate.
Horizontal section
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It was at last the opportunity to use my Tek 180A time mark generator !
I restored it nearly 3 years ago. Would it still work ? I checked each and every time interval and yeah, still works 100% !

So cool to adjust that old Tek scope with period test gear, the very instrument that the manual mentions !

With these sharp pulses/marks, it's easy to check the scope and see that it's impossible to make every time mark line up with every division of the graticule = the horizontal section of the scope is not 100% linear. I remember than when I restored/calibrated my little Tek 317 scope nearly 10 years ago, the manual DID tell you to check and adjust, as best you could, the linearity (and wishing you good luck with it as it's impossible to get perfect). However in the 502 there is no way to adjust it, so they don't even mention it in the calibration procedure. It is what it is...
New problem unveiled : manual tells you to calibrate using 10us and 1us markers. The scope displays markers just fine all the way down to 10us (100kHz), however it fails to trigger on 1us markers (1MHz). So I interrupted the calibration procedure and did a quick experiment : fed the scope with a sine wave of comfortable amplitude, 5Vpp, and made it span 6 vertical divisions. Just so I am sure the trigger section had no excuse for not triggering. Then I swept the frequency to see what would happen. I started at 100kHz to mimic the repetition rate of the 10us markers. It triggers just fine there. Then I increased the frequency in 10kHz steps and bingo... the scope can't trigger at 1MHz. It can trigger reliably up to 400kHz. At 410 and 420 the picture gets a bit blurry/fuzzy, and at 430kHz it abruptly, solidly fails to trigger, no more picture to look at....
I get the exact same results on both channels, so it's unlikely that the problem comes from the trigger pickup tube that's part of each vertical amplifier sub-assembly. The problem must be in the trigger section per se, in the upper deck. I think I had a similar issue with the Type 317, was a weak tube. The 502 has a B/W of 1MHz so it would be nice if it could trigger at least up to that figure. Usually scopes can trigger even beyond the B/W of the scope, since the scope is still useful a bit above its B/W spec.
Sweep length : it was spanning the graticule but not more. The calibration procedure doesn't talk about this, but the scope does provide a trimmer to adjust it. So I lengthened it. It now spans the CRT width fully. However you can't go too crazy with this trimmer because it makes only the RIGHT side of the sweep longer, the left side is not affected. Which means that the more you lengthen it, the more you have to turn the position control CCW to centre the trace... which means that now, the trace is NOT centred anymore when the position control is centred. It does bug me....

Now that the trace spans the CRT fully, I now notice a new "problem" : see picture. Where the trace is outside of the graticule, on the right side the brightness of the trace remains constant all the way until it disappears off screen, nice. However on the left side, the brightness decreases, it's quite dimmer. Bugs me again

Maybe a CRT problem ? Not fixable then...
Vertical AmplifiersWell this side of things was all over the shop so a cal was much needed indeed.
First adjust the DC balance, coarse then fine.
Then adjust the Differential balance, something I had never done before as it's the first time I work on a scope with diff inputs. So that was cool to do.
Then adjust the gain. That was the opportunity to use just the perfect thing for the job : the Tek cal fixture I acquired just the other day !

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You have two adjustments, one for the 0.2V/cm range and another one for the 0.2mV range, the most sensitive one the scope offers. The former was the one that was way out, especially on channel one : I would get only 4+ divisions instead of 5...
The latter however, despite being the most, ultra sensitive one, the one you would think was the most "fragile"... turned out to need no adjustment at all...
[RANT MODE] : when you adjust the lower beam / channel #2, the trimmers are on the UNDER side of the scope, so you have to lay the scope on its side : screwdriving the trimmer with one hand, hoping it won't slip and short things out and kill you in the process, while trying to look at the CRT dead on, while at the same time adjusting constantly the vertical position control to centre the trace each and every time you touch the trimmer.
It's fun. But it's about doable. However when you are done and put the scope back on its feet, you realise that.... the CRT is not aligned properly anymore, it moved a bit ! I told you, the CRT rotation adjustment thing is crap, the worm screw is only held at one end, the other end is flapping in the breeze, so the CRT is never really locked solidly in place...

Oh, and to make me really, really happy... the CRT HV decided to resume messing with me : it started misbehaving again... not the intensity flickering as we had before, no. This time, it looked like the HV was dying : the picture would "shrink" inward, making any calibration useless. It would do it for a few seconds every other minute. So I had to often stop the calibration waiting for it to recover, then resume the calibration, stop again... resume, stop...
Feels like a bad/intermittent connection somewhere but.... WHERE ! The random nature of it makes it impossible to track down, it's a nightmare !
CONCLUSIONWell I think I will leave it at that for now. I will take note of the issues that remain so that I might come back to it later if I feel like it, but for now it's good enough.... I will now resume work on the other 502 as it's got more important problems : remember, fails to trigger. That's a show stopper so I want to fix that. Once done, I will see if it works better, overall, than the other one, and I will decide which one will be declared the "main" 502, and which one will be the "backup" 502.
Also, the reason I don't want to spend 2 years investigating and fixing every minute detail of these scopes is the fact that, as we have seen while working on them, the 502, as much as I love them, are too flawed, they drive me nuts. I now realise that the 502A was a VERY worthwhile update from Tek at the time, it's a much more viable option for doing actual "work". It's no wonder that the 502 was short lived and soon replaced by the 502A, which did enjoy a much longer life than the 502. So I want to get these two 502 to a decent state of restoration, but not make them 100% perfect because I anticipate I will be using the 502A instead, in practice. So the goal is to wrap up this pair of 502 in the near future, then move on to restoring one of the x3 502A, and make this one real good. Then later when time and circumstances allow, I might come back to the two 502 to work some more on them... but not right now.
Sue me... these are my scopes, and I am the one doing all the work and spending valuable time (and money at times) on them so... sorry, but I get to decide how far I want to go with this or that particular scope in my stable...
