OK, some more...
1) I lifted a leg on each of the 4 Sprague caps in that circuit... they all look absolutely fine, and measure OK. They measure high as they often do, but nothing extraordinary. They certainly have zero sign of catastrophe failure...
All resistors look and measure OK as well.
I can see black smoke on the side of the ceramic strip, facing the transformer... so the magic smoke must have emanated from there but... for the life of me I can't find any component that smoked, other than this disc red ceramic cap we have already discussed, which is now out of circuit.
And again, that cap still measures something sensible, and even if it were dead it's in series with a Meg range resistor so that would have kept the corresponding secondary winding from burning, I would think.
So basically I am in one big, big mystery. I am puzzled....

I don't think the transformer smoked because the black smoke is only covering the ceramic strip per se, there is zero trace on the transformer itself nor the wires coming out of it.
2) I checked the resistance, in situ, of the 2 primary windings. I do see an anomaly there, but a strange one : one of the windings reads twice higher than it should.. rather than be shorted or open-circuit. I am not too sure what to make of that, it's weird... how could that happen...
I took comparative measurements on the other 502, and on a 502. Both measure the same (as you would expect, but I wanted to make 100% sure...) : the main winding; between pins 4 and 6, measures about 4.3R, and the smaller windings between pins and 4 and 3 is about 1.3R.
Well on the sick 502 do get a similar reading for the main winding, but the smaller one reads twice too high, at 3.4R instead of 1.7.
What could that be ?!
3) I tried to check if the HV rectifiers were not dead : lifted a pin on each of them so I could power the heaters from my lab power supply.
I set the supply to 6.3V, the tubes did glow but not the expected dim orange, but rather a super bright, bright yellow, very much like an actual light bulb. Weird. Tried all 3 of them... all did it. Geez... tried it on a 502A donour... same result. Hmmm... strange. Pulled the datasheet for these tubes... oh oh... heater is not supposed to run at 6.3V but only at 1.25V ?!

This it the last time I do electronics.... I am not worthy

So I set the supply to 1.25V and checked them again. One is glowing the normal orange, phew, but the other too still are bright yellow, so less bright of course, but still... So I might have to replace them further down the line, we shall see. I have plenty of those so not too worried. Lesson learned : not ALL tubes run on 6 or 12V !

I think I even have "brand new" Mullard rectifiers somewhere. Could still retrofit solid state diode as well, though I don't see the need for that, and prefer to stick to the original tech if it works, for originality.
4) I checked the power supply : CRT circuit runs on +485V UNregulated. I do get that, and unregulated for sure it is.... with 10Vrms ripple. So 30Vpp I guess. A bit much... probably a tired cap in the power supply. But not a show stopped I would think.
That supply is further filtered, locally, with R808, a beefy 1.2K resistor that reads a bit low (for a change), at 1.0K, and capacitor C808, 100nF.
In every Tek scope of that era, that I have seen, Tek somehow insists on rating it at only 400V when it sees nearly 500V, so often it fails.
But somehow it looks and measures just fine in this scope. I also have a problem : there is little ambiguity as to the what cap it is, physically : there is only one 100nF electrolytic cap in the CRT section, and it's soldered right besides the resistor. So pretty easy eh ? Well, in this case how comes the top side of that cap is NOT wired to the resistor ?!

I assure you, it's not. Neither is it in all my other 502/A. So again a big mystery.
The top side of that cap measures at NEGATIVE 150V ! rather +485V. This scope is driving me a bit crazier with every passing week-end...
5) I measured grid voltages at the oscillator tube. The main grid, pin 3&6, that's used as I understand it for the feedback from the auxiliary/smallest winding of the transformer, is supposed to get -50V and I see -10V. The other grid/ "screen" is connected to the feedback from the HV side of things. Grid is supposed to see +60V and I get +185 V ! ISTR that the way it works, if the HV drops that grid voltage gets higher, so it's consistent : zero HV means mega high voltage fed back to the grid. Problem is that I also remember people saying that too high a voltage on that grid can damage permanently that oscillator tube ?! I don't know how many ms or sec or minutes it takes to cause damage, so I don't know it that tube is dead or not. I swapped that tube and also the other one, V814 used for the HV feedback, but no change. So they aren't the culprit.
6) CONCLUSION So that's all I have tried today.
To sum it up : the more I dig, the more mysterious it becomes, it drives me nuts. These old scopes are supposed to be simple to fix, they aren't rocket science nor 100GHz voodoo.
So I am getting tired of failing to fix that HV !
So I am switching gears now, I have no choice but do some electronicking. Yes I know I just said I would not anymore, but I lied.
The main problem is that it's closed loop system, it's a chicken and egg problem...
My idea is to try to divide and conquer. The worked great for the Romans 2,000+ years ago, hopefully it will work for me as well in 2024.
I want to open the loop, make the oscillator run with no feedback from the HV side of things.
I plan on disconnecting the screen / pin 1 of the oscillator tube, where the red cross is, and feed +60V to it from my lab supply.
Since this is the typical voltage that tube sees when things work, it should at least allow for it to get going.
This way, with no feedback to drive me crazy, the oscillator should be much easier to troubleshoot : it will be reduced to what's circled in green on the schematic. That is, not much. Just the oscillator tube, the two primary windings, two resistors and two caps...
I have not to worry, care , about anything on the secondary side of things, and anything about the HV feedback.
No. I will have nothing.. nothing but the oscillator itself, nothing else.
This will also allow me to make sure the grid never sees dangerously high voltages, until everything is fixed. Things is, I can't blow these tubes by the bucket load because unlike the 6AU6 and 6DJ8 which are plennnntyful in Tek scopes (they inundate all amplifier stages), the tube Tek uses for the oscillator in their scopes is... unique. Whatever tube/pentode they use, they never, ever, ever, use it anywhere else in the scope. Never. Plus, not all scope models use the same type of tube either. So you better look after them...
Anyway, I will work on the thing until it manages to oscillate, then when it does oscillate, I will check that the secondary side of things produces the expected HV, and when it does, I will then be able to close the loop again, at which point if it stops working again somehow, I will know the problem comes from the HV feedback. Divide and conquer. Check and fix all 3 parts of the loop methodically, one at a time : oscillator, secondary side, then HV feedback.
First, I need to make sure that my two oscillator tubes are still alive despite having seen +185V at the screen... will plug them in my other 502 and see if that scope still can produce HV with these tubes.
That will be for tomorrow hopefully. If not, next week-end.
Sorry it takes so long to fix this damn thing, but it's a tough ride
