(classic) Tektronix 502 scope repair / restoration

[Vince]

OK so I just had a quick go at the horizontal amplifier.
First had to figure out what I would have to be looking for exactly...  that short line on the CRT, what does it mean exactly, concretely...
It means we have noise on top of either or both of the horizontal deflection plates.



So, that means that I can do a very quick simple test : I grabbed the DMM and simply measured AC voltage at the very end of the amplifier, where it drives the plates. If the amp is at fault then I should see an AC component somewhere... and I did.

On one plate the DMM would measure 0.25Vac and... 4.7Vac on the other ! Hmm... that means of course that peak to peak is much higher than that, maybe 10 or 20Vpp who knows.
I don't have in mind the precise sensitivity of the CRT horizontal deflection plates, but it's usually IIRC at least 10 volts per centimetre, so surely 10 or 20V or whatever, is pretty much ball park for the short line we see on the CRT, which is about 5 to 10mm long.

So I got excited... wanted to see more of it : I scoped that plate signal and.... yeah, problem found !

We do have lots of "stuff"  on there... but it's not noise, it's a signal !  it's a beautiful, perfectly clean and stable... 40kHz SINE WAVE !

Yes we already know what that is : the oscillator that generates the HV for the CRT ! 

So I put the metal shield back onto the HV section, and also put the shield back onto the oscillator tube itself (it's often missing but on that particular scope, all tube shields were present), and hey presto, problem SOLVED !!! 

Look at that ! OK the picture sucks sorry, looks fat and blurry, but in the flesh all I see is a perfectly stable and clean and sharp picture  ! 



I measured AC again on both plates, and now both of them are equal, and much lower : they are both at 25mV with the DMM, no more 0.25V never mind 4.7V ! So, good enough for our purposes.


That's strange though : how comes one of the plates picked up 20 times more noise than the other plate ?! Both are right next to each other...
Also, how comes it's the first time I am hit with this problem ? I mean I never had this problem with the other 502... and it too has no shield(s).


So this scope is getting there ! 

Next issue : trace intensity  is now flickering quite badly.... but it's probably a simple issue.
It's random which makes it difficult to diagnose....

So the plan now that the scope is pretty much good now, is to wash it.  "Hopefully" this will kaput the intensity for good, make it a solid failure rather than intermittent, which will make it so much easier to diagnose !

So next week-end, bath time for the scope !

Oh yeah almost forgot, I will also have to sort out the cooling fan issue discussed earlier. No big deal...

The week-end ends on a very good note then ! HV is back, scope is mostly sorted now ! 

[tautech]

Next issue : trace intensity  is now flickering quite badly.... but it's probably a simple issue.
It's random which makes it difficult to diagnose....

Good progress Vince.

First thoughts would the a dodgy Intensity control pot but IIRC they were often wirewound versions or the balance trimmer.
 

[m k]

Earlier starting point for V800 pin 1 was 185V, so something is different.
Maybe 135V was a heating time.

Yes that's what I am trying to sell to myself ! 
Makes me feel better and worry less...

Maybe you should leave it on for longer period, not over night though, and not if you're not around.
Maybe also add a self lighting voltmeter showing V800 pin 1 voltage.

So HV Adj is still equally away from 350V and HV should be the same.
Maybe those rectifier tubes were the cause.
I'd say that putting old tubes back for testing purposes is not conclusive, too many variables, like bad contacts.

But if V800 pin 1 stays high something can still be somewhere.
Can you still get those trafo winding resistances?

Maybe you should also check the shape of that unreg 485V before any capacitor changes.

[Vince]

OK, took the scope apart in preparation for its bath in the coming days. If the weather forecast is right, which it never is, I might have a window of opportunity Thursday and Friday. We shall see.



Cooling fan : its wires clearly have been messed with, the solder joints on the ceramic strip underneath the deck, are ugly.



Might have a clue, a lead at least, as to why it's so quiet / slow : maybe the guy didn't wire it properly ?
I have five of these scopes but it's the only one with this type of fan motor. The other 4 scopes have a 3 wire motor (wires black, brown and red), with two wires being connected to a big can cap (I assume that's what it is), to help it get going at power up I guess. This cap never appears in any of the manual, "of course"...
.. and only one wire going under the deck to the (3 way) ceramic strip.

The present scope however with this different motor, has no cap, and only two wires, both of them black, and both go straight to the ceramic strip.
There is no trace of mounting screws for the can cap on this scope, which means I guess that this big black motor was the original type of motor for this particular scope. We can at least deduce this much. So although the fan clearly has been removed from the scope at some point, and might r might not have been replaced... at least we know it's the correct type of motor for this particular scope. So might as well try to retain it and figure out why it's slow, and try to straighten its mounting plate so that it sits straight in the chassis and the fan blades stop hitting the air grill....


CRT rotation mechanism : the white nylon clamp that grabs the neck of the CRT, and rotates it using a worm screw... is broken in two, snapped, as you can see.  Sadly it's a common issue on these scopes. Luckily however, it's a standard part used by any and all 500 series scopes, so no worries, I will be able to replace it when I put the scope back together again.




Dirt-wise, not much to be seen on this scope, it's really well preserved. As usual the worst of it is the upper side of the lower deck, and even then it's hardly as gross as the other 502 we cleaned before. A little bit to clean at the back of the CRT as well, but that's about it.. So it should be quick and easy to clean this scope.

[Vince]

Oh, I see it's 00;10 here in Frog land, so 25th it is. Happy Christmas people. Yes I know Tautech, in Kiwi-land it's been the 25th for a few hours already...

[Vince]

Happy new year people, some news.

Washed the scope last Friday I think.
Sorting some things while it's drying.

Started with the failed power switch, which is combined with the graticule illumination pot. It failed in a permanent OFF state.
Tried squirting some contact cleaner but not easy as it's almost sealed. Only a couple minuscule holes, so small that I am not even sure the cleaner even managed to get inside.

So I had to resort to replace it, which I was reluctant to do because removing the 5 wires going to the thing is not fun : the insulation on the wires in these old Tek scopes just loves to melt at the speed of light, and go haywire as if you lit gunpowder... it's impossible to remove and put them back on without burning at least some of the wires, and then they look absolutely horrible, disgusting, as well as being a potential safety hazard as the wire is now a lot more exposed and usually close to the chassis or some other 'hot" wire. In short, the less you mess with those wires (same goes with the ceramic strips), the better ! 

So I was anxious to say the least.  Took every precaution and managed to take it off with no harm, and put it back on with only one wire melted. Still, that's one too many 

The key is to do all you can to be as quick as possible with the iron, so as to heat up the insulation as little as possible.
Access is not great either which doesn't help. The switch is in the centre of the scope. It looks easy on the picture, but when you need to fit big hands like mine, and tools, and be accurate and quick, and do it 5 times in a row... no it's not so easy 
I really hate having to mess with those wires...

Anyway, out if went. Before replacing it I though hey, nothing to lose now, might as well try to take it apart to see if it can be fixed, and if nothing else it would be educational as I have never taken apart a pot/switch combo before.

It was easy to remove the pot (wire-wound) part, but then once you get to the switch assembly per se, it didn't look like it was meant to be taken apart/serviced. I eventually managed to do it but I doubt it's an orthodox way of doing it...
In the end the assembly was a bit difficult to understand, I would not have managed to put it back together, had I to, I think...
But well, it looked like the contact in there was at the least very black / carbonized, and possibly plain broken, not sure.. again the design was not so obvious to my eyes...
I did marvel at the quality of the thing though : it contains a minuscule aloy die cast part, that switch would probably cost a fortune if it were available today.

I did wonder about the specs of that switch though ! The markings on it say 250V 1A, but the scope draws 300 Watts !! Hum.....

Anyway, I got a replacement switch/pot from the 502A donour, put it in the 502 and bob's your uncle. Well, I mean the switch tests good with the DMM, so I hope it works fine.

That's it for now...

Next to sort out : the broken white nylon clamp for the CRT, and the cooling fan that's bent and running too slow probably.

Oh, I also need to find replacement incandescent bulbs for the graticule illumination, it doesn't work at all. Service manual states "Type 47", and that's it, nothing more, no power rating or anything, how helpful. The bulb themselves aren't much more talkative, they just read  : " GE 47 ".  So I guess "47" refers to a then standard bulb type, and that General Electrics did make them back in the day. I will try to find some place that sells these bulbs, and buy a bunch of them....

[m k]

"Illuminate."
What movie.

[factory]

Oh, I also need to find replacement incandescent bulbs for the graticule illumination, it doesn't work at all. Service manual states "Type 47", and that's it, nothing more, no power rating or anything, how helpful. The bulb themselves aren't much more talkative, they just read  : " GE 47 ".  So I guess "47" refers to a then standard bulb type, and that General Electrics did make them back in the day. I will try to find some place that sells these bulbs, and buy a bunch of them....

The number 47 lamp is a standard incandescent type, it would have been made by many brands, searching for "#47 lamp" may help find some.

Mouser have some #47 lamps (6.3V 150mA) from both VCC & JKL, I could add some to a future order & send them with the HP CRT socket cover if that would help. Maybe local suppliers have them too. 
https://www.mouser.co.uk/c/optoelectronics/lamps-holders-accessories/lamps/?q=%2347%20lamp

David

[Vince]

Thanks David.

I had a quick Google, looks like these bulbs are everywhere in the USA but much harder to find in Europe. Google did find me some on-line shops in the Euro zone that carry them though. Mostly shops into vintage audio equipment.

Like this German shop for example :

https://www.banzaimusic.com/47-Light-Bulb.html

Will try that route. If no luck then I would buy some from MouserKey as I think I am likely to use them soon to order lots of electrolytics to recap these two 502 and then a 502A soon after that I think.

[Vince]

Wow, 3 weeks since my last message, time flies ! At that rate I will be 6 feet under in no time ! 

I spent all week-end on the thing...

I put it back together, it's all in one piece now... the bench is now much less messy, a good thing.





Cooling fan

I started with that. I tried to straighten the motor bracket so that the fan sit more upright and stops hitting the air filter/grill, which was a show stopper previously. It's not that easy but the little bit that I managed to achieve, coupled with massaging the fan blades so they are about equal, did the trick.
Runs quietly now, no more hitting. Other problem was the fan shroud that was not oriented / clocked properly. There is a big flat on it so there is only one way to orient it properly. So I tried to do that and... oh, something is wrong. You can do it because if you do, the wires coming out of the motor are now too short to get to the ceramic strip onto which they are soldered. Hmmm.. so that means whoever messed with that fan, must have removed the motor and clocked it incorrectly on its bracket when putting it back in place, making the wires now too short... forcing him to correct that by instead messing with the orientation of the shroud. So I removed the motor... its two retaining nuts were barely finger tight ! Yes, no doubt, someone was in there... The shroud is now oriented as it should, I can tick that problem off the list...
So then that meant I had access to all 4 mounting holes for the grill. Previously only two holes were accessible because of the shroud problem, so the guy put only 2 screws back in. Sadly I was able to only put 3 out of 4 screws.. the last one would not bite, guy messed also with that apparently, he must have damaged the "thread" (it's a self taper screw). So yeah, this whole fan thing was quite a fuckery. Now much better.

CRT rotation mechanism



The white nylon clamp was broken in two, snapped, as mentioned earlier. So I replaced it. I did it in situ which proved to be quite a challenge, took some fiddling...
Doing so, I noticed yet another difference between the 502 and 502A. In the 502A, as logic would dictate, the worm screw is held at both of its extremities, so it's held in place properly. Well on the 502 not so ! Only the tip of the screw is held in place. The head of the screw, where the red knob is, what yo interact with... is not secured/ guided at all, it's just flapping in the breeze ! 
Blew my mind.
Anyway, I soon would have bigger problems to deal with : when I worked that screw to align the CRT, it was not very responsive to say the least... had to turn it 20 times before it eventually, sliiiiightly, rotated the CRT. Something is wrong... So I looked at the clamp while I was working the screw, and the clamp DID move as it should... but the CRT was NOT ! 
Oh no !! The glue securing the plastic base to the neck/glass of the CRT must have dried and is starting to fail ?! 
So that means the wires inside the base got twisted ! Luckily from memory, they are made of a very tough metal. Still, I bet they must be twisted / under stress in there, even if I can't see them... so they might fail one day, possibly.
Of course that means I can't adjust the rotation of the CRT. I did it by hand by trial and error, but it's a pain, time consuming and the result is not perfect, as you can see on the screen shot further down. So what can I do ?
Glue it back ? put a bead of super glue all around the base, where it meets the glass ? I need to restore the functionality... adjusting the rotation is just not viable without this mechanism.

Graticule illumination



That didn't work, so I ordered x20 type 47 bulbs. Very common in the USA but rare in Europe. Had to import them from that random vintage audio German shop that Google found me.
While I was at it, I added a green filter to the scope, didn't come with one. Yeah I admit... as much as I love the blue phosphor, a filter improves the contrast and comfort quite a bit... ideally I would like to fit a blue filter, so that I can have the best of both worlds... I don't think these can be found easily never mind for cheap, so I guess I could make one myself with some tinted plexiglass or something ??
Then with some care, drill holes and makes cutouts for the light bulbs and the 4 studs.

Trace Intensity

Remember before the wash, it was flickering very badly... Well up reassembly it still flickered, so I tapped the back of the intensity control pot on the front panel. Used the handle of a black "ESD safe" brush... and got zapped. Like, 3kV zapped... I didn't like it at all...
Will not make that mistake again... I nearly won a Darwin award...
Not flickering any more though ! 

Vertical amplifier

The lower beam was not showing on the CRT ! I feared it might due to wires inside the CRT base, that got twisted and broken maybe...
So in order to quickly figure that out, I measured the voltage straight at the deflection plates. If voltages are correct then CRT is toast. If not, the amplifier is kaput so there is hope, it can be fixed. Well, voltages were not correct at all. One plate was stuck at max voltage, the other was stuck at the lowest voltage. In short, the beam was there, but way off screen. So I spent 3 hours on the vertical amplifier, probing DC levels, and doing a lot of tube swapping experiments... Eventually narrowed it down to the pre-amp / first stage. I think there is a combination of weak tubes (a couple 6AU6), and some resistors must have drifted. It's not just the tubes I think. For, I replaced them with the two matching tubes from the working channel, and it did not fix the problem... but putting the tubes from the defective channel, into the good channel... did make the latter misbehave in the expected way. So the problem followed the tubes, but only in one direction so to speak.. so it's a bit weird, hence why I think the bad channel must have more than tube problems... so drifted resistors are probably at play here. Anyway, after some trial and error, using tubes from other scopes and some from my tub drawer... I eventually, by chance, after a while, just as I was about to give up and call it a day... managed to find a combination of tubes that worked well. Now when the vertical position control is centred, so is the trace on the screen. Miracle.
To be honest I am tired of this 6AU6 "fun"... it's a recurrent problem in these scopes. They are all over the amplifiers and create gain and offset issues. I have lots of scopes hence lots of 6AU6 but of course they are all in various states of wear, so I basically I am swapping used 6AU6s with other 6AU6s, hoping it will fix the issue... trying lots of them, hoping for the best.... takes lots of time and you never know how long it will be until you find good (enough) tubes.

I am bored playing this game 
I am not rich but I think I will try and buy a few NOS 6AU6, and label them as such (on the glass itself), so that I can never mix them with all the other 6AU6 coming from scopes. I also want to take it a step further and be able to quantify the degree of tiredness of all these 6AU6 in my scopes, so that I am not working blind any more, and can diagnose things with more certainty and fix these scopes faster.. because I have a bunch of them to restore ! ... and I can't afford to spend then nest 10 years doing so... have many other things to do than just restoring these scopes...
So I am thinking of getting a tube tester so I can measure gain/emission. I don't fancy old tube testers are they are so crude and large and large and mega expensive here. The idea is use my Tek 575 transistor curve tracer with some adaption, so it can test tubes.
This way I get more than just a needle dancing on a vintage tube tester, but an actual curve I can compare to a NOS tube and the 6AU6 datasheet.  Yes... that's what I want to do. So I will soon resume work on my pair of 575 tracers...I think.


Conclusion


Anyway, that scope is pretty much done, cleaned inside out, put back together, and fixed / working. There is still more work I could do to it of course, and might do later, but it's now basically back to life and working. Remember the idea was to get this one working, enough, so that I can take measurements to help me fix the other 502 which has a no-trigger issue. So that's what we will do... go back to that other scope and make it work...

[tautech]

Good progress Vince. 

Looking at the screen pics the thing that stands out is the horizontal gain is a touch out and H Position needs correcting a touch to the right also so the trace is evenly overlapping the graticules.
Should be minor tweaks to correct this......

[max.wwwang]

What a beauty. Well done.

[m k]

Used the handle of a black "ESD safe" brush... and got zapped. Like, 3kV zapped... I didn't like it at all...

Can you remember where your other parts were, accurately enough for tracing the zap?

If I remember correctly your table frame is solid wood, that wont zap you.
Maybe other part of your arm was grounded.

If you had muscle feelings afterwards you can be pretty sure that this feeling muscle was a major participant of the incident.
It can also give you a hint how far in you the zap was zapping, maybe you had a subconsciously sparky style "pocketed" other hand.

To be honest I am tired of this 6AU6 "fun"...

Manual has that V414 and V444 are selected, maybe other machines have the same situation.
V493 then is used differently, maybe it is also worn differently, maybe other machines have these too.
So maybe your initial situation is that you have three different kind of used tubes.

ps.
Lenina, 3p hint for the movie.
It's getting old though.

[TimFox]

Do V414 and V444 form a differential pair?
If so, they may be matched for transconductance and grid voltage.

[Vince]

Looking at the screen pics the thing that stands out is the horizontal gain is a touch out and H Position needs correcting a touch to the right also so the trace is evenly overlapping the graticules.
Should be minor tweaks to correct this......

Oh well yes sure, I need to calibrate the whole thing of course !
To begin with, the DC balance is completely out of whack, makes the scope unusable ! 
That will be the perfect opportunity to use some Tek gear for their very purpose...

My Type 180A time mark generator to adjust the horizontal amplifier, then the calibration fixture I just restored the other day, to calibrate the vertical amplifiers.

If I remember correctly your table frame is solid wood, that wont zap you.
Maybe other part of your arm was grounded.

Not solid wood but wood nonetheless. It's a 16mm thick OSB panel. Have had it for 10 years now, so it's fair to say it's well dry by now...

If you had muscle feelings afterwards you can be pretty sure that this feeling muscle was a major participant of the incident.
It can also give you a hint how far in you the zap was zapping, maybe you had a subconsciously sparky style "pocketed" other hand.

There is not much current capability in the intensity circuit luckily... I felt the pain only on my finger tips, not more than than... yet enough to convince me that it was not a good idea !   

Manual has that V414 and V444 are selected

Oh.. I didn't see that in the schematic but...I just looked in the parts list and indeed they they are selected... makes sense for a pre-amp I guess, especially on a scope with such ultra sensitive inputs (compared to a "normal" 500 Tek scope that is)

Do V414 and V444 form a differential pair?
If so, they may be matched for transconductance and grid voltage.

Diff pair ? Schematic attached...
I am no expert. If it is, at least the layout of the schematic doesn't make it obvious. The manual doesn't mention it at the least, but maybe they used some cryptic "back in the day" vocabulary that meant the same and it escaped me...

[TimFox]

V414 and V444 are a differential pair and could benefit from matching.

[factory]

Wow, 3 weeks since my last message, time flies ! At that rate I will be 6 feet under in no time ! 

I spent all week-end on the thing...

I put it back together, it's all in one piece now... the bench is now much less messy, a good thing.

...Snip...

Anyway, I soon would have bigger problems to deal with : when I worked that screw to align the CRT, it was not very responsive to say the least... had to turn it 20 times before it eventually, sliiiiightly, rotated the CRT. Something is wrong... So I looked at the clamp while I was working the screw, and the clamp DID move as it should... but the CRT was NOT ! 
Oh no !! The glue securing the plastic base to the neck/glass of the CRT must have dried and is starting to fail ?! 
So that means the wires inside the base got twisted ! Luckily from memory, they are made of a very tough metal. Still, I bet they must be twisted / under stress in there, even if I can't see them... so they might fail one day, possibly.
Of course that means I can't adjust the rotation of the CRT. I did it by hand by trial and error, but it's a pain, time consuming and the result is not perfect, as you can see on the screen shot further down. So what can I do ?
Glue it back ? put a bead of super glue all around the base, where it meets the glass ? I need to restore the functionality... adjusting the rotation is just not viable without this mechanism.

NEVER use super glue for glass, people have tried it for reattaching valve* bases and as it dries hard, they end up with the glass CRACKING, the exact opposite of what you want.

Instead use something that isn't hard once set, such as araldite epoxy, or a non acidic silicone sealant. Try to clean out the old glue without disturbing the base & internal wiring if possible.

*aka tubes for other countries

David

[m k]

How stiff it must be?

Insulated glass elements use polysulfide as one type of final sealant.

[bat]

Super boulot !

[m k]

What is needed for limited 6AU6 tester?

Small UPS has a 14/240V trafo with primary center tap, I can send few, but they are heavy.
60V plate can already see the shoulder.

[factory]

How stiff it must be?

Insulated glass elements use polysulfide as one type of final sealant.

It's for reattaching the loose connector base to the CRT. Think about what will happen with thermal cycling, with a ring of hard glue such as super glue (NEVER use) around the glass tube, this is the end with the heater.
Maybe try it with a dead tube, if you want to find out.

David

[Vince]

NEVER use super glue for glass, people have tried it for reattaching valve* bases and as it dries hard, they end up with the glass CRACKING, the exact opposite of what you want.

Instead use something that isn't hard once set, such as araldite epoxy, or a non acidic silicone sealant. Try to clean out the old glue without disturbing the base & internal wiring if possible.

David

Thanks for the warning.... I know squat about glues, yet I want to keep moving so... right now I think I will just play it safe and replace the CRT, and keep the "bad" one as a spare, and will make sure to put a note on it to remind me that the base is loose, in case I ever do need to make use of it in a few years.

What is needed for limited 6AU6 tester?

I think it's not worth trying to make a limited tester dedicated to the 6AU6.
I already have 575 transistor curve tracers. IIRC it's pretty easy to make an adapter to convert it to a tube tester.
Basically just provide a 6V source to power the heaters/filaments of the tube under test, then just provide a resistor to convert the base current of the transistor, into a grid voltage for the tube and... I think that's about it !? 
My first 575 tracer IIRC I got pretty much working electrically, or not far off at least. The second 575 I got more recently, though I have not yet even tested it, was sold as working, and witnessed to be working by the chap who picked it up for me. So I am not too worried. I remember popping one of the covers and saw that it was already recapped. Of course I am sure it might need some work, but nothing drastic I would think.
So I just need to put those two 575 on the bench and get one of them working just right, fully calibrated etc, so I can actually use it as intended.

[TimFox]

For a somewhat different purpose, I built simple jigs to test vacuum triodes and triode-connected pentodes for grid current, but which gave results also for grid bias and transconductance.
Each jig was merely a tube socket surrounded by banana jacks for each pin, built on single-sided 2 oz Cu 3/32 FR4 board, with another jack for ground (Cu layer).
For DC measurements and to avoid oscillations, each pin was grounded through a disc capacitor to the foil.
I connected a Keithley lab current source to the cathode (through a resistor needed to keep the Keithley output negative with positive voltage at the cathode), the grid connected to a low-resistance microammeter (to ground), the plate through a milliammeter to a positive supply, and normal DVMs from plate to ground and cathode to ground.
The common connection for the two supplies and voltmeters was the Cu foil.  I used a good electrometer for the grid current, since I was interested in grid current, but that may not be necessary for your purpose.  The separate plate current meter is redundant.
Applying a constant plate voltage, I then measured grid-cathode voltage, grid current, and plate current as a function of cathode current (ensuring the current source stayed negative by changing the resistor box as needed, although the resistance itself was not critical).  This gave me the information needed for evaluating tubes for operation around a given quiescent point.
For a pentode, of course, one would need further connections for the screen.
Except for the cathode current source, the measurements can be made with simple 3.5 or 4.5 digit DMMs.

[factory]

The tested & selected parts for V414 & V444 (157-056, 157-073 and 157-0073-00) are listed in the info on Tekwiki; https://w140.com/tekwiki/images/1/14/157-_checked_and_tested_tubes_specs_and_procedure.pdf
They checked those for microphonics, grid current, K bias, differential balance and then paired them.

David

[Vince]

Thanks David !Wow so they tested them quite extensively then, left nothing to chance !

OK I might have "solved" that tube problem, in relation to what you just said.
I remembered that all those pre-amp tubes (4 of them then, in total) had a Tektronix "service" sticker on them.
A nicely made sticker, that stood the test of time.

Well this evening I realised that out of this pair of pre-amp tubes, only one of them had a sticker hmmm.... and one of the tubes in the NEXT stage of the amplifier, DID have a sticker, when it "should" not have... so I though maybe I somehow managed to mix the tubes inadvertently, even though I pinned them on that ESD foam, respecting the layout, precisely to make sure I would put all these tubes back in, their proper place after washing the scope.
Well I guess I failed ! So I now have located both tubes with that sticker, where they came from, in the pre-amp stage and.... offset issue is now MUCH better indeed !    Of course I will keep an eye on it...
So yeah, pairing tubes for a highly sensitive scope such as the 502, is apparently not a luxury but rather a... necessity !

So looks I just bought myself time... but still, in the long run, I want to be able to test tubes on my 575 curve tracer, if just for fun ! 


What else ? I replaced the CRT... the base seems solidly attached to the glass on this one. I was able to make use of the rotation mechanism and adjust it. See below. Alos tweaked the intensity balance trimmer, so I could get what you see... sharp/ dim-ish traces with the same brightness. 5 minutes later the intensity had drifted already and required yet another tweak of the trimmer. It's hell... no wonder why they later improved the design in the 502A and relocated that control to the front panel so it's dead easy and comfortable to adjust as often as needed...
This feature alone is worth getting a 502A over a 502, imho....

CRT HV is going strong BTW.. remember we had to fix it, replace the tranny etc, headaches.... but it seems well now. Running for hours on end solidly.
The flickering intensity is gone as well, fixed.  This scope is almost there... quite happy with it.
So I decided it was now possible to re-assemble it further : put the 3 HV shields back on, as well as the big carry handle.

OK so this week-end hopefully I can do the final touches : calibrate the thing.
IIRC there is also an issue with the -150V reference power supply, where I had to turn the trimmer full lock to get the -150V. That's not normal so I guess I ought to investigate that section of the power supply and hunt for resistors that might have drifted a bit too much...