Hi there,
So this week-end for your enjoyment, some pics and news.
I did as planned and measured grid voltages on the 502A so I can compare with what I got on the sick 502.
The 502A got sick and would not show any life on the CRT any more, whereas before it at least managed to show the two spots, though immobile they were. So I quickly tried to sort that out. I ended up replacing the final driver tube in the horizontal amplifier, the one that drives the deflection plates of the CRT. That not only brought back the spots, but I now have SWEEP as well !

When I got that scope a year ago I remember spending many hours trying to get some life on the screen with no luck, and a year later I fix it on a pinch. I guess I am getting the hang of these things !

So for the first time I saw a trace, hell even a signal (the calibrator output as usual) on the screen. It's a blue phosphor, but very pale blue, looks almost white ! I also notice it's not very bright, you have to push it to the max to get only "adequate" brightness in the lab with the overhead lighting turned off. So either the HV is a bit lacking ( I measured it at -2,900V which is spot on the spec so...), or it's tired.
Will get back to that scope later for sure, as this the one of my x3 502A that I plan on restoring, the other 2 are for parts.
So once it was back to life, I measured grid voltages and found it was quite different from the vanilla 502 I am trying to fix hmmm... so checked the 502A manual and sure enough, I wasted my time : the 502A doesn't exhibit the same voltages as the 502, they aren't the same in this regard, bummer.
That was Friday evening.
So next day, on saturday I switched plan and instead decided to get my other 502 going, so I can compare like for like.
That was the very first 502/A that I bought, 6 years ago, and never worked on it.
So I decided to embark on a "get it going" sprint !!
So first let's give it a quick visual inspection inside out, and give it a quick clean outside, as per usual.
To cut it short... it's 18 months younger than the other one, S/N 6131 versus 4356, and is clearly a much lower mileage unit, it shows everywhere you look. It's definitely the one I should concentrate on restoring, though the other one is nice as well, given all the work and cleaning that went into it, so hopefully I can finish both of them. However of course that means I can't use one to restore the other any longer... I have to rely on the 502A, but I also want to restore one of my 502A of course so... so basically the situation is now this :
To be restored : x2 502 + x1 502A using parts from x2 502A donours.
I hope it can work out....
So the plusses of this 502 compared to the other one :
- Lower mileage, overall better preserved.
- Didn't belong to a heavy freaking smoker : was quick and easy to clean. The wires inside aren't as yellowed, and the colour stripes on them are still very visible and vivid. That also means no oily residue on the lower deck that ate that rubber grommet alive... all grommets are just fine in this scope

- Was stored I guess in a friendlier environment : no mould / white dots on the wiring harness and tube shields, and the lacquer on the top side of the lower deck is still pristine. Unlike the other 502 it's not littered with corrosion spots poking through the lacquer.
- The two big rotary switches for the attenuators are very light in operation, comfortable to use, whereas the other 502 has them extremely stiff, to the point of being a joy killer. Lubricating them didn't help. I think to fix that I need to release some pressure from the metal tab that keeps the ball bearing captive. I think that's what determines how stiff the knob is to turn.
- The rubber mounts that isolate the attenuator sub-assemblies from the chassis (I like this refinement / attention to detail !

aren't sagging.
- No missing tube shields.
- The can caps have not been messed with : no signs of ugly rework on the solder side, and still using their original mounting hardware, and all the condoms on the 3 cans used for the negative supplies, are original / proper.
Now for the similarities between the two :
- This one also had a power cord soldered directly to the prongs of the american socket ! It's a disease !

However this time instead of a super skinny two wire cord, it was a proper 3 conductor, beefier cord, more adequate... though the guy clearly didn't value his life much : he didn't deem it worthwhile to spend the extra 2 minutes to solder also the earth wire ?!

I just will never understand people, I never will....
Anyway, I cleaned that up.
And for some particularities :
- The cooling fan is sagging, making the blades hit the grill, so had to remove the grill to run the scope... better watch where I put my fingers now !

the 3 three rubber mounts look in good shape so I don't know ... the metal work must be bent, will try putting it back into shape, or replacing the fan altogether. Note also the motor for the fan, it's not at all the kind that I usually see in these Tek scopes. No. This one is huge and is enclosed in a black painted metal casing, that looks quite heavy.
I am not 100% sure if this motor is original as I see signs of rework on the soldering of its wires, some flux residue which I am not accustomed to seeing from Tek. So maybe that fan was replaced... so might replace it altogether, and it will fix the sagging issue at the same time... will see.
- The power switch, which is mated to the back of the graticule illumination pot, died while I was working on the scope : on saturday evening when I went to switch off the scope and go to bed...the switch had no action, the scope would still run in the OFF position !

Then next day, today, opposite problem... as I was working on it, out of the blue the scope lost power. Checked the switch with the DMM... indeed it's dead, it's open circuit in both positions... so need to replace that.
- CRT neck : there is a mechanism to rotate the CRT. A white nylon clamp that grabs the neck, and which rotates using a worm screw that you can adjust by hand. This nylon clamp is a known weak point, it often breaks/snaps. Well it sure did on this scope... luckily it's a standard part so I can get one from any of my numerous Tek scopes, not just the 502/A.
But before it died I enjoyed a few hours of troubleshooting and I almost got it going. Here is what got done.
First, I remember that when I got the scope 6 years ago, the seller said that he tested it, was able to get the square wave from the calibrator to show up on the display, but that after a few minutes magic smoke escaped so he pulled the plug and left it at that.
When I received it, according to my notes from back then, I did :
- Removed the covers to look for obvious signs of smoked components, and found nothing.
- So powered it up. Neon annunciators on the front panel were responsive.
- No trace on the screen for 10 minutes then a trace appeared out of the blue.
- Trace stayed for 5 minutes then disappeared, never to be seen again.
So, 6 years later I am back on it...
I powered it up using the dim bulb tester, no short. Left it for a few minutes to give the can caps a chance to wake up / reform a bit, or a chance to see if a short would suddenly appear and the tester light up like a Christmas tree.
After 10 minutes, no drama, the brightness of the bulbs remained the same all throughout, and never varied/flickered the tiniest bit. The scope looked therefore to be in a safe/stable condition. Inspired confidence so I did away with the tester and plugged the scope straight to the mains.
No drama. As could be expected, nothing on the CRT to be seen. I went straight to the CRT HV section, and immediately noticed that the 3 rectifier tubes in there, were not glowing... so I measured the HV and sure enough, zero. So that's our main problem.
I checked closely the transformer and all components in the primary/oscillator circuit. Everything looks fine but for that one small ceramic disc cap, red. It looks strange to me... its surface looks shiny/greasy instead of matte, and there is a couple "bubbles" on its surface, strange... As you can see it's wired in // with a 2.2M resistor. I checked the manual and neither the cap nor the resistor, are to be seen in the schematic !!

Taking a deep breath...
So that's my first suspect then, removing that cap to check it.
Will be checking all the black/red Sprague caps as well while I am at it, and all the resistors.
Then checked the power supplies. OK I should have checked that first....
I was in for a shock : the voltmeter was losing its mind, the readings were dancing widely all over the place, it could never get a stable reading, with every refresh of the display, I got a new story ! Like 350V, 200V, 5V, 125, .... the meter was going crazy and so was I !

I could not make sense of it, but luckily after a few minutes it calmed down and I got steady readings at last.
They showed that every supply was way off, like 10% too high at least.
Then measured the -150V reference rail.... yep, was at -165V, 10% off.
So I adjusted it to -150V and hey presto all the other supplies followed suit, and are spot-on. A joy.
However I had to crank the trimmer all the way CCW, which means there is an underlying issue that I will need to fix later in that supply. I guess some resistor drifted. but for now it will do.
But now was not the time... I was focused on the grid voltage measurements I needed to make to help me fixing the other 502.
So, I scoped the trigger/sweep/amplifier chain.
I could see that the sweep generator was working just fine in free-running mode, and I got a nice sawtooth out the horizontal amplifier.
However the trigger section was kaput : calibrator signal was getting to it, but no trigger pulses were coming out.
Was easy to fix. The first tube was bad, the comparator/amplifier that shape the input signal and compares it to the trigger level pot.
Replaced that tube, now all is fine.
So, even though there was no life whatsoever on the CRT, I was nonetheless able to scope the scope... to take my measurements.
First I measured the grid DC level when free running, I got I think -49V or so, close enough to the -50V indicated in the schematic.
Then when not free running, I got I think -58 or -59 maybe can't remember for sure, which is again what the manual states.
As I said I am not sure because I didn't have to time to write it down, as the scope right at that moment decided to lose power out of the blue... But I think these were the voltages I measured. Which means they are conform to the manual, which means the issue in the other 502 might indeed be incorrect DC levels at the grid. So I will keep searching in this direction...
So there you have it..... you now have TWO 502 restorations for the price of one, lucky you !

Sorry if that makes this thread confusing and difficult to follow...

All I can do is keep the x3 502A out of the topic other than mentioning the donour here and there. But as for the scopes actually being restored, this topic is only going to detail and show pics of the two 502. I already have a thread open for the restoration of the 502A.
Maybe I should rename those two : the "first" 502, in chronological order, the one I started restoring this week-end, and the "second" 502, I got very recently and which started this thread.
So what's next for next week-end ?
Get the "first" 502 going :
- At least try to sort out the power switch issue so I can use the scope again to take measurements to help me diagnose the second 502.
Will try contact cleaner but not holding much hope. Might have to replace the switch but it's not a 2 minute job from the looks of it, not a two hour job either... but if contact cleaner is enough, will be even quicker. The quicker the better...
- Might replace the cooling fan so that it's not sagging anymore and I can put the grill back in place, if just for my safety/ fingers..
- Try to fix the HV oscillator so we get (hopefully) life on the CRT...
Some piccies. First the scope before cleaning. A bit dusty but otherwise in really good shape overall, especially inside.