Author Topic: (classic) Tektronix 502 scope repair / restoration  (Read 15180 times)

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Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #175 on: January 22, 2025, 10:31:28 pm »
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, now we know these tubes were tested and paired.... maybe it was a factory sticker to identify these special tubes, rather than a sticker to say they were tested 10 years later when servicing the scope. Yeah, that sounds more plausible...

 

Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #176 on: January 25, 2025, 09:13:24 pm »
OK worked on it most of the day, to calibrate it. Studied the manual, followed the procedure in order :


Power supplies

Well 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 !  ;D 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 output

It'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




It was at last the opportunity to use my Tek 180A time mark generator !  8) 
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 !  :D

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 Amplifiers

Well 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 !  :D




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...  :palm:

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 !  >:(

CONCLUSION

Well 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...  :-//

« Last Edit: January 25, 2025, 11:08:40 pm by Vince »
 

Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #177 on: January 26, 2025, 04:41:51 pm »
OK I switched to the other 502. Started taking measurements to investigate the no-trigger issue, which match my recollection of my last measurement, and after a couple hours running, waveforms started being afflicted with a huge ripple making them dance all around the shop. Quick check of the power supplies oops.... the 350V supply is at 400/405V, 100V is at 115 and the reference -150V supply is at...  -167V ?!  So 15% off, which is consistent with the shift of the other supplies. So yeah, I guess this was the scope I had trouble with in the supply department, not the other 502.

So this scope is definitely in need of much more immediate attention than the other one...

So looks like the problem is then limited to the -150V, so I will work on that.
I tried working the trimmer from lock to lock with zero result : doesn't cause any change in output voltage what-so-ever !  :wtf:

So I guess that's a good clue.... supply acts as if it lost connection with the trimmer and is sending full juice all the time. Will look into that shortly... I guess that will be my menu for next week.
This is also the scope that got messed with previously in the can cap department. So I guess it's not too surprising that we now have some human induced fuckery to put straight.... Let's see what horror got done in the past to this poor power supply...
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #178 on: January 26, 2025, 05:00:17 pm »
I've lost count of what is what.
One had a bad trafo, one is not triggering, what else?
I'm pretty sure there are more.

Red rectangle focus is still fine, so only intensity is incorrect.
Maybe the beginning of deflection ramp is so high that partial beam is hitting the plate.

Can you see any difference if one beam is somewhere higher and other somewhere lower?

I guess incorrectly shaped primary side pulse of T801 is a bit far fetched.

ps.
Rat burger, 1p hint.
It's so difficult because it's so far from the real thing.
Advance-Aneng-Appa-AVO-Beckman-Danbridge-Data Tech-Fluke-General Radio-H. W. Sullivan-Heathkit-HP-Kaise-Kyoritsu-Leeds & Northrup-Mastech-OR-X-REO-Simpson-Sinclair-Tektronix-Tokyo Rikosha-Topward-Triplett-Tritron-YFE
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Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #179 on: January 26, 2025, 09:29:09 pm »
OK that was quick !

1) Power supply "fixed" : it was not reacting to the trimmer because... I was acting on the wrong trimmer ! Don't laugh... there are two identical looking trimmers place on the same spot of the lower deck, both in the power supply section, except they are on opposite edges of the deck. If you have one of these scopes you will understand...  :palm:  That other trimmer adjusts the amplitude of the calibrator output.
So I laid the scope on its side and checked all the rails... only to find they were now good and the -150V was spot on, and the trimmer was set half-way, so perfect. Ripple on all can caps is good or good enough. So put the scope on its feet, still working fine.... So we have a power supply that works fine... until it doesn't, then fixes itself ?! So, a random issue.... a bad connection somewhere... but where...

2) Trigger "fixed" as well.... remember previously I had a fuzzy trace and I traced it down to the absence of the HV shield on the CRT oscillator section ?
Well, I thought hey, maybe it's triggering fine and I just have a super, mega, horribly blurry trace that makes it look like it's not triggering properly ? Takes only seconds to refit the shield so I gave it a try... problem fixed !  :-DD

So the scope is working now....
I I had a quick go at calibrating it. Could not get the DC balance to behave with the lower beam (upper one is OK)... tough luck, impossible to adjust gain then   >:(
Maybe again an issue with tired and mismatched pre-amp tubes.  Well at least the upper beam works...

I did the same test as the other 502 : fed it a sine wave and checked the performance of the triggering and B/W of the scope at the same time.
This one has no issue, it's a winner. It can trigger perfectly up to the 1MHz B/W of the scope, and can still trigger up to 3MHz if you don't mind a fat/fuzzy trace.
Scope can still display the signal at that frequency, incredible. Much diminished of course, but still !
So yeah that's a good one...

Graticule illumination : noticed it stopped working for some reason ?! I am pretty sure it was working before non ?!
Bulbs are fine, but they are not getting any voltage somehow ! It's just a pot across the 6V supply for the tube heaters, how could that go wrong ?!  :-//
I measured at the terminals of that pot, and got 0.000V on all 3 terminals  :-BROKE
How can something that simple... not work.
The 3 wires bugger up the scope, all the way to the CRT section... will have a look at that tomorrow. Should be easy to fix I guess !

Fan is... yeah, it's really, really... really loud... with a noisy/rattling bearings to boot, though sometimes the rattle is barely audible.
Main concern is really the noise. It's like the fan is spinning way faster than it should, it's weird.
Maybe it was wired incorrectly and was fed 240V rather than 120V.... checked for that but no luck. Appears to have proper voltage.
So maybe it's the wrong type of fan, one designed to spin faster ?  :-//
I will look into that...


I've lost count of what is what.

Well.... I don't blame you ! I too am starting to mix them up  !  :-DD

But not to worry : I think I will stop working on them for now. I got them 90% of the way : cleaned inside out, tidied them up, checked and fixed the fundamentals, got them running for many hours on end, they basically "work". They are rescued.  I have now reached the point of severely diminishing returns... it's time to move on.
I will keep working a little bit on this one this week, to fix the graticule and work on the fan maybe (try to measure its speed and compare with the other 502 that's super quiet)  then next week-end I think I will move on to the restoration of a 502A, and this one I hope I can get real good, as I would definitely be using it.

So I will write down the current state of both of these 502 so that if/when I come back to them later, I know what each of them needs needs to be looked at.

So, dear MK and all, here it is, the conclusion (for now at least...) of this thread / 502 adventure. Current state of them :

The older one, SN #4356

- Lower beam : can't adjust / calibrate the DC balance, pre-amp tubes ?
- Power supply works fine but sometimes, randomly, the -150V shoots to -170V. Bad connection probably, but where ?
- Evidence of previous work on the power supply : can caps have been messed with.
- Fan (3 wire type) is very noisy + bearings noisy as well.
- Horizontal position control : trace is offset  3 divisions to the right = weak tube in the amplifier ?
- Graticule illumination doesn't work : no voltage present at the terminals of the control pot.
- Replaced a dead rectifier tube in the power supply
- -6.3V supply is a bit low at -6.15V.



The newer one, SN #6131
- CRT HV randomly fluctuates / picture shrinks
- Lack of brightness on the far left of the CRT, outside of the graticule, by the centre line.
- Replaced the CRT THT transformer and all x3 tube rectifiers.
- Replaced the power switch / graticule pot combo.
- Replaced the CRT, base was loose / glue let go.
- Replaced the CRT clamp, was broken.
- Replaced both graticule illumination bulbs.
- Did a full calibration, horizontal and vertical.
- Fan (two wire type / big black) is super quiet...is it normal or is it weak / too slow ?


« Last Edit: January 28, 2025, 11:53:44 am by Vince »
 

Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #180 on: February 08, 2025, 09:38:51 pm »
OK spent 3 hours investigating the graticule illumination issue, where I had no voltage going to the terminals of that control pot.
Took forever to trace the various wires.. eventually decided that they were fine, so I was mesmerised, so I powered up the scope and... well it works now ?! :wtf:
These scopes will drive me nuts...

However it did not work for long : after a very short while, light bulbs were flickering...coming and going.
I noticed that if I put finger pressure on the tabs / terminals of the bulb sockets, I could get them to behave hmmm....
Tried reflowing the wires, no joy.

Tried exerting some pressure onto the bulbs, to "exercise" the spring contacts inside the holders. No joy.
Removed the bulbs and shined some light inside the holders to see if the spring contacts were maybe corroded or sorry looking. They were not at all, they were very shiny.

So maybe the other contact, the body of the holder itself that is, where the bayonet goes to, might not be as "clean" as required to make a reliable contact, and/or the old bulbs themselves might not have clean enough metal. So I threw a couple brand new bulbs in there, so as to have brand new shiny clean metal to stick into those holders and.... problem solved ! Better than that even : the illumination is now quite a bit brighter than before. It looked OK with the old bulbs before, but that goes to show that they go dim over time even if you don't notice it off hand.

So, I guess that's one lesson learned as far as graticule illumination is concerned : don't waste time doing any troubleshooting, just start by replacing the bulbs with brand new ones, and go from there...

OK so I put those two scopes back together fully, they are now back on the storing shelves, just a couple feet away from the bench...
« Last Edit: February 09, 2025, 10:02:37 am by Vince »
 
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Online tautech

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #181 on: February 08, 2025, 10:25:44 pm »
Inspect filament connection to the internal wires.
Sometimes these are either poor , will break or the filament winding go intermittent open.
A magnifying glass can reveal these issues.....
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Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #182 on: February 09, 2025, 10:06:16 am »
Yeah problem solved as I said I put new bulbs in  8)
From now on I will replace the bulbs on any Tek I restore, that will solve the issue you mention among others.
These bulbs are cheap and easy to replace so no reason for me to waste time and pull hair on this particular subject... they will now get replaced systematically...

 

Online VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #183 on: February 09, 2025, 03:06:03 pm »
OK so as I said, I am now moving on to restore one of my 502A.

I already have an old topic about this, like 6+ years old, when I got my first one, so I am now reviving that topic.

Here it is :  https://www.eevblog.com/forum/repair/(classic)-tektronix-502a-scope-repair-restoration/msg5812043/#msg5812043

See you there for those interested.
 


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