Author Topic: (classic) Tektronix 502 scope repair / restoration  (Read 15343 times)

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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #25 on: October 27, 2024, 02:32:19 pm »
Thanks for the advice George, didn't know about that.

Well I mean of course I knew these HV high value resistors in the CRT circuit could drift quite a bit, have seen it in an old HP scope I worked on... but I could measure it when cold/ powered off. I didn't know they could be fine when cold and drift only when in operation.

OK so I have some more news : I meant to video the scope when powering it up, so you can see what it does exactly and how long it takes for the traces to disappear, and in what way they do so but... Murphy gets you every time : the scope doesn't want to fail anymore !  :-DD  I let it run for like 20 minutes and the two traces were still there, bright and sharp !  :-DD

Oh boy... so I guess this means the CRT section is not in that bad a shape after all. It's probably just some caps in that section that needed some time to reform/heal a little bit, to wake from 30 years sitting in that attic ! Of course it means they are borderline and probably will fail eventually, for good this time, and need replacing. But... for now they are hanging to life just enough to do their job and allow me to progress in my troubleshooting.

So I will leave it at that for now.

During these 20 minutes, I played with the controls and noticed 3 problems that need fixing as well !  ;D

1) A minor one, low priority issue, that I can fix later on, not a show stopper, but I thought I would mention it anyway because it does annoy me somewhat...
It's about the intensity control. There is a single control that acts on both beams. The problem is that I found out the two beams are not perfectly "in sync" so to speak. That is, say we start with the intensity control turned fully CCW. None of the beams show up of course. Then I slowly increase the intensity, to get a slightly dim but very sharp trace. Well that's were the problem comes in : as I turn the control, the two traces don't show up at the same time ! The lower beam comes up first, then I need to turn the control a hair more before the upper beam also shows up ! This means that the two traces don't have the same brightness and sharpness ! I have two choices : adjust the intensity to get a sharp and slightly dim lower beam, and have NO upper beam at all... or increase intensity just a hair so that the upper beam shows up, but then that means the intensity of the lower beam has increased a hair and is therefore not as sharp as before !

Call me picky all you want, but this just drives me completely nuts, triggers my OCD big time !  :-DD
So... there is an imbalance between the two beams. I guess it could be two things :

a) The cathodes of the guns have uneven wear, one is more tired than the other.

and/or

b) Those many HV high value resistors in the intensity control circuitry.. have drifted and created that imbalance.

Either way... I guess there is an easy fix ! I just looked at the schematic and noticed that it seems Tek did think of this very issue, and provided a convenient trimmer to adjust the intensity balance !  :D Look at the schematic below.  How thoughtful of you Tek, thank you very much ! Gonna locate that trimmer in the scope (not on the front panel nor the side panel) and play with it... hopefully it's got enough range to fix the problem. Should do it I think, the imbalance being very slight.


2) Horizontal control : I notice it's got a big offset : when the control knob is centered, the traces on the screen are not at all ! They start at about 3 major divisions to the right. The excursion of the control seems correct to me : plenty of it, but just offset to the right, big time. So it's purely an offset issue not a gain issue, nor a mix of the two, me think.
I think I already know what might be wrong here... as I had a similar issue on a another Tek scope. It was in the vertical amp of a Tek 317, but the principle applies to H amps as well I think. Each stage of the amp is made of two tubes, in a symmetrical arrangement. One tube pulls the trace toward one side of the CRT, the other toward the other side. I had one tube more tired than the other, which made the trace pull to one side.
So I guess I can just replace one tube in the H amp somewhere, and that might well fix it...


3) Now the last problem, this one top priority / high  impact : scope fails to trigger, at all. Doesn't matter how large the signal is on the screen, it just won't catch it, even in the "Automatic" mode. The only way to have traces show up on the screen is to turn the Triggering control fully CW, to put it in the "Reccurent" mode, as Tek called it. ISTR reading about this particular mode. From what I recall, it's a relique, something used only in ancient/early scopes but that soon disappeared (none of my many many other hollow-state Tek scopes, have this "recurrent" mode, they only have the "auto" mode). Basically what this mode does is it shows up the trace but it's free running, it does not try to trigger.


So getting the scope to trigger is my next job... wish me luck.
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #26 on: October 30, 2024, 07:59:00 pm »
OK, been working on the triggering issue... deep, deep dive.... taking time.

Probing is not easy, since the interesting points are often trapped between the tube sockets and the forest of passives mounted on the ceramic strips.
Sometimes you can make do by probing some passive on the ceramic strip, easy access then, but not always.

So had to lift a few passives on the ceramic strip so I could get to the tube socket pins, then solder a piece of wire onto which I could then hook a scope probe or DMM... then solder the passives back onto the ceramic strip, power on, probe... what a pain.

Doing so, I was able to say that

1) The trigger circuitry works just fine and generates proper trigger signals.
2) The sweep circuitry works just fine, and if I set to"recurrent" / free running mode, it works just fine, and at its input I get the same waveform as indicated on the schematic, shape and amplitude... except the frequency is much lower than expected : I measured it at about 150Hz but the schematic suggests more like 5 times higher than that...

So it should work eh ? But no, it doesn't. The scope can generate proper triggering signals, but somehow they don't trigger the sweep generator, even though the latter works just fine when free-running. Mesmerizing...

But there is hope !

There is a small 22pF ceramic cap that connects the circuitries, that's C131.
I can see proper triggering signals on one side of that cap, but on the other side... nothing, no signal, just DC.

Great ! That cap must be open circuit somehow, and I just need to replace it and bob will be my uncle.

So I extract that cap a few minutes ago and... it's not open-circuit, and measures spot on 22pF like it's supposed to.

I am... devastated. My only hope at understanding the issue, just vanished ! :(

Could it read OK yet still somehow turn into an open circuit when in operation ? I guess that's a bit of a stretch...

I am now totally clueless, it's very depressing... everything should work.. but it doesn't !  :rant:



 

Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #27 on: October 30, 2024, 10:31:49 pm »
Are the DC voltages anywhere near what the diagram gives for both sections, note the control/switch settings change for each test and the 'sweep' page is using recurrent for the waveforms (not auto).  :-//
Waveforms for the 'timebase' page are line at line frequency, i.e. 50 or 60 Hz.
The 'sweep' page is using ext DC & recurrent setting and should be faster, generated by the sweep gating multivibrator.

Oddly the 1959 manual has deleted the waveforms & voltages from the sweep page.  :-//

Oddly I can't see the germanium? diode in the diagrams, maybe there is a later manual, it seems to be connected to the same point as the 22pF capacitor, but it's hard to tell, could it be shorted?

Edit: The germanium diodes appear in the later 502A, wonder what is going on here? Does the other 502 have this? Might confirm an error in the manual, or modification, or it's something later than 1959.

David
« Last Edit: October 30, 2024, 10:36:45 pm by factory »
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #28 on: October 31, 2024, 12:35:04 pm »
There is a small 22pF ceramic cap that connects the circuitries, that's C131.

So right side is sourcing so much energy that left side can't do a dent?
Maybe your tester is too gentle.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #29 on: October 31, 2024, 07:15:56 pm »
Are the DC voltages anywhere near what the diagram gives for both sections, note the control/switch settings change for each test and the 'sweep' page is using recurrent for the waveforms (not auto).  :-//
Waveforms for the 'timebase' page are line at line frequency, i.e. 50 or 60 Hz.
The 'sweep' page is using ext DC & recurrent setting and should be faster, generated by the sweep gating multivibrator.

Yep I am following the instructions of the schematics to the letter.
The trigger section works absolutely perfectly in all conditions. Text book.
Waveforms shape and amplitude and DC levels are perfect. So I know that this liaison cap gets proper trigger pulses on one side.

On the other side, the sweep generator, waveforms are also fine in shape and amplitude, but not frequency as I said earlier, I get 150Hz which is 5 times less than what the time scale suggest on the waveforms. However.. I am not sure the info is ultra reliable because if you look carefully, the waveform indicate a time base of 0.5ms / DIV whereas the "instructions" say it's 0.1ms / DIV ?!  :o  So I am not too worried about the frequency I get... at least for now, I don't think that's my problem...

DC-wise, there is a slight discrepancy, but not sure how meaningful that is :

It says that in recurrent mode I should get -50V at the input, and I get close enough, -47.7V.
However it says that with the trigger knob set all the way to the left, but not in auto mode, I should get a bit less, -58V, except I get the exact same -47.7 as before, and I do so no matter what the position of the control is, even if I set it to AUTO. However, here too, I am not sure I should be concerned at all, as I don't see how the AUTO mode, or the trigger level, have any influence on this sweep circuitry. Either you turn it fully CW and you activate free-running / recurrent mode, or you don't. And if you don't, then it doesn't matter what the triggering level is set to, even auto mode, as this as no influence on the sweep circuitry that I can see. So in short, I think me having -47.7V both in recurrent and not recurrent, is exactly what I should get...

So in short, I think the sweep apart works just fine indeed, just like the trigger circuitry.

Oddly the 1959 manual has deleted the waveforms & voltages from the sweep page.  :-//

Oh no.... just realised I was using the wrong manual all along  >:(
The cover page on the late manual suggests the change point was serial number 3XXX. Both my units are newer than that.
The one I am working on is early 1961 and SN 4XXX and the other one even newer, mid-1963 and SN  6XXX

OK so I just downloaded the correct manual... indeed they got rid of the waveforms and voltages !  :o
Glad we have the older manual then, this way at least I had something to work with !  :-+

Oddly I can't see the germanium? diode in the diagrams, maybe there is a later manual, it seems to be connected to the same point as the 22pF capacitor, but it's hard to tell, could it be shorted?

Yes it's connected to that cap. Germanium indeed, I just checked it with my DMM, it's not shorted and drops 0.25V.

Edit: The germanium diodes appear in the later 502A, wonder what is going on here? Does the other 502 have this? Might confirm an error in the manual, or modification, or it's something later than 1959.

I looked in my other/newer 502, and that diode is NOT there, it is conform to the schematic.
So, I removed that diode and.... no change, still doesn't work. Pfff...

 

Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #30 on: October 31, 2024, 08:26:28 pm »
OK the voltage is stuck at -47.7V, is this recurrent switch supposed to open when the trigger is turned almost fully left (not in auto)? This could drop the voltage to somewhere around -40V?
The position of the "stability adjust" control may have some influence on the readings too.
And all the other voltage/waveform measurements on the "sweep" page are present & correct?

Something else that I'm drawn to, might not be related, the circuit description states the NE-2 in the miller runup part should be dropping 60V when on, the striking & maintaining voltage of neons are known increase with age.

David
« Last Edit: October 31, 2024, 08:30:49 pm by factory »
 

Offline George Edmonds

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #31 on: October 31, 2024, 09:23:23 pm »
Hi Vince

The last time I worked on or used a Tektronix 502 series was in May 1965 and it was the RM502A.  The main reason that these scopes were being used was their ability to trigger down to DC levels which few other scopes at the time had the reliable ability to do so.

As I do not have your 502 in front of me I can only make some general comments at this time.  First it looks like there are at least two versions of the 502, plus a number of in manufacture modifications.  It may help identify just what you have if you quote the serial number.  In the days of the 502 Tektronix used to provide individually serial numbered manuals with all of the relevant errata and updates included.  (This may no longer be relevant given your recent posting)

The first thing that Tektronix did when getting a scope back for service/repair was to wash it so as to make certain that any leakage paths from the ceramic tag strips to ground were removed, again without sight of your scope it is impossible to say if any such  possible leakage paths are present.

In order to provide a stable and calibrated scope with good triggering Tektronix made extensive use of fully stabilised accurate low impedance power supplies, so using a Variac carry out all of the power supply regulation tests detailed in the manual.  From my recent experience repairing a Tektronix constant level signal generator and time mark generator I would expect to find a number of failing high voltage electrolytic capacitors present, after all they are now some sixty years old.

I have looked at both the early and late versions of the 502 manual.  What appears to be the earlier version the expected voltages are given in the manual text and on the schematics whereas the later version has them on the schematics only.  PLEASE be aware that some voltages are measured using a 20,000 Ohm per volt meter and some a constant 11M input impedance meter.  There will be differences particularly at lower voltages.

Personally I would check the power supplies and then removing one Valve/Tube at a time and run them all through my Valve/Tube tester.  Whilst each Valve/Tube is being removed, insert and remove them a number of times to clean the Valve/Tube pins and their bases.

If this fails to restore the 502 to correct order then you must, using a second scope, fault find the 502 in a logical way.

G Edmonds
 

Offline George Edmonds

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #32 on: October 31, 2024, 09:34:33 pm »
Hi 

The possible problem with the NE-2 is know as the dark effect, they will not correctly strike in the dark.  To overcome this they are doped with a very small amount of a radio active isotope, the problem is that the half life of this radio active isotope is only some 11 Years.

G Edmonds
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #33 on: October 31, 2024, 10:12:07 pm »
Hi chaps,

Thanks a lot for your help !

George it's incredible that you used that scope back in the day when it was brand new ! Reading you feels like I am in a time machine !  ;D
I hope you enjoy this thread then, and am now extra motivated to post about it... and will make sure to try and revive my other 502 for extra content !  ;D

So I finally "fixed" it ! I will cut the story short but in the end yeah, Factory was right, the only "problem" was the stability adjust trimmer that needed a tweak !  :o

I apologize if you feel that I might have wasted your time  :(

... but as for me I don't consider it a waste of time as it allowed me to dig much deeper into that subject than I have ever done before.
I learned things, which is an important part of working / fixing these old scopes which were alien to me just a handful of years ago when I bought my first tube scope, or tube anything.

This knowledge will serve me reaaaal soon might I add : 2 or 3 years ago I presented on TEA my second 502A. Started work on it and gave up after many, many hours of head scratching, trying to understand why the freaking scope would not trigger when everything seemed alright....  the prospect of having at long last a solution to this issue has me overjoyed, and I shall therefore revisit this scope real soon !  :-+


So, to my defense, here is why I didn't play with that trimmer sooner :

1) It's a trimmer, it's got no knob, it's recessed and requires a screwdriver.... and the general rule when working on these old scopes or old anything, is to not mess with trimmers unless you know what you are doing, because it's likely you are going to throw a calibrated scope completely out of cal and create a lot of pain for yourself later.

2) The manual explained what this trimmer was for : make sure that the sweep generator free-runs ONLY when asked to do so, i.e. when the trigger control is set fully CW to the "recurrent" position, and NOT at any other time.
And well, the scope was doing just that as is ! Scope was free-running just fine when set to recurrent mode, and would never free-run under other circumstances.

So, I thought that whatever it was set to, it was most likely set properly since it was doing its job ! :-//


Here is a picture showing the issue fixed.... we have a nice stable trace, well two of them, of the calibrator output. It bings me joy just looking at that screen  8)
.. means that scope it now working fine and now we are down to fine tuning, checking for calibration of everything, and chasing offsets / gain issues / tired tubes in amps.

So that's what we are going to do then...

Let's start with :

- The horizontal amp : I measured the frequency of the calibrator, it's pretty much 1kHz, clocked at 1,0139Hz , however as you an see the trace on the graticule occupies only 4.4 DIV , so 8.8ms, that is 1,136Hz so about 10% too much.

- Still in the horizontal amp, as stated earlier : the position control is heavily biased toward the right side. A tired tube somewhere in the amp probably.

- Vertical amp : the calibrator is set to 10Vpp but the trace as you can see (2V/DIV) is more like 4.2 DIV so 8.4Volts not 10V... the calibrator could be out of adjustment, I will check/adjust that first, but I doubt it's out by THAT much, so the vertical amp(s) most likely need to be checked for gain and/or tired tubes.

- The trace is not perfectly aligned with the graticule, tube needs to be rotated just a hair CCW.

- The intensity balance trimmer, I tried to adjust it but failed... I mean it does have an effect but I failed to adjust it properly due to the very poor mechanical implementation of it, so to speak.... --> when you shove a screwdriver threw the hole in the metal shield of the CRT circuitry, the trimmer runs away from you !
It's not supported at all, it doesn't stay in place !  >:(  I will remove the shield and see what's what, but I think the problem is likely that the trimmer has all of its 3 pins on the same side of its package, and all 3 must be soldered on the same ceramic strip. So if you push on it with the screwdriver, the trimmer just bent downward  :(   To be more clear... it makes this trimmer absolutely unusable. I think Tek eventually realised this, because I noticed that on the later 502A, they relocated that trimmer to the front panel, where it "just works".
So I first need to find a "fix" to this problem, find a way, a safe (with regards to the HV present) and durable and reliable way of holding that trimmer in place, so that I can at last adjust it properly.


OK. So before I do that, now that the scope is basically working, I think it's now time to give the inside of the scope a good, good clean, to revive its former glory, make it more pleasurable to work inside, and also give all the contacts / switches a good dose of contact cleaner, as well as, as George said, clean the tube sockets and exercise all the tubes in their sockets. This will fix many small issues from the get go. This way if I am seeing an issue, I know it might be an actual issue that needs diagnosing, rather than just some dirty contact somewhere, a red herring that would waste lots of time needlessly.

Will try and do that this week-end...


So, looking back.... in the end, this attic find that looked so terrible... needed in fact nothing but a new rectifier tube and a tweak of a trimmer on the front panel  to get going again !!  Not bad at all eh ! Could have been WAY worse !

Hmm... this restoration is going really well, motivation is at a record high !  ;D
« Last Edit: October 31, 2024, 11:33:09 pm by Vince »
 
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Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #34 on: October 31, 2024, 10:59:51 pm »
Good to see it's working.  :-+

I know what you mean about the later 502A & trigger problems, mine failed in the same area, unfortunately for me the solid state parts of that trigger circuit seem very unreliable.

David
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #35 on: November 01, 2024, 12:00:27 pm »
OK ! Bank holiday today so... I am motivated to clean that scope.

I have just finished taking it "apart", to get better access and be able to do a better job more easily.

I removed the cooling fan, CRT and all the tubes, all 39 of them, and the two covers on the CRT HV areas.

First / last time I restored / cleaned an old Tek scope, it was my little type 317. Back then I used styrofoam to hold the tubes in place in their actual layout.
But styrofoam is a bit messy I find, and also it doesn't hold the tubes that reliably, as when you push them into the foam, then let go, they always spring back / out a little bit and in the end they aren't secured all that well, I find. So this time I tried something else instead : ESD foam. I had a left over that was just large enough to hold all the tubes.
Well it's a success. There is no mess compared to styrofoam, as it doesn't fall into bits on its own, and also there is zero spring back : once you stick a tube in there, it just stays there 100%.
I love it. The foam could benefit from being a tad thicker and a hair softer, so might experiment with that one day.


Some "before pictures" before I start cleaning it, see you later for the after pictures, once I am done with it.


« Last Edit: November 01, 2024, 01:05:30 pm by Vince »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #36 on: November 01, 2024, 12:03:05 pm »
Some close-ups of the CRT stuff

 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #37 on: November 01, 2024, 12:05:55 pm »
Some close-ups of the upper deck.

 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #38 on: November 01, 2024, 12:13:41 pm »
Some close-ups of the lower deck, as always in these scopes, the worst part of the scope....

« Last Edit: November 01, 2024, 01:03:14 pm by Vince »
 

Offline med6753

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #39 on: November 02, 2024, 01:23:57 pm »
Does your restoration include replacement of the power supply capacitors?
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #40 on: November 02, 2024, 06:21:31 pm »
At some point yes... especially as I said the one that's on the -150V since it's really bad with 500mV ripple against a spec of 5mV, and it's the reference rail.
Ripple on the other rails is either within spec or not far off, but might as well replace them all for long term peace of mind, since I do intend to use this scope.

« Last Edit: November 02, 2024, 06:23:29 pm by Vince »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #41 on: November 03, 2024, 05:35:52 pm »
Some piccies after the bath. Happy with the result overall, given that it's a 65 year old attic find, but it's not 100% new, corrosion did take its toll on some things : the 3 trannies in the power supply section, as well as the butt of a couple pots behind the front panel, and also the top side of the lower deck that's pitted all over the place, I think the clear coat was starting to fail. Not much I can do about that other  sanding it down, which would remove all the printing on it as well as ruin the finish. So I will just leave it like that. It's clean, it hurts nothing, and doesn't even show from a couple feet distance. It's mostly the well lit macro shots that make it look terrible on pictures. In real life I am happy to live with it.

I removed also the HV shield behind the front panel, where the CRT controls are located. I also debated removing the big handle bar at the top of the scope, to get slightly better access to the CRT HV circuitry, but eventually decided that the pain was not worth it.

As for the hardware, I noted :

- CRT shields, countersunk screws going into the rear panel of the cabinet : they just love to get mangled, so one of them just did. I managed to extract it by stabbing a flat screwdriver into it, works quite well. Will be replacing that screw.  I guess it uses old Philips style head which doesn't play well with my more modern Pozidrive screw drivers. Don't have Philips screw drivers, it's ancient stuff I have never come across other than in these old Tek scopes...

- There is a big cap (?) next to the cooling fan, that it's wired to. There are two screws holding it in place, on a panel. One of the screws seems improper to me. It's a Philips head rather than the slotted head of the other, proper screw. Also, note how the bracket holding that cap, is mislocated : it overlaps partially the text printed on the panel ! I guess it was a Friday evening scope...

- Can caps : more Philips screws in there, 3 of them that I can see. Will be fixing that when I get round to replacing those caps. Also, note how one of the 3 insulated are inconsistent : 2 of them have the old black paper cover on them, which is proper, but the third one has a more modern white plastic cover. It's not right... also note how that plastic cover doesn't even fit the cap properly ! It's not tall enough, the bottom of the can is still exposed ! So clearly someone has been messing with these caps... but we already knew that from prior pics, where we saw obvious signs of rework on some pins of those caps.

- Intensity balance control : see close-up. Once the cover is removed, we can see it... confirms what I imagined : the trimmer has all its 3 legs on the same side of its package, and there is nothing to support it, so when you stick the screwdriver to adjust it, of course it bends and sinks into the abyss. So I need to find something to support it, that will stand the test of heat cycles, and no cause any issue with the HV. Also, the situation is even worse than that : note how the trimmer does not even fit between those two resistors ! So they had to tilt the trimmer to make it fit ! So it's tilted on two axis in fact ! No wonder I was having hard time getting anything done with that trimmer !  It's a disastrous implementation !  :palm:   I guess I could replace the outer resistor so that I can give it more lead length, and clear that trimmer properly.

- Upper deck, cooling fan, where its wiring harness goes through the deck. See prior pics. That area was the ugliest of the deck, dark thick oily / greasy looking deposits... the rubber grommet there was in a horrible state, turned into a very soft sticky mess that was falling into bits... oil and rubber never got along so no surprise here. Of course it's gone now that I cleaned that area. Will be replacing it : some time ago I bought an assortment of rubber grommets... well they look more soft plastic than rubber, cheap they are, but it's better than nothing so it will have to do. If someone knows a source of actual / good quality rubber grommets... please share.

Scope has been drying in the heated living room for 48H now. Will give it one or two more days before I dare powering it up again. I am scared....
Me has an adequate dim bulb tester and variac though, I won't let it explode I promise !  >:D


But overall I am happy, it's in good nick. Zero corrosion on the tube socket pins, nor the big connector for the neck of the CRT.
« Last Edit: November 03, 2024, 05:38:55 pm by Vince »
 
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #42 on: November 03, 2024, 05:40:35 pm »
Some close-ups of the "damages" / minor issues listed above.

 
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Offline Robert763

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #43 on: November 05, 2024, 07:55:44 pm »
Hi Vince,
If you are working on old American TE you really must get at least a No1 and a No2 phillips screwdriver or hex bits.
Pozidrive drivers will damage phillips screw heads. Phillips head screws are not obsolete. They are still used in in America and in a lot of aviation products.
Hex bit's are widely available including longer ones for tight spots.

Robert
 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #44 on: November 05, 2024, 08:18:13 pm »
My pozi tips are pretty pristine.
Here it's practically '+' on not.
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
(plus lesser brands from the work shop of the world)
 

Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #45 on: November 05, 2024, 08:37:57 pm »
I have some Craftsman “Reed & Prince” screwdrivers (only one size) that I use on older HP and other test equipment.  They have a different blade shape than Phillips drivers .  However, I normally use Phillips-head machine screws for electronic construction, and Robertson square-drive for wood construction.
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #46 on: November 05, 2024, 10:28:53 pm »
OK OK !!!

I looked in my old FACOM 1/4" ratchet set ... only PZ bits in there.
Looked in the bit set that came with my cordless drill... no luck either.

Then I remembered that a year or so ago I bought a comprehensive set of security bits because I needed that to crack open an old Game Boy that I tried to fix for a friend.
The repair failed so I forgot about that set but looking at it now... the set was not just security bits, but more of a general purpose kit to work on modern electronic shit like smartphones tablets laptops etc. So lots of small bits.

Look below. Looks like I have lots of Philips bits !  0000, 000, 00, 0, 1 and 2 !
I tried #2 and looking closely at it, it fits these Tek screws perfectly indeed, and I was able to put them on and off with no drama / damage.

OK I am convinced boys.... I will get meself a proper PH2 and 1 screwdriver from .. somewhere.

« Last Edit: November 05, 2024, 10:30:44 pm by Vince »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #47 on: November 09, 2024, 09:39:36 am »
OK some news....

Been 7 days since I washed the thing, must be pretty dry now. So last evening I spent 4++ hours putting it back together and giving it a test ride.

- Replaced the missing/deteriorated grommet in the lower deck, power supply section. Was a pig of a job, but it came out OK in the end. Those old wires don't like being desoldered.. insulation melt like butter, and on the lower wires of the thermal cut-out switch, the wires were now exposed, and real close to the chassis !  :scared:  I bent the tab a bit to bring them a 1/4" away from the chassis...

- Put the fan and CRT in place.

- Cleaned patiently every tube, gently, trying to preserve what was left of the markings on them.

- A squirt of contact cleaner in every tube socket, then exercised each tube in their socket half a dozen time.

- A squirt a contact cleaner in every accessible unsealed pot (control only of course, not trimmers...  ::) ), and wafer switches. Exercised them all.

- Replaced a missing shield on a couple tubes in the vertical amps... however I am still missing one shield, on the oscillator tube for the CRT HV section. This is one is larger than the other ones, it's the only tube in the scope that uses this size.. and sadly my 502A donour is also missing that shield  :(
Will have to dig deeper, later, in my other scopes, hunting for one.

- Found x4 tubes bearing with a cool little sticker from Tektronix, attesting that some Tek service centre tested those tubes at some point.
They aren't some random tube, there is a pattern to them : they are the two tubes forming the first stage/ front-end of each vertical amplifier.
The ones whose heater is powered by that dedicated, low noise DC , solid state low power supply.


Looking all shiny and complete now.

Then brought it back to life via the dim bulb tester just in case. It woke up with no drama : no short, no sizzling noise nor smoke / fire / sparks / fireworks / flames / explosions, and we had signs of life on the CRT.
So I got rid of the bulb tester and then powered it up again, this time to test it properly.

Well it's not good, it's very sick, both in the horizontal and vertical departments. It needs some work... I hope I can fix it.... looks way worse than it was before....  :palm:

- Sweep is present only 0.1% of the time, randomly, and when it does show up, it does not look good at all... only partially spanning the screen.

- Lower beam, the spot is there but it's totally unresponsive to input signals or vertical position control.

- Upper beam does show signs of life : it reacts to input signals, attenuation and position control... however at regular intervals, the spot will all of a sudden decide to move upward, going off screen/disappear.. then a moment later it will come back into place, where it was, and start responding properly again, then again a moment later it will disappear again, then come back... there is a rhythm to it. Maybe some capacitive effect, an old cap that's on the way out, who knows.

So to sum it up, it's now clean and put back together, and nothing dramatic, severely wrong, scary happened upon powering it back up... but it does need some work to make it work again.

Hopefully I can find what the problem(s) is/are... stay tuned...


 
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #48 on: November 09, 2024, 03:00:10 pm »





OK making some progress I think....

Back to basics, I started by checking power rails.
The reference -150V is fine.
The 100V as well.
The 350V is a bit high at 362V or something, but not a show stopper for now.

However the -6.2V is kaput at -3V or so.
It powers the heaters for the front-end tubes of both vertical amplifiers.
So with a quarter of the power, yes I can imagine that signals have a hard time coming through.
The circuitry for the vertical position knob is also part of the first stage, so that adds up nicely.

Not quite sure though, how comes the upper beam kinda works, and not the lower beam.

Anyway, let's fix that power supply and go from there.

Think I found the problem : the two socketed small signal transistors. I messed up it seems. When exercising them in their socket, turns out I bent the centre pin on one of them, V644. I tried to straighten the pin but it snapped  :-\
The are 2N214, Germanium of course, and I don't have such things in stock. Have a few power / T03 Germaniums, but no small signal ones.
They are nothing special, 25V 100mA thingies, so could replace it with a similar silicon tranny, however that means the output voltage of that supply will be 0.5V more negative, and I don't know if the heaters of the 6AU6 will be happy with that or not. I guess it's not that big of a difference so I could give it a try.
Or, another idea, is to swap these two transistors : I could take the other, intact and working (I checked it on the chinese component tester), so I replace like for like, and then V634 I can replace with a silicon tranny because according to the schematic, it should not make any difference...

Yeah, will try that and report...

« Last Edit: November 09, 2024, 03:49:36 pm by Vince »
 

Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #49 on: November 09, 2024, 03:56:42 pm »
I thought you had a 502A spares unit, doesn't it also use those trannystors?

If I was to do the same with my 502A (the cleaning of sockets), I suspect I would break quite a few vacuum bulbs, it's a lot worse condition, once they've got stuck with corrosion they don't pull out easily.

David
 


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