Author Topic: (classic) Tektronix 502 scope repair / restoration  (Read 15334 times)

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Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #125 on: December 03, 2024, 03:04:14 pm »
070-382-02 p.187/201 is taking them away.

070-090 p.59/110 is doubling C850, C866 and C868.
So later machine then, and good to have early version of 502A manual around also.

Right leg of domino seems to be connected to underneath disc and nowhere else.
Front is C820 & R820 and back C805, R805 & R806 then C800 & R800 and R809, R808 & C808.
So C807 is missing, but there is also that disc.
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Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #126 on: December 03, 2024, 04:52:06 pm »
C807 is the Mica domino, the disc underneath it is C808, there should be wires to the transformer too.

David
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #127 on: December 05, 2024, 08:50:34 pm »
There is something odd about the circuit with the bubbled disc capacitor, in parallel with a 2.2M, connected to a 1.5M, these are not in either '502' manual. I seem to remember Vince saying the available manuals aren't quite right.

Yes as I said earlier these 3 components are not anywhere in the manuals...

Those 3 parts don't appear until you look at the '502A' manual, they are C847, R846 & R847,

Oh... I did check the 502A manual but somehow didn't spot these components as I was sure they were physically located in the oscillator circuit / primary side of the transformer.... but they are on the secondary  !  :palm:

Thanks, I can see them now...


they probably won't be responsible for the no HV fault.

Yes I thought the same...
So I lifted the upper end of that network of components, to take the whole thing out of circuit, thinking the scope ought to still work fine since they are just an add-on to older scopes, not a mandatory circuit. I checked both resistors, they measure just fine. The cap measures at 200nF instead of 100nF but maybe that's because I left the resistor across it in circuit, might have caused the measurement to be off. Or maybe the cap is indeed twice the value but still not a show-stopper I think. At any rate it reads ballpark and is neither open-circuit nor shorted, so no drama there.

Anyway, with these components disconnected from the scope... I powered it up and... as expected, still no HV.

So the search continues.


« Last Edit: December 06, 2024, 11:11:25 am by Vince »
 
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #128 on: December 07, 2024, 11:07:50 pm »
OK, some more...

1) I lifted a leg on each of the 4 Sprague caps in that circuit... they all look absolutely fine, and measure OK. They measure high as they often do, but nothing extraordinary. They certainly have zero sign of catastrophe failure...
All resistors look and measure OK as well.

I can see black smoke on the side of the ceramic strip, facing the transformer... so the magic smoke must have emanated from there but... for the life of me I can't find any component that smoked, other than this disc red ceramic cap we have already discussed, which is now out of circuit.
And again, that cap still measures something sensible, and even if it were dead it's in series with a Meg range resistor so that would have kept the corresponding secondary winding from burning, I would think.

So basically I am in one big, big mystery. I am puzzled.... :(

I don't think the transformer smoked because the black smoke is only covering the ceramic strip per se, there is zero trace on the transformer itself nor the wires coming out of it.

2) I checked the resistance, in situ, of the 2 primary windings. I do see an anomaly there, but a strange one : one of the windings reads twice higher than it should.. rather than be shorted or open-circuit. I am not too sure what to make of that, it's weird... how could that happen...

I took comparative measurements on the other 502, and on a 502. Both measure the same (as you would expect, but I wanted to make 100% sure...) : the main winding; between pins 4 and 6, measures about 4.3R, and the smaller windings between pins and 4 and 3 is about 1.3R.

Well on the sick 502 do get a similar reading for the main winding, but the smaller one reads twice too high, at 3.4R instead of 1.7.

What could that be ?!  :-//

3) I tried to check if the HV rectifiers were not dead : lifted a pin on each of them so I could power the heaters from my lab power supply.
I set the supply to 6.3V, the tubes did glow but not the expected dim orange, but rather a super bright, bright yellow, very much like an actual light bulb. Weird. Tried all 3 of them... all did it. Geez... tried it on a 502A donour... same result. Hmmm... strange. Pulled the datasheet for these tubes... oh oh... heater is not supposed to run at 6.3V but only at 1.25V ?!  :palm: :-BROKE
This it the last time I do electronics.... I am not worthy  :-[
So I set the supply to 1.25V and checked them again. One is glowing the normal orange, phew, but the other too still are bright yellow, so less bright of course, but still... So I might have to replace them further down the line, we shall see. I have plenty of those so not too worried. Lesson learned : not ALL tubes run on 6 or 12V !  :palm:   I think I even have "brand new" Mullard rectifiers somewhere.  Could still retrofit solid state diode as well, though I don't see the need for that, and prefer to stick to the original tech if it works, for originality.

4) I checked the power supply : CRT circuit runs on +485V UNregulated. I do get that, and unregulated for sure it is.... with 10Vrms ripple. So 30Vpp I guess. A bit much... probably a tired cap in the power supply. But not a show stopped I would think.
That supply is further filtered, locally, with R808, a beefy 1.2K resistor that reads a bit low (for a change), at 1.0K, and capacitor  C808, 100nF.
In every Tek scope of that era, that I have seen, Tek somehow insists on rating it at only 400V when it sees nearly 500V, so often it fails.
But somehow it looks and measures just fine in this scope. I also have a problem : there is little ambiguity as to the what cap it is, physically : there is only one 100nF electrolytic cap in the CRT section, and it's soldered right besides the resistor. So pretty easy eh ? Well, in this case how comes the top side of that cap is NOT wired to the resistor ?!  :o  I assure you, it's not. Neither is it in all my other 502/A. So again a big mystery.
The top side of that cap measures at NEGATIVE 150V ! rather +485V.   This scope is driving me a bit crazier with every passing week-end...

5) I measured grid voltages at the oscillator tube. The main grid, pin 3&6, that's used as I understand it for the feedback from the auxiliary/smallest  winding of the transformer, is supposed to get -50V and I see -10V.   The other grid/ "screen" is connected to the feedback from the HV side of things. Grid is supposed to see +60V and I get +185 V !   ISTR that the way it works, if the HV drops that grid voltage gets higher, so it's consistent : zero HV means mega high voltage fed back to the grid. Problem is that I also remember people saying that too high a voltage on that grid can damage permanently that oscillator tube ?! I don't know how many ms or sec or minutes it takes to cause damage, so I don't know it that tube is dead or not. I swapped that tube and also the other one, V814 used for the HV feedback, but no change. So they aren't the culprit.


6) CONCLUSION

So that's all I have tried today.
To sum it up : the more I dig, the more mysterious it becomes, it drives me nuts. These old scopes are supposed to be simple to fix, they aren't rocket science nor 100GHz voodoo.

So I am getting tired of failing to fix that HV !
So I am switching gears now, I have no choice but do some electronicking. Yes I know I just said I would not anymore, but I lied.
The main problem is that it's closed loop system, it's a chicken and egg problem...
My idea is to try to divide and conquer. The worked great for the Romans 2,000+ years ago, hopefully it will work for me as well in 2024.
I want to open the loop, make the oscillator run with no feedback from the HV side of things.
I plan on disconnecting the screen / pin 1 of the oscillator tube, where the red cross is, and feed +60V to it from my lab supply.
Since this is the typical voltage that tube sees when things work, it should at least allow for it to get going.
This way, with no feedback to drive me crazy, the oscillator should be much easier to troubleshoot : it will be reduced to what's circled in green on the schematic. That is, not much. Just the oscillator tube, the two primary windings, two resistors and two caps...
I have not to worry, care , about anything on the secondary side of things, and anything about the HV feedback.
No. I will have nothing.. nothing but the oscillator itself, nothing else.
This will also allow me to make sure the grid never sees dangerously high voltages, until everything is fixed. Things is, I can't blow these tubes by the bucket load because unlike the 6AU6 and 6DJ8 which are plennnntyful in Tek scopes (they inundate all amplifier stages), the tube Tek uses for the oscillator in their scopes is... unique. Whatever tube/pentode they use, they never, ever, ever, use it anywhere else in the scope. Never. Plus, not all scope models use the same type of tube either.  So you better look after them...

Anyway, I will work on the thing until it manages to oscillate, then when it does oscillate, I will check that the secondary side of things produces the expected HV, and when it does, I will then be able to close the loop again, at which point if it stops working again somehow, I will know the problem comes from the HV feedback. Divide and conquer. Check and fix all 3 parts of the loop methodically, one at a time : oscillator, secondary side, then HV feedback.


First, I need to make sure that my two oscillator tubes are still alive despite having seen +185V at the screen... will plug them in my other 502 and see if that scope still can produce HV with these tubes.

That will be for tomorrow hopefully. If not, next week-end.

Sorry it takes so long to fix this damn thing, but it's a tough ride  :-\

« Last Edit: December 07, 2024, 11:21:18 pm by Vince »
 

Offline med6753

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #129 on: December 07, 2024, 11:51:27 pm »

3) I tried to check if the HV rectifiers were not dead : lifted a pin on each of them so I could power the heaters from my lab power supply.
I set the supply to 6.3V, the tubes did glow but not the expected dim orange, but rather a super bright, bright yellow, very much like an actual light bulb. Weird. Tried all 3 of them... all did it. Geez... tried it on a 502A donour... same result. Hmmm... strange. Pulled the datasheet for these tubes... oh oh... heater is not supposed to run at 6.3V but only at 1.25V ?!  :palm: :-BROKE
This it the last time I do electronics.... I am not worthy  :-[
So I set the supply to 1.25V and checked them again. One is glowing the normal orange, phew, but the other too still are bright yellow, so less bright of course, but still... So I might have to replace them further down the line, we shall see. I have plenty of those so not too worried. Lesson learned : not ALL tubes run on 6 or 12V !  :palm:   I think I even have "brand new" Mullard rectifiers somewhere.  Could still retrofit solid state diode as well, though I don't see the need for that, and prefer to stick to the original tech if it works, for originality.



We all make mistakes. Trust me, I've made my share. That's how we learn. I would not attempt to use the 5642 HV Rectifiers that you blasted with 6.3V. Chances are the filaments are damaged. They are readily available on Ebay or as you mentioned one of your spare 502's or any other 500 series scope you might have.

I have upgraded the HV supply in Type 547's to SS rectifiers and it works well. So that is something to consider too.
An old gray beard with an attitude.
 
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Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #130 on: December 08, 2024, 11:47:29 am »
This it the last time I do electronics.... I am not worthy  :-[

For the forum the content is still much better than many other.

Early 502A manual 070-382 p.105/110 has C808 as .02 and C824 as .01 in rectangle(blue outline), first left of high voltage trimmer.
Rectangle means that value has changed.
070-090 p.88/110 has C824 as .1 and obviously without any change indicators.

So C824 has had its old value a bit before that 502A manual and probably also 100nF.
Then what factory said earlier is spot on.
The topmost of the column is C824, underneath disc is C808 with new value, domino is C807 and no leftovers.

You can get higher winding value only if its wire has more resistance.
Means that part of the wire must be pretty thin, not very usual thing.
Seams that you must disconnect the trafo and use your new variac for the real thing.
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Offline factory

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #131 on: December 08, 2024, 11:55:20 am »
You can get higher winding value only if its wire has more resistance.
Means that part of the wire must be pretty thin, not very usual thing.
Seams that you must disconnect the trafo and use your new variac for the real thing.

The variac is no use here, the HV transformer runs at higher frequency (60kHz according to the manual), low frequency will just burn it up.

Those oscillator tubes seem to have no European equivalents, they were widely used in 50s TVs in the USA. But over here with TV/radios using series strings for the tube heaters, we only had higher filament voltage versions.

David
« Last Edit: December 08, 2024, 12:07:24 pm by factory »
 
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Offline med6753

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #132 on: December 08, 2024, 01:06:54 pm »


Those oscillator tubes seem to have no European equivalents, they were widely used in 50s TVs in the USA. But over here with TV/radios using series strings for the tube heaters, we only had higher filament voltage versions.

David

Correct. Typical application for the 6CZ5 was Vertical Output Amp for B/W TV's. They are dirt cheap on Ebay but shipping to EU might be an issue.
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Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #133 on: December 08, 2024, 04:13:01 pm »
The variac is no use here, the HV transformer runs at higher frequency (60kHz according to the manual), low frequency will just burn it up.


Yes, I meant that low variac output is used to check that idling trafo can output something.
But better use something else that can't be destructive and after a second thought Vince probably has few around already.

For tube types,
seems that number-letter-number types are generally so that first number part is heater voltage range.
Was that ever a standard?
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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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #134 on: December 08, 2024, 04:27:28 pm »
Thanks for the help people. I have been able to make progress...

Thanks to MK for the help with identifying the caps and looking through the various manuals... he was 100% right and it cleared all confusion. It's all perfectly clear now, hooray...

It appears that my 502 obeys more to the 502A manual than it does either early or late 502 manuals, thanks Tek...

I chased wires to confirm it all. It's all as MK says. The filter cap is indeed not that 100nF / Sprague, but a 20nF like the 502A manual states. It's.. DISC CERAMIC cap ! I found it. It's in great shape and measures spot on. I guess lowering the value to only 20nF allowed the use of a disc ceramic which in turn allowed to achieve excellent voltage rating/reliability, and do away with electrolytic caps that were used to blow in this place, I found out on other scopes.

As for the that 100nF Sprague cap indeed it's C824 located in the HV feedback branch.... and I should have known better because it DOES show up in the 502 manual, However it only shows in the LATE manual. It's completely absent (not a different value, but completely inexistent) in the early manual. Point is, when I printed the schematics the other day, I used the early  manual because that was the only manual I had downloaded on my computer 6 years ago when I bought that scope. I don't remember there being a late manual back then. I think I will throw away those early schematic and print the ones from the late manual, and when I don't find something, or find something odd... I will rush to check the 502A manual(s) to see if that helps...

While I was in there I checked C807, that domino mica cap that tunes the oscillator frequency. I measured 3nF as it should. No blatant failure there.

Then I did as I said I would do... I  ran the oscillator open loop : I pulled V814 from its socket to disconnect the HV feedback from the oscillator. Then also had to lift the upper pin of R800 to keep the +350V supply from frying the screen / pin 1 of the oscillator tube. Then I was able to feed +60V to that pin from my lab supply (put the two outputs in series, gets me to 60V precisely, not more.. phew) to simulate a working feedback.

Then I looked at the (main) grid of the oscillator tube, pin 3&6, where it takes its feedback from the transformer primary.
I measured DC levels as well as waveform using a scope.

Well we have good news : we do have oscillation, look at that !  It's a bit distorted, with lots of clipping at the top, and at the bottom what looks like a bit of RF noise, but it's basically a "nice" sine wave, that's rock solid both in amplitude and frequency.. so I like that A LOT !  :D We even have GLOWING heaters in the HV rectifiers ! Well two of them have a normal glow to me, but there is one that glows much dimmer than the other, the filament on that one probably didn't survive, will need replacing probably.
I measured HV : we get -560V or so !!!  :D  Of course it's a far cry from the required -2900V but it's A LOT compared to before (zero....), so it's encouraging to me !

So basically the oscillator is going, and the problem is the amplitude.
So I measured the DC voltage at the grid, pin 3/6, and we get -32V or so, instead of the advertised -50V ! That's cool. It's consistent : it's ballpark, just not quite there, just like HV level. So the low HV is probably indeed due to that low feedback voltage... so if we can fix that, HV might come back strong !  :D

Amplitude-wise, the oscillation measure as you can see from the scope close-up, at about 90Vpp. Frequency is 38kHz or so. So about 40kHz which is a lot less than the 60kHz advertised in the manual....

So, I made the same measurements on the working 502, to get a baseline.

Results : DC levels actually match what the schematic says. I get -52V instead of -50V at the grid, and +61,4V at the screen instead of +60V. You will agree it's fine...

As for waveform, the shape is 100% the same, so there is no problem there. Frequency ? Almost the same as well : 42kHz.  So 40kHz or so is normal, don't believe the 60kHz in the manual...
Amplitude ? Yes, it's indeed the culprit : I measured about 140Vpp versus 90Vpp on the sick scope.


So to sum it up, the oscillator is working, and the lack of HV is due to a low feedback voltage at the grid / pin 3&6, -30V+ instead of -50V.
So we need to investigate that.  The feedback branch is circled in red.  So we have at the source : the auxiliary primary winding, between pins 3 and 4, the one that measures at twice its normal resistance. Then we have R805, R806 and C805. I have already measured them and they are fine, and look fine physically.
So I fear that the problem is indeed that transformer !  :-BROKE
I will try replacing it but it's not going to be fun with all those unmarked wires in a tight space.. I am NOT looking forward to it.... but no choice I think !  :palm:

Stay tuned for the next episode next week-end...

« Last Edit: December 08, 2024, 05:01:36 pm by Vince »
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #135 on: December 08, 2024, 04:30:20 pm »
For tube types,
seems that number-letter-number types are generally so that first number part is heater voltage range.
Was that ever a standard?

Yes, this is the american stand, the first digit indeed specifies the heater voltage.
Don't know if the other digits and letters have any intrinsic meaning though.

European type denominations however, I struggle to see any logic to them.
They start with an 'E' so maybe it means 'E'uropean, who knows. But the rest of it, no idea.
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #136 on: December 08, 2024, 04:46:07 pm »
Geez that's the second spam message on this low volume topic in a short time ! What's going on.... looks like we need some kind of vetting from the moderators before we let "people"/ robots on board...

 

Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #137 on: December 08, 2024, 05:20:17 pm »
For tube types,
seems that number-letter-number types are generally so that first number part is heater voltage range.
Was that ever a standard?

Yes, this is the american stand, the first digit indeed specifies the heater voltage.
Don't know if the other digits and letters have any intrinsic meaning though.

European type denominations however, I struggle to see any logic to them.
They start with an 'E' so maybe it means 'E'uropean, who knows. But the rest of it, no idea.

American standards:  For "receiving tubes", the first number is the nominal heater voltage, e.g. 1, 6, 12, 35, or 50. 
Often (not reliably so), the late alpha letters (U, Y, Z, etc.) suggest rectifier diodes.
A series of metal octal tubes such as "6SJ7" uses "S" to indicate "single ended" construction, opposed to "6J7" for a similar tube with grid cap.
Military/industrial numbers range from 5500 upwards, e.g. 5642, 6146, 7586.  The number seems to be only in chronological order for registration.
Earlier transmitting tubes often had numbers in the 800 series, e.g. 807.
Another numbering system for power tubes, e.g. 4D32, has the first number indicate heater power, "4" means 20 to 50 W.

A good reference for American numbers is L Sibley Tube Lore, 1996.

European:  the first letter indicates heater voltage, "E" means 6.3 V.
The next letter(s) indicate type, e.g. "C" for triode or "F" for pentode.  ECC83 is a dual triode, equivalent to the American 12AX7.  Technically, it has a center-tapped heater useful at 6.3 V or 12.6 V.  The "83" is an arbitrary reference.
 
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #138 on: December 08, 2024, 05:29:51 pm »
Thanks for that !

So basically there is no strict standard that encompasses every tube type... but there are "patterns" for this or that subset of tubes. Better than nothing.

The European standard, the way you explained it, looks to be the most useful of the two after all !
Thanks for sharing, will make my life a bit easier for sure, every time I see one of those 'E' something tubes.
« Last Edit: December 08, 2024, 05:32:05 pm by Vince »
 

Offline TimFox

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #139 on: December 08, 2024, 05:35:31 pm »
Thanks for that !

So basically there is no strict standard that encompasses every tube type... but there are "patterns" for this or that subset of tubes. Better than nothing.

The European standards as you explained it, looks to be the most useful to me after all !
Thanks for sharing, will make my life a bit easier for sure.

Not "no strict standard":  rather, many standards.
The European numbers such as ECC83 are reasonable, but many European tubes use different standards.

For more information than you need, there are several editions of the classic Polish compendium  Mikolajczyk & Paszkowski Electronic Universal Vade-Mecum, Wydawnicktwa Naukowo-Techniczne, Warsaw  that is polylingual and includes Soviet tubes along with American and European tubes.
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #140 on: December 08, 2024, 05:47:41 pm »
Oh boy, sounds like I would be overwhelmed with 600 pages....

It's nice to know someone had a go at listing everything... just not sure I have time to read it all  :scared:

I guess it's best for now to stick to learning about the tubes I actually encounter in the scopes I work on...





 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #141 on: December 08, 2024, 07:05:27 pm »
the main winding; between pins 4 and 6, measures about 4.3R, and the smaller windings between pins and 4 and 3 is about 1.3R.

Well on the sick 502 do get a similar reading for the main winding, but the smaller one reads twice too high, at 3.4R instead of 1.7.

What could that be ?!  :-//


Original manual has numbered T801 pins, but one secondary winding is missing.
Other pictures have that missing winding, but no pin numbers.

502A manuals have T801 primary center tapped symmetrically, 502 manuals are different.
Later 502 has new trafo version, but picture is the old one.
Original manual is also missing pins 2 and 5, but has 6 twice.

Are both total resistances between 6 and 3 as they should?

If we assume that second 6 is actually 2 the primary side is missing 5.
Maybe primary is somehow incorrectly connected.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #142 on: December 08, 2024, 08:07:41 pm »
the main winding; between pins 4 and 6, measures about 4.3R, and the smaller windings between pins and 4 and 3 is about 1.3R.

Well on the sick 502 do get a similar reading for the main winding, but the smaller one reads twice too high, at 3.4R instead of 1.7.

What could that be ?!  :-//



Are both total resistances between 6 and 3 as they should?

No, the total resistance is higher oin the sick 502, by the exact same amount that the faulty winding is.

Working 502 or 502A : 4.4R + 1.7R = 6.1 total
Sick 502 : 4.4R + 3.4 = 7.8 total


Sorry about pin numbers, just realised that the late 502 schematic I posted does NOT show them !   :palm: When I wrote my message I was actually looking at my printed copy of the early manual, which does have pin numbers for the transformer.  I am getting tired of all these inconsistencies between manuals, it doesn't help communication...  >:(

If we assume that second 6 is actually 2 the primary side is missing 5.
Maybe primary is somehow incorrectly connected.

Nah that's not it, measurements are perfectly consistent as I said above.
Also, we know that the scope DID  manage to show a trace, 6 years ago when the seller tested it, then when I tested it upon receiving it, it briefly showed a trace before it died for good. So it's not like it's been messed with by someone in the past and it's incapable of displaying a trace.. no. It is capable of producing the proper HV.. it's just that it failed at some point, with some smoke along with it, according to the seller...
Also, I see zero sign of rework / "suspicious activity" in the CRT circuit. It all looks 100% factory to me.

 

Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #143 on: December 09, 2024, 10:57:25 am »
Then one possibility is that trafo connection is disconnected close to winding end and output wire.
The fact that after smoke has had a trace indicates that the disconnected point has a space to move.
Three serial windings where either of lower parts is originally shorted is not rational.
But two parts where lower part is originally two parallel windings has at least some rationality.
Just trying to save the trafo here.

If C847 itself is the cause of its bubbles it has loaded the first secondary winding.
But is it enough for overloading the primary, it's only a 200V thingy behind 1M5 ohms.
On the other hand if C847 is close to its limit even mild over voltage may have been too much for it.

E,
RCA '63 has that 6DT5 grid 2 max. is 285V and 315V for 6CZ5.
But that's history now.

Continuing with double winding, how it can happen.
First second winding is off and trafo can barely provide and HV is adjusted accordingly.
Then something happens and second winding becomes connected and HV goes so much higher that C847 is cooked.
Something else must be cooked also or situation goes back to "normal" when second winding disconnects again.

Can the thing be so on the edge that now unreg 485V is just so unreg that it can't provide anymore?
My impression is that these things are pretty robust.

BTW,
I was shuffling some tube manuals and when full wave rectifier came up I had to search how it operates.
No matter how twisted the resulted wiring is a cathode is pictured to be down, reason or not, but my head missed a center tap, even that the situation in hand is very center tapped, probably something from cathode being a source.
« Last Edit: December 10, 2024, 12:30:03 pm by m k »
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #144 on: December 14, 2024, 10:06:30 pm »
It's this time of the week again ! Ready for your weekly dose of 502 goodness ??

Today I worked on it :

1) Trying my best to NOT replace the transformer, I took some comparative measurements in the feedback loop of the oscillator, to see if maybe the low feedback voltage might be due to a failing C805 Sprague cap.



Although it tests fine with the capacitance meter, maybe it's defective under operation and drops the voltage that we are missing.
It didn't seem like so, but just in case I replaced it with its counterpart from a 502A donour : I see that Tek replaced the 400V rated Sprague electrolytic with... a more modern and confidence inspiring.... yellow FILM cap, rated at 630V at that, rather than 400V. So in it went !
"Of course" it solved nothing, but at least I felt more confident about that cap and could move on to other suspects... the transformer.

2) So I replaced that transformer... oh boy that was NOT fun at all, brrr.... first time I do that, and hopefully the last.
The one from the 502A donour is slightly different : the 502 uses, for each windings, not one but 2 or 3 super thin wires, which love to come off the eyelet/rivet, or break, when you desolder the wires going to them. Horrible. The 502A is different : it uses a single, thicker wire. Made my life easier, and less cursing.







The 502A transformer had one problem with it : the thick insulation on one of the HV wires was badly melted it seems, down to core wire almost !
No it was not me damaging it with the soldering iron, so it sure looks like it you will say ! I certainly don't remember any soldering accident, so I don't know. I guess the evidence is against me !  :-BROKE
Anyway, it was really sketchy so I put a piece of heat shrink tubing over it, for now. If you have a better solution please speak up.





Result ? Absolutely zero change, I still get the exact same voltage from the feedback winding : -30V instead of -50V, and still -500V at the secondaries.

I was.... totally devastated, ready to jump out the window ! Sadly my house is only ground floor so the likely outcome would not have been the desired one.

3) So out of despair I turned off the brain and just did whatever I could think of, just to see what would happen, and expecting absolutely nothing out of it... but I had to do SOMETHING... right ?!  :-\
So I replaced the 3 HV tube rectifiers since I most likely didn't do them much good when I heated them with 6.3V instead of 1.25V. I have a few NOS Mullard rectifiers so I used those.

Result ? It got WORSE : now the feedback voltage is down to -10V or so !!!  :palm:

4) So I went for broke : I reconnected the feedback path from the HV side of things : put the dual triode V814 back in place, removed my external +60V supply, and reconnected R800 to the +350V supply.

Result ? I turned the intensity control full right on the front panel and we DO have a trace now !!!!  :D :box: :-DMM

How so ? No idea. I guess I did something, or I didn't , and after so much fiddling with things, maybe there was a dodgy component somewhere that I unknowingly disturbed just enough to make it work again.... I will never know.


I measured the feedback voltage for the oscillator, grid pin #3, we now get -48V or something..... almost the specified -50V, and certainly much better than the -30V we were getting before !  I measured the HV and we get what we are supposed to : -3000V for the cathodes, and 10% more for the intensity circuit., -3250 or something, perfect. I am very happy with that to say the least...

So we now have a trace, that's marvelous... I can now nitpick and look at finer details... like... it's not normal that we have to turn the intensity to the max or almost, to get a trace. So I measured the voltage from the HV feedback this time, on grid / pin #1 of the pentode. The feedback voltage there is supposed to be +60V when things are working right, and we get... +135V , way too much !!!  :scared:
A minute later or so, I check again and this time I see "only" 80V ! MUCH better. Still quite higher than the expected 60V, but much better.. .at least it's ballpark.

So I check again the response of the intensity control and ...  it ads up : it's now much better, I only need to turn it  50%, centered (why does Firefox dictionary not recognise this word ?!  :o ) to get traces to appear !
I like that.

So I guess maybe we have a problem in the HV feedback path, that we will need to investigate later on to make things right and more reliable...
Maybe the chain of resistors that's used to take a sample of the HV, is faulty.
But for now, it's a huge win.


5) Then I adjusted the intensity balance control... it appears to work on this scope, unlike the other one. However it's extremely, extremely touchy, and takes a lot of back and forth between the trimmer, and the front panel to see the result. After 10 cycles of this game, I managed to get it okay-ish... almost good. Acceptable for now at least.
The 502A has this control relocated to the front panel, and it's also much less touchy in my memory. This improvement alone I reckon, is worth getting a 502A !  :-DD
.. but I love my vanilla 502 anyway.  I need both a 502 and a 502A in my life... why should I have to choose when I don't have to...



6) Now that I had activity on the CRT.. I gave the scope a  2 minutes test drive to check how the fundamentals were doing.
It's actually pretty good !

- Vertical : both the upper and lower beams appear to be fully functional  : they do show up, attenuators work, position controls work.

- Horizontal : we do have sweep, time base woks, trace spans the entire width of the screen. Position control is centered and can move the traces symmetrically about half a screen's worth both left and right.  Magnifier works. Trace brightness is adequate even with the time base at full speed  + x20 magnifier.

- Trigger : Triggers reliably, even when the signal amplitude is only one minor/small division. Lovely. Works fine in auto mode, free running good too.



However as you can see, we have some kind of a problem though : I am feeding the scope the square wave from its calibrator output, as per usual, and see how the trace does not display properly ? The "high" and "low" parts of the waveform actually "overlap" partially ! Weird... I will have to come up with an explanation for this... Also, inside the portion of the signal, where overlapping occurs, see how much vertical noise there is ! Insane...





Also, I slowed down the time base and noticed that the width of the overlapping section is CONSTANT. It's about half a division no matter what the sweep speed is. That might be a hint.
Look at that picture, where I slowed down the time base to the max, so I could take a picture.




However for now I am not too worried... I think we are at the point now, where it makes sense to wash the scope, and pour half a can of contact cleaner all around. Then if the symptoms persist we can start taking them more seriously, as actual failures, and start our investigations... what do you think ?!

So I think tomorrow I will wash the scope. It's a lot cleaner than the other 502 so I will be able to be more gentle with it...

« Last Edit: December 14, 2024, 11:41:38 pm by Vince »
 
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Online tautech

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #145 on: December 14, 2024, 11:41:48 pm »
In normal use Intensity should be ~50% and at minimum settings the trace should be just not quite visible.
The Intensity balance circuit is adjusted to implement this.....maybe values have drifted with age.


Trace jitter is a trigger issue.

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Offline m k

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #146 on: December 15, 2024, 10:23:19 am »
Or calibrator output.
The result is a norm for computer stuff and very visible for slower changes like serial/parallel port stuff.

Result ? It got WORSE : now the feedback voltage is down to -10V or so !!!  :palm:


You had that -10V earlier also.

Earlier starting point for V800 pin 1 was 185V, so something is different.
Maybe 135V was a heating time.
There is also that hygroscopic resistor thing.

So pin 1 went down and intensity control also.
Intensity is not part of the feedback.
Did you adjust HV down also?

Later 502 manual misses HV rectifier output R824 and R834 from early 502A manual.
6n8 capacitors are also 5n types, later 502 manual changes them as modifications.
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Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #147 on: December 15, 2024, 11:30:55 am »
Trace jitter is a trigger issue.

OK I think it does make sense indeed... will try to investigate in this direction.

I am a bit puzzled though, as I checked trigger pulses earlier and from my memory they looked just fine... will check again....
 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #148 on: December 15, 2024, 11:41:30 am »
Or calibrator output.

Yeah I will check that...

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 !  :-DD
Makes me feel better and worry less...

There is also that hygroscopic resistor thing.

Hmm yeah. That's an annoying one. I wonder if this issue can fix itself over time ?
I mean, I assume this issue comes from the scope sitting unused for decades in a row in a damp barn or garage ?
But now that the scope is in my comfy living room 24/7, moisture might come out on its own over time ? Especially as I use the scope which heats them up even further ?


Intensity is not part of the feedback.

Oh stupid me, your are right of course !  :palm:
That's excellent news then, means there is nothing with the generation of HV per se, so I should stop worrying and being paranoid about it ! :D
So I  can instead concentrate on the more simple matter of the intensity circuit, which is much simpler. There is hope then !  8)


Did you adjust HV down also?

Nope, I didn't touch this because the voltages on all rectifiers were conform to what was expected, so there was no incentive to touch anything.


Later 502 manual misses HV rectifier output R824 and R834 from early 502A manual.

Oh yeah indeed !  Guess what ? Yes, my 502 DOES have these resistors !  :palm:
So yet another example that show that even the "late" 502 manual is not "late" enough for MY 502 ! LOL
It's much closer to the early 502A in so many accounts... I should start using this manual as my primary reference really !  ::)

 

Offline VinceTopic starter

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Re: (classic) Tektronix 502 scope repair / restoration
« Reply #149 on: December 15, 2024, 04:48:16 pm »
OK some more...

1) As MK suggested I scoped the output of the 502 calibrator to make the jitter was not coming from there. It's not. The output is exquisite.

2) Then next in the chain of events, as Tautech suggested I started by investigating the trigger section... no worries there either.

3) So next in line is the sweep generator, maybe somehow it's him generating the jitter. Too far fetched ? Maybe but well, when you don't know the answer, you have to test things methodically, one by one, until you do find the answer... right ?
So I removed the input signal, set the scope to external trigger to be 100% the sweep generator would not receive any trigger pulse, then set the scope to free-run. But of course when free-running we can't see a signal, just straight lines.... or maybe not. When I scoped the input of the sweep generator, I measured the frequency of the free-run. It was about 330Hz. So, I set my function generator for a 330Hz square wave and sent that to the 502.
Then I set the function gen resolution to 0.01Hz and fine tuned the frequency so that it matches the free-run as good as possible... this way I was able to observe the square wave even in free-run. Of course I could never make it perfectly stable, but stable enough that I could observe it and take a picture. See :



Yes... we still have 100% the exact same issue !

So now we can be 100% sure that the problem is neither the signal source, nor anything to do with triggering.

But is it really the sweep generator ?

4) To determine that... did the simplest of experiments...
I got out of the free-run mode, yet still no input signal, zero trigger pulses. So I had no sweep whatsoever.
Indeed, nothing to be seen on the CRT... or is it ? I turned the intensity control to the max and oh, look at that !



YES ! Even no input signal and withOUT sweep, we still have the exact same issue !
So that means we have narrowed it down to... the horizontal amplifier.
So I will investigate that. From past experience with my Tek 575 curve tracer which also did weird and wonderful things on the CRT, it could be as simple as bad connections at the CRT neck. So will check that as well.


Week-end is almost over, not sure I will have time to work on it this evening, might be another day or next week-end.

Stay tuned anyway !  >:D

 


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