(classic) Tektronix 502 scope repair / restoration

[Vince]

Hi there,

I thought I would document the restoration of my Tektronix type 502 because... well why not.
I am very fond of it and it's a rare and special scope technically, and not many restoration threads on here about old hollow-state Tek scopes I find.

So I volunteer !   

Here is the page about it on Tek Wiki : https://w140.com/tekwiki/wiki/502

No plug-in, it's the monolithic kind. Dual beam CRT, two channels, high sensitivity inputs, differential inputs, low B/W, XY mode available.

It all started in October of 2018 when I got my first 502 :

The seller said he managed to get it to display the square wave from the calibrator output, but that after a few minutes the scope started smoking (but would still display the trace just fine nonetheless), so he swiftly pulled the power plug before anything worse happened. Good on him.
When I received the scope I gave it a quick test ride : scope powered up, no smoke, and no trace on the CRT. After a few minutes eventually a trace showed up... disappeared only a few seconds later, never to reappear ever again....
I left it at that and never worked on it since then. Don't remember even cleaning it nor even popping the hood to have a look inside. So it's pretty much a virgin.

Fast forward exactly 6 years later... this month I find another 502 for sale ! Yes they are rare over here, 6 years to find the next one ! 
It was clearly a barn find.. well an attic find in this case to be more precise, so looked extremely rough on the single pic the seller had posted.   The seller was upfront about it, didn't try to invent some story. The description was short, I quote : " Old scope, found in an attic. Untested, as is, local pick up only".
So for all I knew, scope may well be void of all of its tubes and whatever other part, full of corrosion and mouse poop. I had to assume the worst. However I didn't care because the intent was to use it for parts to help me fix my first 502, and I got it for cheap, only 50 Euros.
Cost me 80 more Euros to get it to cross the country to meet his new owner though... Seller would not ship it of course, as per usual, but I found this website in France, www.cocolis.fr , where people who travel for XY reason, for work or hols say, offer to pick up objects along the way, and deliver them to you. Helps them pay for travel expenses, it's win-win.  I got lucky and within only 2 days I found someone who departed from the sellers' town, and was heading my way (and beyond). Incredible. Cost me 80 Euros as I said, but well, it was worth it to me.

The scope as you can see is... well it sure looks like an attic find indeed ! As you can see the top of the cabinet looks beyond saving, I thought the covers would need repainting. The power cord is... well, previous owner soldered a very skinny two wire cord to the prongs of the original American socket, super safe and tidy eh ?!     Also, the removable part of the fuse holder is missing, how practical...

Some piccies. First one are the two 502 side by side. My first one is on the left. The "new" one, the attic find is on the right, the one with the green CRT filter. Then some pics of the attic find, so you can see how lovely it looks... 

[Vince]

That attic find was so gross that it was not even fit for storage (especially since the "storage" room for now... is also my living room !).
So I decided to give it a quick clean just for hygiene purposes. However, I thought well, if I clean it, might as well try my best and see how good it might come out, knowing the paint work was beyond saving anyway.

Well, the "quick" clean ended up lasting 2 and a half hours !  Boy that thing was so gross, I hit every part of it THREE times with detergent, and a stiff carpet brush to work the product into the fake "grain" of the covers.... let me tell you the result was not a pretty sight ! What came out of every square centimetre of this scope was nothing but a thick dark brown juice ! I can only assume the previous owner was a heavy, heavy freaking smoker !!!   

HOWEVER , look at the end result !!!
This scope came back to like ! It's not just clean, it actually looks really nice to boot !
The paint, even on the top side, was in fact savable, incredible !
It even still has its original light and glossy blue which most of my other Tek scopes have long lost !

Oh look at that thing, a thing of beauty ! I have never had a more incredible, more satisfying before/after experience when cleaning one of these old Tek scopes !  It is completely transformed ! 

Which brings me to the point of this thread (finally !    --> this scope now looks so good, there is no way I am using it for parts to fix the other one ! It obviously deserves to be restored as well ! Well unless of course there is a show stopper hiding inside... haven't looked in there just yet... one thing at a time 

So there I am.. switched from just a quick clean before putting it into storage to... embarking into its restoration, what a change of plan !

Am starting with the exterior... replaced the missing fuse holder piece, and got rid of that crappy power cord and tidied up the original socket.

[Vince]

...

[the Chris]

Wow, that cleaning marathon was well invested time. It also speaks highly for the materials and paints that were used back in those days. Good luck with the internals, hopefully they survived the attic conditions in a state that makes repair a sensible option.

[med6753]

Wow, that cleaning marathon was well invested time. It also speaks highly for the materials and paints that were used back in those days. Good luck with the internals, hopefully they survived the attic conditions in a state that makes repair a sensible option.

There's 3 conditions where restoring a 500 series scope isn't worth the effort:

1. It was stored in an extremely damp environment and the aluminum frame is corroded.

2. The chassis is bent/damaged to an extreme or it was left open for rodents to get in there and piss all over it or chew the wiring.

3. Some clown robbed all the vacuum tubes, unless you have a good tube stock.

I've restored 500 series that were full of sawdust and quarter inch thick dust.   

[Vince]

OK !

Fasten your seat belt, for I have taken the plunge inside this thing, here is the report ! 

In short : very happy, no disaster in there. No massive corrosion nor mildew, no missing or broken part, it's perfectly restorable if that's a word !

As often in these scopes, gravity being what it is... the bottom side of the decks is pretty much like new, no exception here 

- Top side of the upper deck : light dust, looking very good. CRT shroud does need a good clean though.

- Side panel, holding the XY switch. This one is of course accessible from the outside, so it was hit big time with the smoke, looking gross.

- Top side of the lower deck : as always this the part of the scope that's looking the worst, no exception here but nothing that can't be clean.
Some areas of the deck are really gross, I hate smokers. Looking completely black and thick, disgusting... but it should clean fine I think so not too worried.

That's for the cleanliness part of it.

Now for the quick visual inspection, here are the things that caught my eyes :

- Date code : found a can cap with a date code of 52/1960, so I guess the scope was made in very early 1961.

- Serial Number is 004356

- Signs of rework on some pins of some of the can caps...

- Cooling fan : looks too nice to be true, zero corrosion, and the rubber mounts are not as aged and worn out as they should be (also, they are GREEN, how weird... never seen anything other black mounts so far...). Must have been replaced at some point. The sticker on it bears what might look like a date code, if it is then it means it was made in 1974. Now whether it was actually replaced in 1974, or replaced in 2015 with a 1974 salvaged part from some other scope, who knows. But it was definitely replaced... for I can see obvious signs of rework on the solder joints for the fan wires on a small ceramic strip underneath the scope ! Looks ugly too !    Will need to clean that up.

- A couple tubes are missing their metal shield.

- One of the two big 5AR4 rectifier tubes in the power supply section, is clearly dead : inside, the top part of the glass is covered in that white stuff you get when the glass leaks its vacuum...   I looked in my tube database, somehow this particular rectifier tube was used ONLY in the 502/A scopes ! 
Glad I have several of these scopes to steal parts from, I knew it would come in handy one day ! 

- The power switch, which is part of the graticule illumination pot, is seized... it just won't budge. However all the other switches pots and knobs are all smooth as butter, a joy. Weird to have such a contrast.
EDIT : if you apply brute force, it DOES move (a bit). I tried my chance with contact cleaner, and to my surprise it did wonders in just a few seconds : switch/pot now as smooth as all the other ones, one problem fixed already ! 

- It's old enough to have a Selenium rectifier in there. It's used for the low voltage, low power supply, fully solid-state, that powers the heaters of the two 6AU6 pre-amp of the vertical amps, with DC current. I guess that's to reduce induced noise on this very sensitive front-end (200uV/DIV, and even better, 100uV on the 502A). I am wary of these ancient Selenium things, so I did a quick test in situ : good I did : both diodes in it are shorted both ways...  I hope the transformer winding feeding them, survived...  will check for that later of course. I can just replace it with modern silicon diodes... don't even need to add series resistors as it's a low voltage low power. Tektronix did do their own silicon diode conversion kit to retrofit older scopes like mine, and they didn't add any resistors IIRC. Sadly I don't have the specs of that Selenium rectifier (nor Tek's silicon replacements). It's an SR642 and Google couldn't find me a datasheet for those. But it's not too difficult to make a good guess based on the schematic. It only needs to feed the current for the heater of two 6AU6. This is a known quantity, a few hundred mA, will check the exact value. It's much less than one amp total that's for sure. Voltage-wise.. it's a 6V supply and the secondary windings are only 9.5V RMS, so it won't be too difficult to find a diode that can handle that. However there will be a big surge current due to the big 4,000uF filter cap, so need a diode that can reliably sustain that kind of "event". An old engineer friend of mine just advised me today against the cheap 1N400X series, as they are known to be quite fragile in this regard, apparently.
Or, I could look at the schematics of my other classic Tek scopes of the era, and find those that do have silicon rectifiers from the get go, and lookup their datasheet to help me find an equivalent. Or I just put whatever beefy diode I have in my stock of salvaged parts...

Now for some piccies...

[Vince]

OK so here are some :

- The side panel looking gross
- Upper part of the scope : light dust on the deck, and the CRT shroud that needs a good clean
- Under side / power supply, clean
- Right side
- Left side
- bottom side of upper deck : clean.

[Vince]

Now some close-ups.

- Power Supply tubes : one can easily spot the big rectifier tube that failed snow style.
- Can caps : signs of rework on some solder joints and bare wires.
- Replaced cooling fan : looking too nice to be true, its weird looking green rubber mounts + pic of the matching solder job on the ceramic strip underneath the deck

[Vince]

One can just download the service manual from Tekwiki but I thought I would make it easier for you... here is the schematic for the power supply that uses that selenium rectifier.

[Vince]

OK just before I go to bed... I thought... if Tek made a retrofit kit with silicon diodes for later 502 scopes then surely, the 502A scopes, which came later obviously, must have had silicon diodes from the get go. So let's just look at the 502A schematic then !

See below. As you can see, luckily they didn't reinvent the wheel. The schematic is 100% the same as the 502, and they didn't indeed add series resistors with the rectifiers. Just a direct swap.
SR642 has now turned into D642A and B. Looked them up in the parts list, they are 1N2862. Can't find a datasheet for them, too old.

Did find some basic info on this website :

https://www.web-bcs.com/diode/dc/1n/1N2862.php?lan=en

It says they are rated for only 400mA, even less than I thought.

It also gives a representation of the package. I sure recognise it, I saw this type many times in the power supplies of my other Tek scopes. It's a quite beefy metallic package. I could pinch one from one of my parts scopes for sure.

Google also found me an old scanned book from 1975 which gives substitutes for diodes. They cross ref that diode to a..... 1N4004. Yes, precisely the one I was advised against ! 
The 1N4004 comes in a tiny plastic package, certainly not as beefy as the metal can of the 2N2968... so I think I will play it safe and get a real one of those from one of my other scopes. It will also look more genuine/period than modern looking crappy plastic bodied 1N400X... won't it.

[Vince]

Hmmm...I looked at the manual for a few of other scopes, and then realised it may not be a good idea to blindly steal a diode from them because... the filter cap in all of them is much smaller than the 502 : 125uF, 300uF tops, compared to the 4,000uF in the 502 ! So can't be 100% sure that they will handle the surge current. Can't take the risk.
But stupid me... I can just steal the diode from one of my x3 502A scopes, this way I am 100% sure the diode will be the right ones, whatever they are  !
I have already started working/restoring my first two 502A, but the third and last one is really very rough, so I will declare it my official donour from now on... sorry little guy ! 

OK can't wait to give that a go.... later. 23h20 here, time to get some sleep...   

[med6753]

Vince...yes, get rid of the selenium rectifiers. And the 1N4004's will handle the surge current just fine although as you stated won't look original. But to me that's not important. Function over original looks.

[Vince]

OK I just worked a little bit on the scope....

- I looked at the diodes in the 502A, and unlike what that website suggests, they aren't the beefy metal case at all, they are just humble looking, skinny plastic jobbies !
So yes, Papa smurf was probably right and a 1N4004 might have done the job.

- I desoldered all 3 wires from the Selenium rectifier, so I could test it 100% reliably : glad I did because.... actually they aren't shorted at all, they are perfectly fine !    Both diodes in it drop about 430mV

- That allowed me to also test the transformer secondary winding, to see if it was burned / open circuit : well what do you know, it's not ! It measured real low, barely one ohms, but still, not a short at all.
I guess the low resistance is normal given how low a voltage it outputs, so not too worried about it. I can always compare the measurement with the 502A donour anyway.


So.... what the hell is this magic trick ? What's going on here ?! 

Looking at the schematic, the  filter cap / Can cap, because of the winding that's almost a short... is pretty much wired in // with the rectifier.... so a shorted cap would explain it all.

So, I pulled the positive wire of that cap, to isolate it and tested it : will it be shorted ? Not at all ! It's high impedance, and as far as capacitance it measures at about 5,000uF which is well within spec : 4,000uF -20/+50%

So the lesson is... don't assume anything, just be methodical, and test, test...

SO the good news of course is that the transformer is fine, PHEW !!! 
The rectifiers and big, huge can cap as well, great as well.

So now, I need to scratch my head, and look at that schematic again to find what other path might lead to shorting the rectifiers.

So starting from the top of the schematic, the cathodes of the rectifier, we have two possible "entry" points for a short : the emitter of Q647, and then those two 15ohms resistors in //
Then for the return path, to the anodes of the rectifiers, we have C635 and the emitter of Q644.

Need to do some more testing, stay tuned.... place your bets ! The winner gets a virtual cookie.

[Vince]

OK, some more :

I measured the short accross that power supply.... it's not a dead short, it shows 9.5 ohms to be exact.

So as a quick sanity check I measured it on the 502A donour.. guess what..... yes, also "shorted", and also 9.5 ohms precisely !!

What are the odds that this other scope also has a shorted such supply ? Not much. And that the "short" is exactly the same value, down to the tenth of an ohm ? You get the picture.

So I started to think I was chasing a red herring and that something simply escaped me, because not so clever, I failed my B.Eng for a reason I guess 
No, it was a different reason actually... it was complicated.

Anyway. So I decided to try my chance and I powered up the scope, using the dim bulb tester of course.
The latter instantly lit up super bright and solid. Very.... solid.

Then next idea was to isolate all power supplies from the transformer : I disconnected once again the Selenium rectifier, and also pulled all the rectifier tubes from the other supplies. 3 such tubes.

Bulb tester still lights up hmmm...

At this point, all that's left connected to the secondary of the transformers are all the tube heaters. There are x6 6.3V windings that power the heaters. So that means I would have a shorted heater ? That does not compute tome does it ? How could a filament, be it a heater or a light bulb, be "shorted"... it's already a short when cold. Either it's alive and heating, or it's burnt and open circuit no ?

So, I think it looks like there is nothing wrong on the secondary side of this transformer, at all.
So, the primary might be shorted ? Doesn't look like so : it measures at 5 to 6 ohms, just like my other Tek scopes. It's what they are... 6ohms or so.

So I am starting to think that scope has no short anywhere, it's just fine, and it's just the bulb tester playing tricks on me because...because as always the bulb in it is not powerful enough relative to the power of the scope. Bulb is only 77 Watts. I don't have a bigger one.

I bet if I plugged the scope with the dim bulb tester it would work just fine ! I mean, it would explode nor catch fire at the least.

But still, I don't feel quite cheeky enough to do that... I like this scope a lot and it's too rare and special to me, I don't want to take the risk to blow it up...

So I think I will try to find a more powerful bulb, or find more bulbs and put them in parallel,  or/and buy at long last a freaking variac...... if you know of a brand that offers decent value for money.... up to 100 Euros tops please... if it all possible.

At that price it will probably be a Chinese one but hey, I am not a millionaire...

23h23 here, good night.... 

[jdragoset]

T 801 and V 800 (under HV box) need careful attention!

[Vince]

Hi,

What kind of attention do you mean ? What should I be looking for, doing or not doing ?

[squadchannel]

Perhaps @jdragoset is referring to the moisture absorption issue with HV transformers.
The HV transformers used by 500 series have a problem with the epoxy used absorbing moisture and degrading the insulation.
My (not at all worked on) 515A has it as well. Fortunately, a new one was available in Italy, so I will use that.

[Vince]

Ah OK. Well in this case there is not much I can do about it. Being careful will not do anything to that transformer.
It might kill the oscillator tube but if so, it's most likely already dead.

I am surprised you had this issue with your 515, since to my knowledge, the problem affects only the 547 as its transformer was the only one that used epoxy.
Never heard this issue talked about on any model other than the 547.

Anyway, even if the 502 were using epoxy, there is not much I can do about it. All I can is power the scope up and see how the transformer and oscillator tube are doing / operating.

But, first we must power up this scope..... then asses the situation. Diagnosis will be simple enough : if we get a trace, and a bight one, and it doesn't die after a few seconds or minute or an hour... then de facto the transformer and oscillator tube are fine (enough) to get the job done.

[factory]

This 502 must have been made sometime between 1959 & 1963 (I see the extra control for the later 502 version), did they even use epoxy then? My 502A HV transformer looks to be wax coated, would have thought paper capacitors on the secondary would be more of a problem.

Thankfully it only uses 3 vacuum filled transistor bulbs (they are listed with the tubes), that's an improvement over the later 502A.

P.S. Variacs offer no current limiting, or visual indication of a sudden or gradual short, the lamp limiter of course does (dependant on bulb used), I prefer lamp limiters myself for this reason. I now have a selection of bulbs from 8W up to 375W (photo flood special).

David

[Vince]

This 502 must have been made sometime between 1959 & 1963

Yes indeed. As I said earlier, I checked the date code on a can cap and it read last week of 1960, so the scope must have rolled off the production line in very early 1961 I guess...

(I see the extra control for the later 502 version)

Didn't even know there were different versions of the 502 ! What control is it that they added on mine ?

P.S. Variacs offer no current limiting, or visual indication of a sudden or gradual short, the lamp limiter of course does (dependant on bulb used), I prefer lamp limiters myself for this reason. I now have a selection of bulbs from 8W up to 375W (photo flood special).

Too bad, I just bought one and received it this evening !   

It's one more thing in my arsenal.  Of course it's a cheap(ish) chinese one. I had a quick look for used one, but found only 3 or 4 in France and all were expensive low power ones, as is, and local pick-up only. So not much choice, chinese it had to be.
For less than 100 Euros, shipped, delivered in just 36 hours, I picked the big 3kVA model, being wary of chinese anything-rating of course !
Weighs 10kg. Hopefully it should be able to power my most hungry Tek scope for a few minutes without catching fire. That would be my 585 rated at 710W and 890VA ! 

But in //,  I also explored the dim bulb tester route : I searched for more powerful light bulbs to replace my 77W one. They aren't sold in Europe anymore, so I searched for used one. Found some, but just out of curiosity I checked on the web and what do you know... the ban on light bulbs appears to apply only to "physical" stores ! On the web it took only a fdew clicks to find on-line merchants selling you every light bulb you may wish were ! Talk about hypocrisy ! 
So I bought x10 pack of 150W bulbs ! ... and received them today as well, like the variac, what a coincidence, but what a blessing : in time for me to play with the scope this week-end !!! 

So I left the variac alone for now, and kept going with my dim bulb tester, replacing my 77W bulb with a 150W one. It's better, internal voltages rose up, but the bulb still lights up bright and solid. Bummer !  Need a solution, and fast, right here right now... browsed the website of my local H/W store, discovered they had something I didn't even know existed : an E27 socket "splitter" : plugs into one E27 socket, and gives you TWO in return ! I bought two of them...
They are rated at only 100W but who cares... if they start glowing red, melting and then smoke and catch fire... I will be there watching them anyway, so I can just pull the plug and move on to some other solution...

So I put one splitter, and two 150W bulbs. It's better still, less bright, but still quite bright.  300W total, which is precisely the advertised power rating of this scope BTW.

So I added a THIRD bulb, now 450W total, by cascading two splitters ! Yeah... sketchy as hell but no big deal : if they light up bright hence might melt the splitters, I would just be pull the plug immediately anyway since the scope is going nowhere. The only circumstance where I might leave them powered up for longer, a few minutes, an hours top, would be if the scope is walknig up, in which case the bulbs would be very dim hence drawing very little power hence no risk of overheating the splitters. So basically it's a non-issue eh ? And it's just temporary of course.

Anyway, so with 3 bulbs, at last things start to change : they are now very dim and at last starting to see signs of life on the CRT, which is encouraging for sure !!!    I could see a bright spot for the upper beam, which I could move up and down and left to right, but no lower beam to be seen, and no sweep.
So not quite there. But the scope was clearly not shorted anywhere by the time I got to that point so... I took the plunge and got rid of the tester, gave the scope full power ! .... Scope is now alive for good ! I now get sweep and the lower beam now shows up ! It basically works ! 

However after a few minutes the traces disappeared ! Bad transformer ? Don't think though. I 100% agree with Dave. The epoxy transformer only hit the 547.
Also a fading trace is extremely common in these scopes, any of them this time, just due as Dave said to failing caps in the CRT circuit. I have seen that many times in my own Tek scopes. It's routine as far as I am concerned, I consider it almost a given on any scope I buy...
So, not worried at all at this stage.

However I will tell you what worries me much more !

Now that the scope was clearly alive, I immediately proceeded to check all the power rails, as you do :

- The selenium supply, that lower power -6.2V rail... it is SPOT ON at -6.2V  ! So clearly the selenium rectifiers are still good enough. Might replace them anyway later on, but for now they are doing their job just fine, so it's good enough to keep going through the basic / early stages of this scopes's resurrection.

- The -150V, the reference rail, is pretty good at -148.5V ! For an attic find, not complaining ! Ripple ? I did a sanity check with my DMM in AC mode, it's at 0.17V or something. I guess one needs to multiply that figure by about 3 to get an idea of the peak to peak value, which would then be about 500mV, which is 100 times more than the specified 5mVpp in the manual, but still good enough to carry on debugging this scope. Cap is tired and will be replaced, but it's not the priority at the minute.

- The +100V rail is pretty nice at 101.8V and only 7mV ripple RMS, so about 20mVpp which is "only" double the spec (10mVpp). So plenty good enough for now.

- Now I am about to tell you what is REALLY of IMMEDIATE concern my friends !!!
The +350V supply sits at +460V !!!! Yes you read that right, no typo !!
Oh boy as soon as I saw that, I rushed to power off the scope before half of it gets fried ! 

465V is the unregulated power that the regulated 350V is derived from, using two pass triodes wired in // , see schematic below. So that means one or both of these tubes would be shorted ? I can't compute how a vacuum tube can be shorted ? There is freaking vacuum between the anode and cathode FFS ! 
Maybe scope got dropped and that caused an internal mechanical damage inside the tubes, causing the anode and triode elements to contact each other ?!
I don't know ! 

But sure enough this is my one and only concern right now ! I must fix this supply before I do anything else to this scope...

OK so to sum it up... all the fundamentals look good, this scope basically works. Just need to fix that power supply then go for round two of the diagnostics, and look at things in finer detail, check every feature and check for calibration. But it basically works so I am happy. It's definitely worth restoring that's for sure ! 

I am planning on building a better dim bulb tester of course, with more real estate so I can fit 4 or even 6 individual bulb sockets. 6x150W = 900, should be enough to wake up my 585 scope before I risk plugging it in straight to the mains. I would put individual switch to select which bulb I want to wire in circuit, and maybe add a voltmeter and an ammeter, or at least 4mm posts so I can plug a couple DMM in there to measure that.


EDIT : oh, I see there is a resistor wired in  //  with the pass triodes, typical it is at that (to do the bulk of the work and lower the power requirements on the pass triodes, IIRC), I should have thought about it. Maybe it's fried and shorted. Will check for that in a minute...

[Vince]

OK, I am already back.... took only a few minutes to the following sanity checks, with the scope powered off :

That ballast resistor... is not fried at all. It's a very big WW resistor bolted to the underneath of the deck. It's in perfect shape and measures spot on. So that's not it.

That resistor being in // with the two pass triodes, therefore means that the latter are also not shorted (I measured directly on the tube sockets to be 100% sure, in case there was a wiring issue somewhere).
Indeed these triodes are not shorted.


So, that means the problem is not in the pass elements, but rather in the behaviour of the circuitry as a whole... the regulation just is not regulating anything.
So I have some finer troubleshooting to do, it becomes more interesting, hooray ! 

Stay tuned...

[factory]

This 502 must have been made sometime between 1959 & 1963

Yes indeed. As I said earlier, I checked the date code on a can cap and it read last week of 1960, so the scope must have rolled off the production line in very early 1961 I guess...

Guess I should have re-read this thread before making that comment.

(I see the extra control for the later 502 version)

Didn't even know there were different versions of the 502 ! What control is it that they added on mine ?

After the comments about the HV section, I had a read of the Tekwiki page, it's there I read about the differences between the early production 502 and the rest. They added another secondary winding & HV rectifier for the later 502, the wiki describes it better than I could.

From Tekwiki The early 502 uses a single supply for the upper beam and lower beam CRT cathodes (HV transformer 120-114). Later models (1959+, see 070-090) and the 502A have separate supplies for the two CRT cathodes (HV transformer 120-150). This improvement allows slight differences in horizontal CRT sensitivity between the two beams to be cancelled out in step 8 of the calibration procedure.

Here is a picture from the later 502 manual, it seems Tek were just as bad as HP at not updating the internal pictures to match the circuit changes. Compare with your scope, it's not hard to miss.

- Now I am about to tell you what is REALLY of IMMEDIATE concern my friends !!!
The +350V supply sits at +460V !!!! Yes you read that right, no typo !!
Oh boy as soon as I saw that, I rushed to power off the scope before half of it gets fried ! 

465V is the unregulated power that the regulated 350V is derived from, using two pass triodes wired in // , see schematic below. So that means one or both of these tubes would be shorted ? I can't compute how a vacuum tube can be shorted ? There is freaking vacuum between the anode and cathode FFS ! 

Probably the +350V regulator circuit is not working & the pass tubes are fully turned on, not good for the pass tubes or rectifier, which may red plate if left drawing too much current in this state.
I'm sure your checking for open/drifted resistors & the one 0.01uF capacitor in this circuit isn't leaky, as I'm typing this, V746 might be bad too.

P.S. The variac is still a useful addition for other uses, just not the first thing I grab for bringing stuff up, I didn't think you would find anything new that isn't Chinese, I think they have killed off the well known brands that were made here and probably others. Don't forget to give it a safety check for missing ground connections to the case and wring quality, fuses actually in circuit etc.

David

[m k]

they are 1N2862. Can't find a datasheet for them, too old.

For diode and stuff.

1972 Motorola Semiconductor Data Library Reference Volume
https://archive.org/details/bitsavers_motoroladaSemiconductorDataLibraryReferenceVolume_51803030

You can also use bulb and variac, and measure voltage over bulb.

Incandescent restriction is for regular stuff only, you can still get others.

[Vince]

Thanks David for the info and thanks MK for the datasheet ! 

I have already fixed that 350V power supply, got lucky ! 

I started by checking all the resistors in situ. The ones that could be measured reliably read OK, as for the others I don't know but I didn't find any that measured something unplausible.
So I just tried my chance and replaced V746, the tube that does the regulation. That didn't fix it.

Shortly after I found the actual problem ! See pictures below.

It was not easy to spot at first, because it's quite dark in there due to the scope laying on its side and the overhead lighting of the bench is therefore pretty useless.

The issue is this bare wire indicated in yellow. It connects the unregulated 485V coming from the cathode of the rectifier tube, to the anode of V747, one of the pass triodes, on pin 9. Doing so, it is routed above a couple unused pins of the tube socket as you can see, namely pins 8 and 7, being close to them. Well, when I looked real close at those pins, with good lighting from a torch, and at the right angle... yes, it became evident that this wire was touching pin 7. Still it's unused right ? No, look at the schematic... it's actually, internally connected to pin 2, which is the GRID ! So yes, that pass triode was turned on REAL HARD because of that, problem found ! I gently pulled the wire a little away from that pin, powered up the scope, and hey presto it's fixed indeed : that supply now delivers the expected +350V ... to be precise, we get 357.6V which is not bad is it, and ripple is 3mV RMS so about 10mVpp which is spot on the specified ripple in the manual ! 

So we are all good now, the fundamentals of the scope are good, we can now leave it powered up for as long as we want and start troubleshooting the finer details : round one is a victory ! Round 2 now starts !

OK so... the traces on the CRT still disappear shortly after power up, problem is still there... so that will be my next job.
I will go straight at the HV caps in the oscillator circuit as it's such a recurrent issue on these Tek scopes.
10 PM here, getting tired.... I have a safety rule to never work on my Tek scopes too late when I am tired... a bad move could be fatal when probing in there, not worth it...
I only work on them when I feel "fresh" and alert.

[George Edmonds]

Hi Vince

As you appear to have fixed one problem I will concentrate on you following portion of your posting only

OK so... the traces on the CRT still disappear shortly after power up, problem is still there... so that will be my next job.
I will go straight at the HV caps in the oscillator circuit as it's such a recurrent issue on these Tek scopes.

If I understand you correctly you are saying that following initial turn on of the scope you get a normal intensity display which then over time gradually fades away.  Although you make a valid point regarding the HV caps I suspect that they are not the problem.

If you turn the scope off and then back on again after say within one minute has the display returned?  If this is the case it is most probably not the HV caps but is most probably being caused by the resistors in the Intensity control circuitry or possibly the Focus circuitry.

Three is a little appreciated problem with older types of resistors, although sometime seen now days with the resistors in the primary side of SMPU’s, in that the go high in value when a voltage is applied across them, this change in value can take from seconds to up to say 20 minutes.  It normally does not occur with resistors below say 50K but increases as the resistor values increase.

With your scope you have resistors up to 15M present and voltages up to some 3KV there is every reason to suspect the resistors of changing value over time with applied voltage.  This used to be a common problem with older type valve/tube communication receivers where they gradually lost volume as the screen resistors went high in value.

It is almost impossible for you to check your resistors for this effect as theyA will only do it with a high applied voltage.  Fortunately Vishay in their VR series, for example, still provide suitable high value/high working voltage resistors.  So my best advice is start changing all of the resistors in the intensity and focus chains.

Hope this helps

G Edmonds         

[Vince]

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 !    I let it run for like 20 minutes and the two traces were still there, bright and sharp ! 

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 ! 

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 ! 
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 !  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.

[Vince]

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 ! 

[factory]

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

[m k]

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.

[Vince]

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 ?!    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 ! 
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...

[factory]

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

[George Edmonds]

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

[George Edmonds]

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

[Vince]

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 ! 
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 ! 

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 ! 

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 
.. 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 ! 

[factory]

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

[Vince]

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.

[Vince]

Some close-ups of the CRT stuff

[Vince]

Some close-ups of the upper deck.

[Vince]

Some close-ups of the lower deck, as always in these scopes, the worst part of the scope....

[med6753]

Does your restoration include replacement of the power supply capacitors?

[Vince]

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.

[Vince]

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 !     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 ! 


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.

[Vince]

Some close-ups of the "damages" / minor issues listed above.

[Robert763]

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

[m k]

My pozi tips are pretty pristine.
Here it's practically '+' on not.

[TimFox]

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.

[Vince]

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.

[Vince]

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 !    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.... 

- 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...

[Vince]

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...

[factory]

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

[George Edmonds]

Hi Vince

It is important to maintain the -6.2V supply at -6.2.  The front end tubes/valves have a design heater voltage of 6.3VAC so 6.2VDC is right.

This power supply is only partially regulated as R646 and R647 are in parallel with V647 (the regulator transistor)

If you use Si transistors change the value of R648 to adjust the supply back to -6.2V.

G Edmonds

[Vince]

OK, noted.... no Silicon then...

[Vince]

I thought you had a 502A spares unit, doesn't it also use those trannystors?

Oh stupid me, yes of course ! 
Pinched both trannies front the 502A and... same result ! Strange... needs more digging. 

I will keep those transistors though, as they don't use the same package as the 502.
They are more modern looking TO39, with much longer pins as well. They fit perfectly into the sockets, no need to bend the pins.
The component tester confirms they are Germanium as well, no worries.
Not sure they are 2N214 though, despite what the manual states.
Their packages are extremely crusty so can't read their part number. On one of them I think I can about guess the digit '8'...

OK so more investigation is required then...

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

Wow, sounds pretty bad they way you talk about it !    ... like it stayed outside in the British weather for the past 40 years.
I think if you showed us pictures of it I might faint....
I guess you will keep it for parts rather than attempt a restoration ?

[George Edmonds]

Hi Vince

Commonly used replacements would be 2N404 and 2N1304 as they are both TO5 devices

G Edmonds

[Vince]

TO5 ?! Didn't know that one... just googled it, found an interesting page about it on Wikipedia...

https://en.wikipedia.org/wiki/TO-5

So it looks just like a T039, and predated it. Apparently even back in the day manufacturer themselves were calling one by the name of the other ! 
So no shame for feeling confused then...

OK so these must TO5 not TO39, are Germanium and most likely the original transistors, nobody replaced them further down the line.

One oddity I noticed when testing them, is that if my component test is right, the BASE is connected to the can ! Isn't that weird ? It's usual the drain/collector, where the juice is coming from, that's connected to the can.

Article even explains what "TO" means in all these packages designations... stands for "Transistor Outline". Makes sense.... you learn a little bit new every day ! 

[Vince]

Making progress....

-6.3V supply : the upper beam is now kaput as well, it's now equal with the lower beam. At least it's all consistent now... I had a hard time understanding why I would get some life from one beam and none from the other.

I measured the voltage from the two transformer windings. I get 9.5V RMS ... which is exactly what the schematic says. So that's good ! The most vital part of this scope, along with the CRT... is in good nick, it's not our problem 
Then measured ripple across the filter cap / rectifier output : no significant ripple, which means both diodes in it must be good / working.
Then measured DC voltage across that cap... and I get only 8.5V !!! That can't be good, it's a bit too low I think for that supply to regulate at 6.3V ... don't you think ?
So something might be dragging the voltage down.

As I was poking around that supply, I noticed that the -150V reference supply somehow decided to drift ! Was just fine 2 hours ago and now somehow it jumped to -160V !    That's not good, need to fix that.


At that point I needed a break from the power supply stuff and started looking at the front of the scope and... eh...uh.... I got SWEEP now ?! And looking good / perfectly fine as well, not some crappy "signs of life " kinda thing ! 
Then 4 seconds later sweep stopped and I Was back again with just two spots on the CRT... but then noticed that if I tap the face plate, we have a semblance of sweep happening hmmm !!!  A dodgy connection then ?! Cool, some hope ! So I turned my attention to the upper deck, where all the tubes for the trigger  and horizontal stuff are lined up. I wiggled every tube methodically, one by one, starting from the front of the scope (trigger section) then progressing to the sweep generator, then the horizontal amplifier. I got lucky and was able to quickly find the offending tube ! It's V161, and indeed it's located in the sweep/ time base section. Look at the schematic below.
It's indeed a vital tube : half of it is the cathode follower that send the sweep signal to the horizontal amplifier, it adds up nicely ! 

So I will check the pins of that tube for corrosion, and see if I can tighten the female parts in the tube socket, somehow. Maybe I can stick a tiny screwdriver in there, I will see what I can do.

So that's great new, sweep problem is I think tackled, and I am now left with the "easy" (-er) stuff : two power rails to fix.

Stay tuned ! 

[factory]

Low voltage supply problem might be the rectifier, they can end up with a higher voltage drop with age, you could probably test this separately.

Well done at finding the bad connection with the tube socket, wiggling the tubes about is a frequent thing Shango does with old TVs & radios, to find intermittent sockets, it's a common problem.

David

[TimFox]

According to one 2N404 germanium PNP datasheet, the metal can is connected to the collector.
I seem to remember that older germanium transistors had the substrate be the base connection, so that might explain some connecting the base to the metal can.
I believe that the difference between the TO-5 and TO-39 packages is that the TO-5 has a metal base, but the TO-39 has epoxy fill at the bottom of the can.
Otherwise, they are the same dimensions.

[George Edmonds]

Hi Vince

May I suggest that you fix the 6.2VDC  heater supply before you do anything else, running the front end Valves/Tubes with a very low heater voltage can damage them by stripping the Cathode or contaminating the Cathode.  I would suspect that the Tektronix selected ones were selected for low microphony

First step is to replace those Selenium rectifiers with Silicon ones, unlike Silicon rectifiers Selenium ones have a limited life before they start to fail with a much higher forward voltage drop.  Finally they will self destruct and produce a most vial smell, failing X rated made by Rifa produce  only a very small fraction of the obnoxious smell that Selenium rectifiers do, it is certainly a bad smelling experience that you will never forget.

As far as replacement transistors go in Europe a suitable small signal PNP germanium  replacement would be an AC127 or AC125, however the pin out will be different.  In the good old days you could get little plastic transistor mounting pads that rearranged the pin out for you.

All the best

G Edmonds

[BILLPOD]

Article even explains what "TO" means in all these packages designations... stands for "Transistor Outline". Makes sense.... you learn a little bit new every day ! 
[/quote]


Nice find Vince.  I always wondered about that 'TO'.

[Vince]

Worked some more :


-6.3V supply :

- Replaced the Selenium rectifiers with 1N4004 : no change.
- Removed the x4 6AU6 tubes from the amplifiers = output voltage now went from -3V or so, to -13V !
That supply doesn't like to be unloaded it seems... so either I keep those tubes in place, hoping they will survive long enough for me to fix the supply,
or I replace them with a dummy load ? The datasheet for the 6AU6 states a heater current of 300mA under 6.3V DC or AC.
So with 4 tubes that's 1.2A under 6.3V, that would be a 5.25 ohms resistor dissipating 7.5 Watts.
I will see if I can find something close to that.

- What I notice is that according to the schematic, the two ballast resistor across the pass transistors, should drop about 3.5V. If they did, actually the output voltage would be in spec.
So maybe that's a lead. Why would they drop more voltage than the should ? Because too much current flow through them, which means the pass transistor is not conducting enough. So either that transistor is tired (or completely kaput / open circuit, for all I know ! ) or it works fine but its base is not being driven properly, so the surrounding circuitry is misbehaving.


Sweep :

Looked at that tube.. pins all looks fine to me. Straight, smooth, corrosion free.
Feels snug when I insert it.
Googled for " Howto tighten tube sockets", found only audio amp guys, and their sockets look a different / crappier design that what I see in that Tek scope.
The socket in the scope look just fine to me...

However the plastic base the contacts sit in, by design, is loose inside its metal base. So when I wiggle the tube it's not the tube wigglinh inside the contacts, but the tube makein the base wobble. This of course stress the solder joints on the socket terminals. So I looked for craked solder joints. Thought I found one, see picture below. Pin 9 of the socket is soldered to a tiny thick bare piece of wire, very short, to the metal base of the socket. So, it's very rigid, no give at all.. perfect recipe for cracking a joint then. So I wiggled the tube and saw the socket pin slide along that piece of wire... at least I could swear that. So I reflowed the joint at both ends of the piece of wire, adding a tad of fresh solder.

Then powered the scope back up and.... fixed ? NO ! It's now worse somehow ! Instead of having no sweep and sweep when I wiggle the tube.. now I have no sweep PERIOD ! Wiggling the tube doesn't restore sweep. I now have a "reliable", solid... NO sweep condition ! 

Oh boy...

Tried replacing that tube, no change.

What I did notice though, is that now, the neon bulb (B 167) that's part of that tube circuitry, does NOT light up any more !
I guess the chances of the tube dying all of a sudden, are nil. At best it would degrade very progressively if losing vacuum, I guess.


I guess I should not have washed it. From now on I will not clean my old Tek scopes any more. I will just wear long sleeved rubber gloves and a COVID mask when I want to use them, and that's it !
 

No... hopefully I won't have to resort to that... I like my Tek scopes to look their best.

[m k]

Maybe re-seated tube connection is now on oxidized spot.

[factory]

The leads of that neon used for the sweep looks quite rusty (they are usually copper plated iron), I find they crack at the point they enter the glass, this might be the problem here, you might be able to confirm by measuring the voltage across the tag strips at that point, if it's more than the normal striking voltage the neon in no longer in circuit (cracked leads). I'm used to finding the broken off neons in the bottom of scope cases.

The -6.2V supply, I was thinking the rectifier might be OK, it would be odd for it to suddenly drift like that, I'm not convinced Se rectifiers fail with smoke, unless there is a gross overcurrent fault, or they are under specced, i.e. those in consumer grade stuff. I would check all the voltages for the transistors, to see if something has gone open.

The 502A here seems to have those ancient looking 2N214 things & the Si rectifiers.

P.S. the Good-All T-600UE capacitor seems to be a plastic film type, one got taken apart here, along with a very useful chart to help identify what is actually paper based;
http://crasno.ca/articles/capdielectrics.htm

David

[George Edmonds]

Hi Factory

Sorry but you are mistaken, your photographs are of a 502A which uses 1N2862 silicon diodes NOT selenium rectifiers.

If you have ever been within 100M of a failing Selenium rectifier, I have on many occasions, you will experience a smell like children’s stink bombs on steroids.

G Edmonds

[factory]

I'm not mistaken, I never said I had a '502' and my post stated the 502A here has 'Si' rectifiers.

Still waiting for one of these de-rated 'Se' rectifiers to fail in my TE. The old radios don't get used anymore, so no danger there.

David

[PwrElectronics]

Don't have any advice on this model scope but...

Circa 1986-91 I worked at a place where one of these was something the boss got for his startup.  I suspect it came from McCulloch, the chain saw company engineering in LA from back when.  Probably used in the engine lab as he had some other junky stuff and had worked for them at one time in the early 80s.  It had a very deep light shroud pipe attachment for the CRT and also a rubber eye shield so it must have been used someplace with a lot of harsh ambient light.

It was not working and ended up in a store room.  I am sure it went to the bin when he sold out and had a auction about 20yrs ago.  I was unable to attend and grab some stuff for my "museum" I had worked on that probably was also junked.

[tautech]

Been watching......
Any of those old carbon comp resistors of high values ..... green last band ......well 
Check them all......

[Vince]

PROGRESS has been achieved !! 

Fixed both issues.

-6.2V supply

Was starting to drive me nuts, so I checked the most basic of things and... that was it.
The two small transistors I replaced. The replacements from the 502A were more modern TO5 packages rather than the old inline style I had originally.
I inserted these "new" trannies the "natural " way : with their pins laid out as a triangle, since the sockets provided this layout as well.
Well turns out the sockets do NOT connect the two base contacts together internally... nor did Tek solder them together either.
So basically the base of these two trannies were floating, not even connected to the circuit !

I bent the base terminal to make it conform to the inline layout of the socket, works perfectly now ! 
It's not great though. I think a best way, more permanent fix would be to solder the two base pins together underneath the socket, so I can let the TO5 trannies sit in their natural, comfortable triangular layout.
But doing so will be a pain as I would need to remove tons of components from the ceramic strips, so I can work on the back of the sockets... and these ceramic strips are better left alone in my experience... the less you mess with them, the happier they are... decisions decisions... 

But at least it's all sussed out now, it works, so I am happy.


SWEEP

I checked the neon bulb that doesn't light up any more. Supply voltages on both sides of it, and wiring, were all good.
So it should be able to light up... but does not.
So since Factory said these things like to break their pins by the glass... I wiggled them with tweezers and... sure enough one of them was broken off !
Replaced the bulb with one from the 502A donour.... sweep is now back !! 


So I am super, super happy ! 


I can now move on to lesser problems... this is what I want to work on, next :

- CRT rotation : as you can see it needs a slight adjustment, no big deal...

- Intensity balance control : can't adjust it, turning the trimmer from lock to lock has zero effect.
It drives me mad : the intensity of the lower beam is less than that of the upper beam. So if I want to see both beams at the same time on the CRT, that means the upper beam is too bright and not as sharp as the lower beam. This is totally unacceptable, I am sure you will agree ! 

- As you can see, somehow we have lost the lower beam !    We do get a trace for it, but vertically it's 100% lifeless. Position control has zero effect, and it's totally unresponsive to input signals, not even noise shows up. The trace is sharp and solid... it's as dead as can be.  Hopefully just a dead tube somewhere in that amplifier.


Stay tuned....

[Vince]

Making progress again, am lucky ! 

I swapped all 6 tubes between the upper and lower beam amplifiers... the issue did NOT follow the tubes... so not a tube problem it is.

So then I had a look for broken wires or such... oh my god.... look at that : I simply did not reconnect the two wires for the lower beam, on the CRT itself ! Why ? Because I did NOT disconnect them in the first place ! Not willingly / knowingly I mean !
Yeah, not proud of myself here, to say the least...

When I removed the CRT prior to washing the scope, I removed all the visible wires from it, which are facing you, hard to miss them... because that's were they are on any Tek scope !



But Vince... this scope is no ordinary scope, it's a DUAL BEAM scope, there is an extra pair of wires you dumb ass ! Didn't think of it at the time, ans since these two wires were really hiding deep inside the scope, 180° away from all the other wires... you just don't see them unless you know about them and look for them.



So, when I pulled the CRT from it's shroud, these two extra wires did come off anyway, because the wires aren't actually opposing a lot of force. However, one of the pins was bent 90° ! I must have hit the shroud on its way out 
Despite the horrible access for both tools and my big hands, I just about managed to straighten that pin in situ.
I hope it did not micro-crack the glass of the CRT... but if the CRT goes kaput one day, at least I will know why...
Luckily with 5 of these scopes in stock, I am lucky enough to have a few spare CRTs 

No need to flame me, I feel bad enough as it is.... at least it's a lesson learned, won't be making the same mistake twice....

So the lower beam is back ! 



It's also in good health : no significant vertical offset, and plenty of up/down excursion when playing with the position control.

I also adjusted the CRT orientation, it's now aligned much better with the graticule.

BONUS TRACK : Oh, and look at that.... I can't quite believe it... this scope had a hidden gem : I happened to remove the green filter and would you believe it, the CRT is NOT green, it's BLUE !!!  Well, cyan. I can't believe it... this scope has a beautiful, rare blue CRT and some moron put a green filter to hide it ?! 
I need to find, or make, a blue filter for it... 





OK so next, investigate the intensity balance control issue...

[TimFox]

Blue phosphors (e.g., P11) were preferred when photographing the trace (usually with Polaroid film), but green (P1) was preferred for direct observation.
Some blue phosphors such as P5 had very short decay times (18 us to 10%).
Other phosphors had two different materials with a short and long decay time:  the observer would see a "flash" followed by a long persistence.
Some Tektronix data:  https://w140.com/tekwiki/wiki/Phosphor

[Vince]

Thanks for the info, didn't know that.... oh well, I like blue anyway ?! 
I guess I will be putting the green filter back on, for now at least, to get better contrast.

So I guess my scope makes sense then, unlike what I originally thought ?
Guy removes the green filter when he wanted to take a picture, and puts it back on when looking at it with his own eyes.
You live and learn ! 

[med6753]

Have you been able to resolve the differences in intensity between the upper and lower beam?

[Vince]

Hey Papa Smurf, don't be so impatient, it's been only one hour since my last update ! 

I will post about it as soon as it's fixed, that goes without saying ! 
Well, assuming I can fix it of course !

For now, if you allow, I will make dinner and watch a documentary on Egyptian archeology ! 

I think I am done working on that scope for this evening... more tomorrow 

[TimFox]

By the way, in the JEDEC number for a CRT, e.g. "5CP1", the P number at the end is the phosphor type, from the table I posted.
In that number, the "5" is the outer diameter of the glass envelope in inches, and the source for the excessive screen size numbers on televisions (a 17-inch TV had the screen size available by masking a 17-inch diameter CRT).

[factory]

You've reminded me I had a similar problem, OK not a Tek scope but one day it was working fine, then nothing on the CRT the next, eventually found the socket loose and not making contact. I'm sure I've seen those neck connections fall off something, due to loose connectors too.

My favourite is the P7 phosphor, it's a slow persistence type, blue that fades to orange. Those scopes I have from a rival brand would have come with two filters (orange & blue), one of which is always gone.

I've only got one P11 CRT, note they burn easier than others, if left with a static spot & high intensity.

David

[tautech]

FYI, a great PDF on phosphor types first posted here by rf-loop is attached.
Enjoy.

[Vince]

Wow that's a looooot of CRT's in there ! 
I guess when it's the CRT era, you do everything you can think of with CRT's.....

I started work on the intensity balance issue.

Tek was nice, it's all very tidy and organised in there, and the intensity control circuitry has its own dedicated area, its own set of ceramic strips.
Only Problem is that it's partially obfuscated by the big handle bar...



So I removed it. Excellent access now !



It was not that hard... I should have done it when I washed the scope, would have made my life a tad easier and I would have been able to clean that area easily. Never too late though, so I proceeded to do just that... clean the tiny bit of grime that escaped the wash.



Of course I was as frugal as possible... I simply sprayed one squirt of cleaner directly into the bristles of a soft brush, then lightly wiped the grime off the ceramic strips and components. Then wiped the excess with a kitchen towel.



Then I started investigating the issue.

I noticed there was a bare wire that was running underneath the trimmer uh.... I lifted the trimmer to have a better look, and confirmed the "issue"... much more of an issue when you see that the trimmer has a huge, long screw sticking out of it, very much likely to be touching that wire !
So I pushed the wire as far down as I could, so that it clears that screw.





Then I measured the value of the trimmer, because there are two manuals for the scope, and early ones are supposed to have a 1M trimmer and the later ones like mine, a 3.5M one.

Mine measured at.... 400K ! 
I don't know it's that's intended or not, could not find markings on the trimmer that looked like a resistance value.
I can't be 100% sure whether it's normal or not, because I noticed this scope does NOT stick to the manual !
They modified the circuitry like this I think.. I tried to make a drawing of it :




As a reminder, here is again the official schematic :




So basically instead of having each beam be controlled by each end of the trimmer, what they did is wire the trimmer with only 2 pins, as a simple variable resistor, and it controls only one of the two beams. For the other beam, they replaced the trimmer with a fixed resistor of 1.8M, which is about half of the 3.5M value that this trimmer is supposed to have... hint hint... so maybe that trimmer measuring at 400K is not normal. I don't know.


Then I powered up the scope to see if lifting that trimmer away from that wire underneath it, would make any difference at all.
It certainly did : I got sizzling noise and a few pops and tiny flashes... then the CRT losing life. Pulled the plug... maybe the kitchen towel did not dry the couple drops of cleaner well enough then... 

So I tried to be more thorough this time. I shoved a micro fibre towel through the components, to reach the surface of the deck, helped with a screw driver.
Then blew some hot air using the hot air rework station.
Then powered up the scope again. Zero life on the CRT.... I killed that scope again ! 

Told you, I should not be allowed to work on these scopes.... still, I want to believe that overall he is still better off in my company.. the alternative would have been the junk yard. At least with me he got a good clean, lots of diagnosis and repairs, and a nice warm dry place to live in.
So I might not be perfect, but I am still much better than the other option...  No, not trying to justify my mistakes, just trying to feel less bad about them 

OK so I hope I can fix it and make it go again.... 

[Vince]

Looking at my schematic, the trimmer needs to be at least 1.8M in order to match the fixed 1.8 Tek put in the other branch of the circuit, so as to get a balanced intensity. Plus, of course it needs to be able to go above 1.8M to be able to lean the other way. So.... that means that trimmer really is supposed to be 3.5M like the manual states... so the measured 400k just is not right.

What could possibly make it measure so much lower ? Boggles the mind.
Idea : since we established that the scope was most likely the victim of a heavy smoker... and since HV attracts particles... maybe the track of the trimmer is covered with a layer or cigarette smoke, contaminating it, lowering its resistance ?

I will unsolder that trimmer, try to take it apart non-destructively and see if cleaning the track restores the nominal resistance...

[m k]

Then powered up the scope again. Zero life on the CRT.... I killed that scope again ! 

Maybe something just reached it's "best before" date.

[Vince]

I don't know...

I checked all 4 power supplies : -6.2V, -150V, +100V and +350V = all OK
That's good.

Then dug out my HV probe. I do get HV so that's good, the HV transformer, oscillator and rectifiers and HV disc caps, are also good, that is.

Manual states cathode(s) voltage should be "about" -2900V and I measure -3,000V, on both of them, close enough I guess.

Then measured the grid(s) voltages / intensity voltage for both beams. manual states "about" -3100V (depending on the setting of the intensity control) , i.e 200V lower than the cathodes.
I measured a lot less than that : almost -3,600V  Say I got -3,550 initially, then after a bit (warming up, I don't know), it got down to -3,575V

Unlike what the manual states, that voltage does not vary depending on the (main) intensity control knob on the front panel. At all.
I checked that pot, looks alright to me : measures 25% high at 2.5M instead of 2M, not the end of the world. Resistance varies smoothly and linearly from lock to lock, as it should.

I played with the intensity control on the front panel, while measuring the grid voltages. Doing that, I can see voltages vary between about -3,200V (intensity control set to max) and -3,450V (set to min). So that means the grids are at least 200V lower than the cathode. Would that be enough to reduce intensity to the point of not seeing the beams on the display ?

I will try to adjust the HV from 3,000 down to the indicated 2,900, see if that changes anything... I guess not, since grid voltage would also move in unison, but well...

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 
Of course the bad news is that since voltages look about fine, that means maybe I killed the CRT... 
I hope not...
I guess the worst that could have happened during that brief fireworks in the intensity control part, is the grid voltage going way less negative than they should, like... zero volts potentially... but if CRT's are like other tubes, a grid at zero should be perfectly fine and sustainable, no ?

Oh my.... 

[tautech]

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 

Yet it may have died on its own.

Post the CRT circuitry for our study please as your reported voltages don't seem right. 

[m k]

So what have you possibly done, pulled those disk capacitor sides closer together?
How long that sizzling noise lasted?
You have those bent CRT pins.

[Vince]

Post the CRT circuitry for our study please as your reported voltages don't seem right. 

Attached.

I desoldered the trimmer. My in situ measurement was wrong it seems... it's not 400k anymore, but 3.6M as it should, and working smoothly. So I put it back.
Checked every resistor in the circuit, all fine. Many read are a bit high, but like 5% high only, so within spec (10%).
The two 15M resistors in each branch of the circuit, read 17M, so again within spec, and since they read the same they aren't introducing any imbalance of their own.

Just as a sanity check, I disconnected all wires from the deflection plates of the CRT, and shorted the terminals, just to make sure the spot was not just lost off screen. It's not, still no spot to be seen.

[Vince]

So what have you possibly done, pulled those disk capacitor sides closer together?

Not sure what you mean ?
The pairs of disc caps ? Tehy are already wired in  // so making them closer will not change anything I would guess...

How long that sizzling noise lasted?

A few seconds, 5 at worst I would guess...

You have those bent CRT pins.

Only one bent pin, and only affecting one beam... unless you mean that as envisaged, when I straightened it, I created a microcrack that compromised the vacuum...  yeah.
I don't see the relation to the sizzling in the intensity control circuit, but maybe the two events, though unrelated, happened at the same time...
I guess I should try swapping the CRT and see for myself....

[tautech]

I don't know...

I checked all 4 power supplies : -6.2V, -150V, +100V and +350V = all OK
That's good.

Then dug out my HV probe. I do get HV so that's good, the HV transformer, oscillator and rectifiers and HV disc caps, are also good, that is.

Manual states cathode(s) voltage should be "about" -2900V and I measure -3,000V, on both of them, close enough I guess.

Then measured the grid(s) voltages / intensity voltage for both beams. manual states "about" -3100V (depending on the setting of the intensity control) , i.e 200V lower than the cathodes.
I measured a lot less than that : almost -3,600V  Say I got -3,550 initially, then after a bit (warming up, I don't know), it got down to -3,575V

Unlike what the manual states, that voltage does not vary depending on the (main) intensity control knob on the front panel. At all.
I checked that pot, looks alright to me : measures 25% high at 2.5M instead of 2M, not the end of the world. Resistance varies smoothly and linearly from lock to lock, as it should.

I played with the intensity control on the front panel, while measuring the grid voltages. Doing that, I can see voltages vary between about -3,200V (intensity control set to max) and -3,450V (set to min). So that means the grids are at least 200V lower than the cathode. Would that be enough to reduce intensity to the point of not seeing the beams on the display ?

I will try to adjust the HV from 3,000 down to the indicated 2,900, see if that changes anything... I guess not, since grid voltage would also move in unison, but well...

So for now the good news is that the scope is still generating HV, I didn't kill the HV circuitry ! 
Of course the bad news is that since voltages look about fine, that means maybe I killed the CRT... 
I hope not...
I guess the worst that could have happened during that brief fireworks in the intensity control part, is the grid voltage going way less negative than they should, like... zero volts potentially... but if CRT's are like other tubes, a grid at zero should be perfectly fine and sustainable, no ?

Oh my.... 

Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....

[TimFox]

A negative grid-cathode voltage controls the electron beam current, and avoids grid current.
A positive grid-cathode voltage would "incite" even more beam current, but substantial current would flow in the grid circuit and affect the grid-driving network.
If you must measure the grid-cathode voltage, it is quite dangerous since you want to connect a voltmeter between the grid and cathode, both of which are at a negative high voltage with respect to ground.
Measuring the two voltages from ground separately, with a reasonable voltmeter resistance, will not give an accurate value for the difference.

[Vince]

Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....

Didn't know that. I thought CRT's were just tubes, and tubes give their full electron flow just fine with the grid tied to the cathode / same potential.
Never saw them needing the grid to be higher than the cathode. Strange. You lost me...
Every tube datasheet I have looked at give current flow being maximum when the grid is at the same potential than the cathode, not higher.
Then if you want to restrict the electron flow you make the grid negative relative to the cathode. The more negative it is, the lesser the current.

I don't understand what you mean 

[TimFox]

Grid voltages MUST be lower than cathode to incite electron flow.
Lower in that ....more positive than minus 2.9kV.

Some divider is open or something.....

That's how CRT's work.....

Didn't know that. I thought CRT's were just tubes, and tubes give their full electron flow just fine with the grid tied to the cathode / same potential.
Never saw them needing the grid to be higher than the cathode. Strange. You lost me...
Every tube datasheet I have looked at give current flow being maximum when the grid is at the same potential than the cathode, not higher.
Then if you want to restrict the electron flow you make the grid negative relative to the cathode. The more negative it is, the lesser the current.

I don't understand what you mean 

In a CRO with CRT, varying the grid-cathode voltage (always negative) changes the beam current and therefore the brightness of the trace.  It is controlled by the "brightness" panel potentiometer.
In vacuum tubes in general, going from zero grid-cathode voltage to positive grid-cathode voltage will increase the cathode current further, but some of that current flows through the grid.
Some vacuum tubes' ratings exclude positive grid voltage, due to excessive current through the relatively fragile grid.
However, high-power tubes used for RF power amplifiers often run with substantial positive grid voltage.
Here is the datasheet for a 6C4 general-purpose triode:  https://frank.pocnet.net/sheets/093/6/6C4.pdf
Note the two graphs of characteristic curves on p. 3 of the datasheet, for positive and negative values of "Ec", the grid-cathode voltage.
The max specs give a limit on negative grid voltage, and another limit for grid current.

[Vince]

A negative grid-cathode voltage controls the electron beam current, and avoids grid current.
A positive grid-cathode voltage would "incite" even more beam current, but substantial current would flow in the grid circuit and affect the grid-driving network.

Do you mean to say that as I suggested earlier, maybe during the "accident", the grid potentially having gone to zero volts or something, might have induced a high current grid, hence might have destroyed it/them ?

If you must measure the grid-cathode voltage, it is quite dangerous since you want to connect a voltmeter between the grid and cathode, both of which are at a negative high voltage with respect to ground.

I don't see the need to do that, but call me stupid (I am) but if I measure that voltage using a handheld/ battery powered/ floating voltmeter, how would that ground reference matter to the meter ?

Measuring the two voltages from ground separately, with a reasonable voltmeter resistance, will not give an accurate value for the difference.

I don't understand ?  The HV probe is rated for 40kV, 1:1000 ratio, 10G resistance.. it draws bugger all current so it should not load the circuit enough to give a false reading I would think ?! 

Still, the voltages indicated in the manual aren't even taken with a 10G probe, they only talk about a "voltmeter", with no particular indication.

Anyway, from what you say it's likely I damaged the grids, so will try swapping the CRT...

[TimFox]

Actually, the "grid" in a CRT is usually not a physical grid, but an electrode with a hole in the center.
If you accidentally shorted it to ground, that might damage it--I've no experience with that.
It might damage the components in the brightness control circuit.

Even though your HV probe has a high resistance, the effective resistance of the voltage divider for grid bias ("brightness") is high and will shift with the probe loading more than the cathode voltage.
In general, deriving a small quantity by taking the arithmetic difference of two very large quantities is bad for accuracy.

A "handheld" voltmeter would measure the grid-cathode voltage accurately, but the entire multimeter package would be at a dangerously high potential--do not hand-hold it!

[George Edmonds]

Hi Vince

I cannot recall the last time I read a topic containing so much incorrect information.

As was standard at the time the scope was manufactured ALL voltage measurements were made using a 20K/V meter, for example a Simpson 260.

Your chances of having damaged the CRT are very low to none existent unless you have caused mechanical damage to it.

 Your biggest problem is likely to be the Allen Bradley resistors as they are hydroscopic. They also ALL drift high in time.

G Edmonds

[George Edmonds]

Hi Vince

The CRT used in the 502 is what is known as a dual gun one, ie there are two independent electron gun assemblies in one CRT envelope.  This was quite common for a time in the 1960’s with most scope manufacturers using such CRT’s, for example the Telequipment D43.

Now I have no intention of giving a full description of how CRT’s work other than to provide the information that will help you.  The 502 series used a considerable range of Tektronix CRT’s some of which I can find data for and some I cannot, it may help if you can provide the Tektronix part number of the one you have, in this posting I will be using the data for a CRT used in the 502A as I have it.

With one exception the actual CRT voltages are not critical as most are adjustable, the 502 uses a Cathode voltage of some 3KV, this exact voltage in not critical, the voltage that is critical is the Cathode to G1 voltage as this controls the intensity of the electron beam, the G1 voltage MUST always be negative with respect to the Cathode, most CRT manufacturers call the voltage that causes complete extinction of the electrum beam the pinch off voltage.  With the 502A CRT the pinch off voltage looks to be about -95V with respect to the Cathode so that any G1 voltage between say -90v and say -40V should show a CRT trace or spot.  Be aware that a very bright spot or trace can and will burn the CRT phosphor if displayed for more than a very short time.

The simplest way of measuring the G1 voltage is to do it directly with a BATTERY powered hand held DMM, however NEVER hold the DMM or touch the probes whilst the scope is turned on.

I am going to try to attach some Allen Bradley information to this posting, it is very relevant information.

G Edmonds

[m k]

So wet resistor values are more than dry values.
But can they be so wet this time that drying is noisy.

CRT beam I understand so that more negative grid is more exporting electrons.
But what high negative potential means, is it more negative than low negative potential?
No positive is present here.

When cathode is heated enough it start emitting electrons, letting them loose from heated material.
(cold cathode is not heated, sort of)
But nothing there is regulated in any other way than how much those loose electrons happen.

Between cathode and grid those freed electrons are bouncing around.
That continues until something catches then or they pass the grid.

When grid material is negatively charged it pushes electrons away and more when charge is more.
If negative charge is enough it also start emitting electrons, what obviously is not an intention.
So grid pushing electrons away is an electron lens.

If basic level of freed electrons in vacuum tube is so high that operation is overshooting it's possible that grid must be positive.
But if electron lens is also needed it must be another grid.

[TimFox]

No:  the relevant control voltage is V_gk, the voltage on the grid with respect to the cathode.
Just as in any vacuum tube, a negative V_gk reduces the cathode current.

[m k]

So grid voltage over chamber to cathode is a closed loop and is sucking freed electrons back to cathode, more than anode.
Then I don't understand the lens.

Maybe I don't understand what negative means.

[TimFox]

No.
The negative grid voltage affects the electrostatic field at the cathode, reducing the number of electrons that can exit the hot cathode.
This was all worked about by Child and Langmuir over one hundred years ago, and is in all the textbooks.
Lacking a textbook, try googling "triode".

[m k]

So grid level is under cathode level and is partially guiding electrons to anode and partially to another "anode" somewhere around a circuit of grid and cathode.
So stream of electrons is turned to two streams.
So mechanical structure decides what part is for what function.

[TimFox]

If you are interested in this, you should look for information on the "electron optics" of CRT electron guns.
Note that the control grid in electron guns is often called a "Wehnelt", and is not the same shape as the grids used in vacuum tubes for amplifiers.
Above all, you need to understand the physics behind thermionic emission and space-charge limited current in vacuum tubes.

[Wallace Gasiewicz]

The cathode in a CRT emits electrons based on the voltage potential.  This would result in a valve "Diode" if it was not controlled with all power available transferred thru the tube to the Anode.  The grid changes the electrostatic environment in the tube and limits the cathode emission.  The grid voltage is about 60 volts Negative compared to cathode.     
The idea is to get the grid voltage to a level so that at minimum brightness, there is just a barely visible trace or spot on the screen. You then increase the brightness control to make the electron beam stronger. The voltage on the grid is maximum Neg with the beam almost turned off.     
This actually regulates the electron emission of the Cathode or "Electron Gun" rather than absorbing or otherwise steering electrons elsewhere.     
The other plates in the CRT "steer" the electron beam to make a trace.   

A Triode tube does the same thing and the grid is Neg with respect to the Cathode usually.  A "Self Biasing" triode uses a resistor from the Negative supply to the Cathode to make the Cathode more Positive with respect to the Neg supply, which is usually chassis ground. That way the Neg supply  is more negative than the cathode and can give you a neg supply (with respect to Cathode) for the grid.   

Without a Negative Bias on the Grid the Triode tube will act as a Diode and pass all the current it can in ONE direction.  There is a reason people call these things "Valves" which I think is more helpful than calling them "Tubes"  (but I speak "United States" rather than English)

I realize this is a simple minded explanation and apologize in advance. I am leaving out the inter electrode capacitances and probably other things that I have either forgotten or am ignorant of.

[m k]

I was around CRT monitors for decades, but unfortunately never really "inside" a tube.
And no wonder my understanding is a bit off when many times everything is anodes without voltages.

One example has positive voltages after G1, maybe it's an error.
It means that all electrons away from center of G2 are attracted by G2 and go more away from the center.
Much more rational it would be if G2 voltage is negative, then it would collect electrons towards the center.
This example has G2 as pre-accelerating anode, maybe its mechanical shape can change the situation.

I remember that long long ago I tried to understand how accelerating anode is actually accelerating, but it was left at that.
Seems that my understanding also includes more parts between cathode and G2 than there really are.
No wonder either when some use general name grid and other anode, and then some has much more Gs and none has voltages.

But it's Vince who's CRT must be made clear and simple.
The control grid hole is not a lens, the hole is a pinhole like a pinhole camera, but energy goes to other direction.
And yes, control grid level is under cathode, is it a must, can't say, maybe it is not, but pinhole start acting differently.
It's also possible that control grid is emitting some rouge electrons, but that's not important and mainly it's a static repelling plate.
When G1 is repelling it's also repelling the cathode, so some electrons are not emitted when repelling force is low.

If G1 has no voltage at all the surface of it is not repelling nor attracting, so it's merely a mirror.
Can't say how that actually behaves, but for sure it's different to situation where voltage is much above what cathode has.
Maybe different is a wrong word, maybe it's just weaker, there's still a route for those electrons, so maybe some electrons are pushing G1 voltage down.

When electron bundle exit from control grid its shape is like over bent umbrella.
So before the spread is too much something must start collecting it towards the center.
But maybe that is for later.

[TimFox]

Still not right.

[m k]

Dang, but new page, side track elsewhere.

Vince has a boat load of handheld meters.
What and how Vince should measure that he can be sure that CRT change is not going to harm the new CRT?

If current measurement is needed can a regular handheld do HV without a clamp?

[Wallace Gasiewicz]

I am with George on this one, I do not think the CRT itself is the problem..   
Having said that there have been problems with transmitting tubes and sometimes with scope tubes due to bad connections in the pins.   I mean the wires from inside the CRT somehow are not connected well inside the pins on the base of the CRT.  The fix is to clean out the solder a bit and add new solder.  There are some reasons given why this happens in some tubes and in particular leads (pins). I do not know if any of these reasons hold water but I have restored a few transmitting tubes by cleaning the solder in the pins.

[m k]

I try to find a reason not to think that vacuum has escaped.
Longer noise is bad, but cracked tube would mean hissing more than crackling or something sparky.

Shorting water or other conducting liquid then means more something sparky than hissing.
And used carbon composite resistor values go higher when moisturized, so no harm there.

How far G1 must go from cathode before it start making sound, is it then like what thyratron do?

Is it probable that the sound came from transformer?

[factory]

If is was slow loss of vacuum, you probably wouldn't hear the hiss over the fan noise.

David

[m k]

Maybe unblanking is not happening.
200V too low intensity would be explained if unblanking lifts it that 200V.

Manual has that for slower sweep unblanking pulse is possibly not needed.

[Vince]

OK people, it's week-end time again, all cold and windy and rainy outside... perfect weather to stay inside and work on some TE isn't it...

Made some progress.

I put the 502 CRT inside one of my 502A : CRT is fine, phew ! 
With that off my chest, I then took some HV measurementson the 502A so I get a baseline.
That 502A is yet to be restored, it looks like junk and has barely life in it, very sick, but it does have enough life in the CRT department to allow me to take the voltage measurements I needed.

So here goes :

- Cathodes : both are spot on -3,000V  just like the 502, so that's perfect.
- Intensity HV : -3,300V vs -3,600V on the 502 hmmm.... 502 is 300V lower than it should hmmm...
- Grids : depending on the setting of the Intensity control, I varies between -2,900 (max brightness) down to -3,150V (dark), and the "normal" brightness level is obtained at about -3,050V, so very close to the cathode voltage, only 50V lower.

On the 502A however we get  between -3,200 and -3,450V, which is the correct span (about 250V variation) but is shifted 300V lower than it should, which is consistent with the intensity HV which is also 300V too low.

So looks like we are making progress. The problem is : why does the intensity control get a supply of -3,600V instead of -3,300V ?
And all because of a few drops of water on the circuitry.

There is no reason to believe that the HV transformer is all by magic producing 300V more than it should, especially as the other secondary windings are just fine...
So somehow it must be to be explained within the passive components of the intensity control circuitry that I cleaned, ahem...
But I already measured every resistor and they check fine. However as Georges said, these resistors can sometimes measure fine when cold, but misbehave when in operation.

Indeed experts here... meaning anyone that knows more than me... which should not be too too difficult to find !

So I guess the only way for a health secondary to produce 300V MORE than it should, would be if say, it was not loaded as heavily as it should ?
So we might have some resistor that goes way up in value when operating ?

Thoughts welcome as always !

In the meantime I will try to suss it out by myself.... crossing fingers.

BTW, to answer a previous question, here is the model of the CRT in my 502 :

T5021-2 , P/N 154-0246-00

And the one in the 502A (also a blue phosphor) is :

T5021-7 , P/N 154-0247-00

Thinking of it, generally speaking we can safely that I am not the only one having problems with intensity control... Tek themselves just kept changing the design !
There are two versions just for the 502 despite its short production run. My own 502 even has yet a third version, that's not even documented, and the 502A has yet another, 4th version !......

[factory]

Could it be possible something that uses around +300V is shorted or now tracking to ground after getting damp and hasn't recovered once dried out?

Did you confirm the unblanking pulse is working? What voltages and/or pulsed voltage are you getting either side of R864?

David

[Vince]

Oh thanks for that idea David !
That was it indeed ! I replaced the tube, V135, that generates that unblanking pulse, and we are now back in business ! 

So the fireworks in the CRT section did backfire all the way to the sweep generator section ! 
The two are connected only by a long piece of coax cable that brings that pulse.

We are making progress for sure, but I guess I must have fried some other stuff, because we have lost trigger as well as you can see ! 
Scopes should be afraid of me.
I did try replacing the two tubes from the trigger section, no luck. That said, of course I pulled them from a 502A donour and these tubes might well be defective for all I know   
Yes, I did play with the "stability" control that caught me initially... but no luck this time, that's not the issue. That would be too easy.
I guess I will have to do some real troubleshooting in the trigger department then ! Luckily I have already been there once, so should hopefully be faster and easier this time... crossing fingers nonetheless !

Stay tuned....

[Vince]

OK worked on the thing.

Scoped the trigger section extensively. It works absolutely perfectly, and sends beautiful sharp pulses to the sweep circuit, at the grid of V135A, see schematic.

... yet, these pulses somehow fail to trigger the sweep, even though sweep itself works fine, when you make it free-run.

So it's basically the same situation I had at the very beginning of the restoration. It was just the stability trimmer that needed tweaking.
Except this time this trimmer doesn't fix the issue.

So I guess either the  stability circuit misbehaves, or the amplitude of the pulses is not great enough.

Maybe the tube is defective in some way and is more difficult to trigger. I am no tube expert so why not. I swapped tubes, no luck.


I do have maybe a hint : I checked DC levels at the grid, and it's a bit off.

Should be -50V in free-run, and I get -51V, close enough I guess.
However when not free-running we should get -58V, 8V difference, and I get only 1V difference, -52 or something.

This DC level I understand is set by the stability circuitry, so maybe that's indeed the problem.
I will check resistors in there, and take comparative measurements on the 502A.

Later... week-end is over.

[m k]

Since grid is around -50V all are generally fine.
And totally well a moment in the past.

Recurrent switch is pulling grid down to -58V through R126.
If stability can't mimic that its effect must be too small.

On the other hand C131 can't do a thing.
So either R131 is too stiff or C131 is too weak.

On the third hand stability is 100k behind R114.
So R131 is not so easily too stiff.

Maybe R114 is too high.

[Vince]

Hi there,

So this week-end for your enjoyment, some pics and news.

I did as planned and measured grid voltages on the 502A so I can compare with what I got on the sick 502.
The 502A got sick and would not show any life on the CRT any more, whereas before it at least managed to show the two spots, though immobile they were. So I quickly tried to sort that out. I ended up replacing the final driver tube in the horizontal amplifier, the one that drives the deflection plates of the CRT. That not only brought back the spots, but I now have SWEEP as well ! 
When I got that scope a year ago I remember spending many hours trying to get some life on the screen with no luck, and a year later I fix it on a pinch. I guess I am getting the hang of these things ! 
So for the first time I saw a trace, hell even a signal (the calibrator output as usual) on the screen. It's a blue phosphor, but very pale blue, looks almost white ! I also notice it's not very bright, you have to push it to the max to get only "adequate" brightness in the lab with the overhead lighting turned off. So either the HV is a bit lacking ( I measured it at -2,900V which is spot on the spec so...), or it's tired.
Will get back to that scope later for sure, as this the one of my x3 502A that I plan on restoring, the other 2 are for parts.

So once it was back to life, I measured grid voltages and found it was quite different from the vanilla 502 I am trying to fix hmmm... so checked the 502A manual and sure enough, I wasted my time : the 502A doesn't exhibit the same voltages as the 502, they aren't the same in this regard, bummer.

That was Friday evening.

So next day, on saturday I switched plan and instead decided to get my other 502 going, so I can compare like for like.
That was the very first 502/A that I bought, 6 years ago, and never worked on it.
So I decided to embark on a "get it going" sprint !!

So first let's give it a quick visual inspection inside out, and give it a quick clean outside, as per usual.
To cut it short... it's 18 months younger than the other one, S/N 6131 versus 4356, and is clearly a much lower mileage unit, it shows everywhere you look. It's definitely the one I should concentrate on restoring, though the other one is nice as well, given all the work and cleaning that went into it, so hopefully I can finish both of them. However of course that means I can't use one to restore the other any longer... I have to rely on the 502A, but I also want to restore one of my 502A of course so...  so basically the situation is now this :

To be restored : x2 502 + x1 502A using parts from x2 502A donours.

I hope it can work out....

So the plusses of this 502 compared to the other one :

- Lower mileage, overall better preserved.

- Didn't belong to a heavy freaking smoker : was quick and easy to clean. The wires inside aren't as yellowed, and the colour stripes on them are still very visible and vivid. That also means no oily residue on the lower deck that ate that rubber grommet alive... all grommets are just fine in this scope 

- Was stored I guess in a friendlier environment : no mould / white dots on the wiring harness and tube shields, and the lacquer on the top side of the lower deck is still pristine. Unlike the other 502 it's not littered with corrosion spots poking through the lacquer.

- The two big rotary switches for the attenuators are very light in operation, comfortable to use, whereas the other 502 has them extremely stiff, to the point of being a joy killer. Lubricating them didn't help. I think to fix that I need to release some pressure from the metal tab that keeps the ball bearing captive. I think that's what determines how stiff the knob is to turn.

- The rubber mounts that isolate the attenuator sub-assemblies from the chassis (I like this refinement / attention to detail !  aren't sagging.

- No missing tube shields.

- The can caps have not been messed with : no signs of ugly rework on the solder side, and still using their original mounting hardware, and all the condoms on the 3 cans used for the negative supplies, are original / proper.

Now for the similarities between the two :

- This one also had a power cord soldered directly to the prongs of the american socket ! It's a disease !
However this time instead of a super skinny two wire cord, it was a proper 3 conductor, beefier cord, more adequate... though the guy clearly didn't value his life much : he didn't deem it worthwhile to spend the extra 2 minutes to solder also the earth wire ?! 
I just will never understand people, I never will....
Anyway, I cleaned that up.


And for some particularities :

-  The cooling fan is sagging, making the blades hit the grill, so had to remove the grill to run the scope... better watch where I put my fingers now !   the 3 three rubber mounts look in good shape so I don't know ... the metal work must be bent, will try putting it back into shape, or replacing the fan altogether. Note also the motor for the fan, it's not at all the kind that I usually see in these Tek scopes. No. This one is huge and is enclosed in a black painted metal casing, that looks quite heavy.
I am not 100% sure if this motor is original as I see signs of rework on the soldering of its wires, some flux residue which I am not accustomed to seeing from Tek. So maybe that fan was replaced... so might replace it altogether, and it will fix the sagging issue at the same time... will see.

- The power switch, which is mated to the back of the graticule illumination pot, died while I was working on the scope : on saturday evening when I went to switch off the scope and go to bed...the switch had no action, the scope would still run in the OFF position ! 
Then next day, today, opposite problem... as I was working on it, out of the blue the scope lost power. Checked the switch with the DMM... indeed it's dead, it's open circuit in both positions... so need to replace that.

- CRT neck : there is a mechanism to rotate the CRT. A white nylon clamp that grabs the neck, and which rotates using a worm screw that you can adjust by hand. This nylon clamp is a known weak point, it often breaks/snaps.  Well it sure did on this scope... luckily it's a standard part so I can get one from any of my numerous Tek scopes, not just the 502/A.

But before it died I enjoyed a few hours of troubleshooting and I almost got it going. Here is what got done.

First, I remember that when I got the scope 6 years ago, the seller said that he tested it, was able to get the square wave from the calibrator to show up on the display, but that after a few minutes magic smoke escaped so he pulled the plug and left it at that.

When I received it, according to my notes from back then, I did :

- Removed the covers to look for obvious signs of smoked components, and found nothing.
- So powered it up. Neon annunciators on the front panel were responsive.
- No trace on the screen for 10 minutes then a trace appeared out of the blue.
- Trace stayed for 5 minutes then disappeared, never to be seen again.

So, 6 years later I am back on it...

I powered it up using the dim bulb tester, no short. Left it for a few minutes to give the can caps a chance to wake up / reform a bit, or a chance to see if a short would suddenly appear and the tester light up like a Christmas tree.
After 10 minutes, no drama, the brightness of the bulbs remained the same all throughout, and never varied/flickered the tiniest bit. The scope looked therefore to be in a safe/stable condition. Inspired confidence so I did away with the tester and plugged the scope straight to the mains.

No drama. As could be expected, nothing on the CRT to be seen. I went straight to the CRT HV section, and immediately noticed that the 3 rectifier tubes in there, were not glowing... so I measured the HV and sure enough, zero.  So that's our main problem.
I checked closely the transformer and all components in the primary/oscillator circuit. Everything looks fine but for that one small ceramic disc cap, red. It looks strange to me... its surface looks shiny/greasy instead of matte, and there is a couple "bubbles" on its surface, strange...  As you can see it's wired in // with a 2.2M resistor. I checked the manual and neither the cap nor the resistor, are to be seen in the schematic !!    Taking a deep breath...
So that's my first suspect then, removing that cap to check it.
Will be checking all the black/red Sprague caps as well while I am at it, and all the resistors.


Then checked the power supplies. OK I should have checked that first....
I was in for a shock : the voltmeter was losing its mind, the readings were dancing widely all over the place, it could never get a stable reading, with every refresh of the display, I got a new story ! Like 350V, 200V, 5V, 125, .... the meter was going crazy and so was I ! 
I could not make sense of it, but luckily after a few minutes it calmed down and I got steady readings at last.
They showed that every supply was way off, like 10% too high at least.
Then measured the -150V reference rail.... yep, was at -165V, 10% off.
So I adjusted it to -150V and hey presto all the other supplies followed suit, and are spot-on. A joy.
However I had to crank the trimmer all the way CCW, which means there is an underlying issue that I will need to fix later in that supply. I guess some resistor drifted. but for now it will do.

But now was not the time... I was focused on the grid voltage measurements I needed to make to help me fixing the other 502.
So, I scoped the trigger/sweep/amplifier chain.
I could see that the sweep generator was working just fine in free-running mode, and I got a nice sawtooth out the horizontal amplifier.
However the trigger section was kaput : calibrator signal was getting to it, but no trigger pulses were coming out.
Was easy to fix. The first tube was bad, the comparator/amplifier that shape the input signal and compares it to the trigger level pot.
Replaced that tube, now all is fine.

So, even though there was no life whatsoever on the CRT, I was nonetheless able to scope the scope... to take my measurements.

First I measured the grid DC level when free running, I got I think -49V or so, close enough to the -50V indicated in the schematic.
Then when not free running, I got I think -58 or -59 maybe can't remember for sure, which is again what the manual states.
As I said I am not sure because I didn't have to time to write it down, as the scope right at that moment decided to lose power out of the blue...  But I think these were the voltages I measured. Which means they are conform to the manual, which means the issue in the other 502 might indeed be incorrect DC levels at the grid. So I will keep searching in this direction...


So there you have it..... you now have TWO 502 restorations for the  price of one, lucky you ! 
Sorry if that makes this thread confusing and difficult to follow... 

All I can do is keep the x3 502A out of the topic other than mentioning the donour here and there. But as for the scopes actually being restored, this topic is only going to detail and show pics of the two 502. I already have a thread open for the restoration of the 502A.

Maybe I should rename those two : the "first" 502, in chronological order, the one I started restoring this week-end, and the "second" 502, I got very recently and which started this thread.

So what's next for next week-end ?

Get the "first" 502 going :

- At least try to sort out the power switch issue so I can use the scope again to take measurements to help me diagnose the second 502.
Will try contact cleaner but not holding much hope. Might have to replace the switch but it's not a 2 minute job from the looks of it, not a two hour job either... but if contact cleaner is enough, will be even quicker. The quicker the better...

- Might replace the cooling fan so that it's not sagging anymore and I can put the grill back in place, if just for my safety/ fingers..

- Try to fix the HV oscillator so we get (hopefully) life on the CRT...


Some piccies. First the scope before cleaning. A bit dusty but otherwise in really good shape overall, especially inside.

[Vince]

The scope after giving the exterior a little cleaning, and sorting out the mains socket.
There were quite a bit of rock hard glue residue at the top of the face plate but with some patience scrubbing and lots of alcohol, they were kind enough to vanish 

Oh, looking at those pics, I notice the CRT bezel is not fitted properly, it's rotated 90° clock-wise, the little hole is on the left side instead of the top side... if only all the problems were so easy to fix...

[Vince]

And then I started working on the scope.

Close-up shot of that dubious little ceramic disc cap I mentioned in the CRT HV circuit. What do you think  ? Overheated ? Might have been the source of the smoke ?

[factory]

Those ceramic caps are wax coated, the bubbles indicate it's got quite hot a some point.

The fan motor is the exact same type I have in my 502A, it might also be a replacement, as some of the fixings look to be in the wrong place. Wonder if it's a later type supplied by Tek?

David

[Vince]

Thanks for the info.

Will check & replace that cap for sure...
Just wondering if it's the actual cause of the problem, or just the victim/symptom of some other problem... so will check every component in there just in case.

OK so that fan is not so out of place as I thought then... good to know.
No idea what led to its use though, as out of my x2 502 and x3 502A, this is the only scope that has this type of fan/motor...

What I notice is how quiet it is, to the point that I am wondering if it spins at the proper speed... maybe it's tired. Its black case, despite its large size, is kinda warm to the touch so I wonder if something is wrong with it that keeps it from spinning faster. The rotor is not seized, I can at least say that much, since the fan itself spins just as freely as any other of these scopes...

[Vince]

Oh, hot from the oven... I just found a problem with the way the fan is mounted, which most definitely proves it's been messed with !
It does not prove that it's been replaced with a different type, though, but still... it's intriguing.

Look at that : the fan shroud is designed with a very large flat on its right side, so that the shroud does not stick out of the air filter housing / be visible. So, there is only one single proper orientation for it.

Well look at how it looks on the scope ! The shroud DOES stick out, and the flat is not oriented at all where it should !

So that means the shroud/ fan/motor have been rotated, hence most definitely removed, so it's possible a different type of motor has been put in there, upon reassembly... isn't it...

This also explains why the air grill / mesh was secured with only 3 screws instead of 4 --> with the shroud mis-oriented like that, on the right side you end up with only one screw hole on this side, not 2 anymore.

[factory]

Actually I believe the 'second' 502 (made around 1961) from the first page of this thread has a replacement fan, note it has a 9 digit Tek part number, other parts in the scope have 6 digit part numbers, the fan motor appears to have a datecode of 7/74.
Tek started using 6 digit part numbers in 1965 according to Tekwiki. https://w140.com/tekwiki/wiki/Tektronix_part_numbers



Also note the picture from the manual, which has the larger fan motor.



David

[factory]

What I notice is how quiet it is, to the point that I am wondering if it spins at the proper speed... maybe it's tired. Its black case, despite its large size, is kinda warm to the touch so I wonder if something is wrong with it that keeps it from spinning faster. The rotor is not seized, I can at least say that much, since the fan itself spins just as freely as any other of these scopes...

Warm is probably fine, too hot to touch would be a problem, maybe the fact it's quiet (bearings in good condition?) & cleaner, indicates something about how much use it's had compared with the other one, i.e. lower hours.
Or they could have put a salvaged fan motor in, we will never know.

Then again an optical tachometer might be useful for comparison, if you think it's slower than the other one. I've only got two, both about the same vintage as this scope.

David

[Vince]

Actually I believe the 'second' 502 (made around 1961) from the first page of this thread has a replacement fan

Yeah I did mention that at the time, based on the massive flux residue/ugly soldering on the wires/ceramic strip, what looked like might be a date code, and the funky looking green rubber mounts...

note it has a 9 digit Tek part number, other parts in the scope have 6 digit part numbers, the fan motor appears to have a datecode of 7/74.
Tek started using 6 digit part numbers in 1965 according to Tekwiki. https://w140.com/tekwiki/wiki/Tektronix_part_numbers

That's just the final nail in the coffin then !
Thanks for the info, didn't know about that.
Tekwiki is such a great resource... so much good info in there.

Also note the picture from the manual, which has the larger fan motor.

David

Ah ! Well that closes the case then ! 
OK, no more guessing then... this fan motor is 100% legit in this vanilla 502.....

[Vince]

What I notice is how quiet it is, to the point that I am wondering if it spins at the proper speed... maybe it's tired. Its black case, despite its large size, is kinda warm to the touch so I wonder if something is wrong with it that keeps it from spinning faster. The rotor is not seized, I can at least say that much, since the fan itself spins just as freely as any other of these scopes...

Warm is probably fine, too hot to touch would be a problem, maybe the fact it's quiet (bearings in good condition?) & cleaner, indicates something about how much use it's had compared with the other one, i.e. lower hours.
Or they could have put a salvaged fan motor in, we will never know.

Then again an optical tachometer might be useful for comparison, if you think it's slower than the other one. I've only got two, both about the same vintage as this scope.

David

Oh yeah ! Didn't mention it but indeed I have been thinking about cobbling together a quick and dirty tachometer experiment to gather some data rather than guessing ! Sounds like a cool little project, will try to get that done this week-end ! I just love this kind of little experiments  !

[m k]

HV section, orange rectangle between black capacitors.
Is its side as it should be?

[tautech]

HV section, orange rectangle between black capacitors.
Is its side as it should be?

This one ?



Looks like dirty waxy coating.....

[factory]

That is either a silver mica domino capacitor, or wax paper type disguised as mica (which is very unlikely for non-consumer grade equipment). Both manuals list a Mica cap for C807.

David

[factory]

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.
Those 3 parts don't appear until you look at the '502A' manual, they are C847, R846 & R847, they probably won't be responsible for the no HV fault.

David

[m k]

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.

[factory]

C807 is the Mica domino, the disc underneath it is C808, there should be wires to the transformer too.

David

[Vince]

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  ! 

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.

[Vince]

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 ?! 
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 !     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 ?!    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 

[med6753]

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 ?! 
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 !     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.

[m k]

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.

[factory]

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

[med6753]

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.

[m k]

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?

[Vince]

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 !  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 !!!    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 ! 

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 ! 
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 ! 

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

[Vince]

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.

[Vince]

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...

[TimFox]

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.

[Vince]

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.

[TimFox]

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.

[Vince]

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 

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

[m k]

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.

[Vince]

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 !    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.

[m k]

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.