Author Topic: Tektronix 317 classic scope - repair and restoration.  (Read 9081 times)

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Offline Vince

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Tektronix 317 classic scope - repair and restoration.
« on: October 29, 2017, 07:12:28 pm »
Hi group,

Next in line : the repair and restoration of my Tek 317.


Background:
------------------

That's my first tube based piece of equipment, probably the last as well since I don't intend to collect them, but I wanted to have at least one of them...
I was born in 1977 so not exactly of the tube era, so I wanted to have at least one instrument of this era so I can see it up close and fiddle with it, study it, and add to my little museum/collection of old stuff.
Electronics is basically 50 years of tubes followed by 50 years of solid state, so I can't possibly pretend to be interested i electronics, and wipe my butt with 50% of its history ! ...  and I just love old stuff and learning about the history of science and technology/engineering, fascinates me, the ongoing march of the human race...

I have a fetish for CRT oscilloscopes and like Tek Scopes particularly. I have one for the early '90s, fully digital (TDS 544A), then a hybrid/combo one from the '80s (Tek 2232), and one from the 70's (Tek 5111) that's transistorized, so I wanted one from the '60s, tube stuff, to complete the collection. I wanted one that looks really old, with a round CRT not a rectangular one. So that would be early 60's not late 60's.  The 500 series scopes seem to be by far the most popular, but they are very big and heavy as well as expensive, and I don't have much money to put in this purchase, nor much space for it in my tiny lab.

So, when I saw a local ad for a Tek 317, which I didn't even know existed, it was love at first sight ! It was small and beautiful to me, found it so cute !  Compact and light enough that I could actually consider keeping it in the lab to actually use it/play with at times, rather than use a hoist to put it in storage and leave it there sitting, out of sight, gathering dust. It looked in great nick overall, and even came with a few original accessories, which added value to it, in my eye. And it was cheap, 50 Euros... though it cost me a fair bit more than that to pay someone to pick it up locally (900kms away from me) and deliver it to me safely. But still, for a piece of Tek history, and a great piece of engineering, I didn't mind. Other thing going for it, was that the picture in the ad showed a trace on the screen ! So at least I knew it was not a basket case, which was good because I know zero about tube stuff, so I would be hard pressed to fix it !  :(  I called the seller, said it was powered up recently when the pictures were taken. So, looking good indeed, so bought it !

Got it a couple months ago, now having time to work on it.  In the meantime I learned some stuff about tubes, mostly thanks to that blog "Mr Carlson's Lab", a guy in love with anything featuring tubes, and very knowledgeable. Learned an awful lot of practical stuff, by watching his videos.
Actually his very latest video is particularly educational :



On YT there are also a few cool educational videos about electronics, that the US army did in the 40's, teaching the basics. Tektronix did something similar too, in the '60s. Came across one about tubes, which I liked, from 1943 :



Short but informative, and of course it's interesting to see how tubes were taught to people back when it was the current/state of the art of electronics, rather than a modern point of view.

The page on Wikipedia is interesting too.

Anyway, getting this old scope was also an excuse to try and learn about how tube equipment worked. I hardly intend to become an expert on the subject, but I wanted to grab at least the basics, so that they are not these magical and mysterious things anymore. I wanted to be able to make some sense of the schematics, follow the signal path, understand the structure of the various circuits.

The above material made that easy.. you gotta love the Internet.

My understanding
-------------------------

Basically it's quite simple. The tube behaves very much like a transistor, a FET transistor more accurately. With 3 active terminals (not counting the heater filament)  anode, cathode and a control grid. There is current flowing from the cathode (which emits electrons thanks to the heater energizing it) to the anode, positively charged, which of course attracts the negatively charged electrons.
Just like a FET would control the flow of electrons in its channel by applying an electrical field between gate and source, that pinches the channel, the tube can control the flow of electrons by applying a negative voltage between the control grid (equivalent of the Gate of the FET) and the cathode (the "Source" of the FET). The drain is of course analog for the Anode of the tube.

So that's it. When looking at the schematics you just have to picture a FET and you suddenly find yourself being able to make some sense out of the schematics, magic... and you realize that it's very much like the modern electronics you are familiar with, same building blocks/basic structures, except that it was implemented using tubes rather than transistors, but that's about it.

What can cause confusion is also that some tubes have more than 3 active elements/pins in them... the basic type, the triode, has 3 terminals as its name implies, but on the schematics there are also Pentodes which as the name suggests, have 5 elements not 3. But nothing to worry about : fundamentally it works just like a triode/FET, it's just that it comes with 2 more grids which add refinement to it, but don't change the way the tube works. In practice, to understand the schematics, you can basically make abstraction of these 2 extra grids and whatever circuitry that surrounds them. The control grid we are interested in, is always represented/drawn as the very first grid (grids are represented as doted lines), counting from the cathode, which is usually at the bottom of the tube symbol.
So to follow the signal path, just look for the cathode, then the first grid from there is the control grid/gate of the FET, where the signal is applied.

The two other grids  are there, as I understand, to improve the noise and bandwidth performance of the tube. The middle grid, second from the cathode, is called the "screen" grid. It helps reduce the cathode-to-anode parasitic capacitance, therefore improving the bandwidth of the tube.
Then the third grid, next to the anode, "plate" as it's also called, is the "suppressor" grid. Its purpose is to reduce the noise a tube intrinsically/by its very nature, generates, as the flow of electrons "crashes" and "bounces" off of the anode/plate. This is called "secondary emissions" (as opposed to the main/primary emission, from the cathode). The suppressor grid applies an electric field whose polarity and intensity, relative to the anode, tends to keep these naughty rebel electrons from flying too far away from the anode.

In practice these two grids are most often tied directly to the anode or cathode, or sometimes to an intermediate DC level using a simple resistor divider, and don't actually interact with the main circuitry/signal path. Very rarely (in this scope I mean, no idea about other types of equipments ! ), are those grids involved in the actual operation of the circuit. One example of that, (thanks to the great service manual which is very educational !) is the LV power supply section, where the screen grid is used to "inject" a signal to the tube, to modulate the input signal in a clever way, to cancel out the ripple voltage from the bridge rectifier.

So basically you have a simple triode/FET for most of the work, and when you require better performance on the noise and/or BW front, you use the extra refinement that the Pentode offers.  For example in the preamp section, the input signal hits a pentode, presumably because like with any preamp you are worried about the noise polluting your fragile weak signal. Then once the signal is cleanly amplified, the output of the pentode feeds a simple triode, which acts as a driver/buffer to safely drive the next stage : the vertical amplifier section.
Sometime, maybe to save space, you would have the Pentode and Triode packed into a single tube assembly. A single tube can also host two identical triodes, which is handy when implementing for example a differential amplifier, also seen in several places in this scope.

So that's my basic understanding of these old tubes. I hope some tube expert will stumble upon this thread and correct me wherever needed. But so far this simple understanding of things proved efficient in helping me understand this old scope !

And I am glad to understand it because... yep you guessed it : it's NOT working ! LOL  Yes, seller said it was working, and the picture showed a (flat) trace on the screen but.... I powered the scope and well, it kinda works yes, but no quite !

So I am glad in some perverse kind of way, because I will have now an excuse to really dig deep into the thing and spend time studying the schematics in detail, and probing all around the place !  Should be educational, and probably useful should I ever have to work on some other tube gear, though again it's not my intention to buy more of these.


So that's it for this long winded preamble/introduction. This old Tek is a work of art, the looks of it, inside and out, the build quality, the nicely laid out internals, the front panel that's elegant and beautiful, those lovely big knobs that feel so nice. This is a master piece of engineering and industrial design. I am so proud to own one of these, and I just can't get enough looking at it and playing with the knobs !   ;D    These old Tek scopes are a pure delight  :D

So, needless to say I will try to fix it and restore/refurbish the thing.

Anyway, let's get to the meaty stuff now :

1) Repair : what's wrong with it and what have I done so far ?
2) Refurbishing : what's needed, what's planned ?
3) Some pics of course !



REPAIR
------------

Before I spend time and money refurbishing the thing, I first want to see if it can be fixed at all ! My priority is to have it working so I can play with it, then only do I want to make it look good.

So, the symptoms are as follows : You power up the scope, it appears to work fine (tried all basic controls), but instantly, the trace on the screen starts to move up and up and up... and eventually disappears off the screen.  At the same time, the brightness of the trace progressively decreases, so much as to eventually become completely invisible. Cranking up the brightness/intensity control only buys you some time, but not much at all.  It all happens quite fast : in practice, once you have powered up the scope from cold, you don't get much time to play with the controls, before the trace is gone.  I will try to make a short video to better show what I am talking about.

From what I can see, it can display a sine wave just fine, clear and crisp trace (focus and intensity work fine), triggers fine, vertical and horizontal position controls work, time base works,  It basically works just fine... if only the trace didn't feel the urge to run away and turn to black !  LOL

My take on it : the fact that the scope works fine when cold, kinda leads me to believe that all the tubes are fine. I reseated all of them for good measure anyway, and not surprisingly it didn't change a thing. From what I understand, the failure mode of tubes is mainly a progressive degradation of performance due to imperfect sealing of the tube (ambient air being sucked in, seeping through the pin holes at the bottom, the glass not making a perfect bond with the terminals). But whatever problem this might cause, I think it would happen from cold, right from the beginning, and it would be constant/stable. This is not the case here : scope works just fine when cold, and the problem changes with time.
A trace drifting could be a warm up issue... but I don't think this is it, because it happens way too quickly ! So I think it must be due to some capacitive effect. A bad capacitor somewhere, or a failed resistor that makes a capacitor not work not in the way it was supposed to.  But something capacitor related, I think. Because the problem makes the trace drift vertically, I assume the problem must lie in the vertical amplifier or preamp. Either that, or a system-wide problem, ie the power supply. Had a first look at this side of things. IIRC there are 5 big "cans" in the thing, each of them actually hosting several caps. I see that people are used to "restuffing" these cans with new, modern low ESR electrolytic caps. I guess it would be good measure regardless. I don't have an ESR meter just yet, but could try and measure their value to get a clue maybe. Don't have a leakage tester either. Whatever, if the filter caps go bad so much that it makes the scope go nuts as much as it does, then surely it should affect the power rails greatly and this should be easy to detect just by measuring them and checking for ripple.
I gather that very old tube gear had paper&wax caps in them, that invariably leak and can cause all sorts of very serious problems because they don't block DC anymore, but this scope is not "that" old, it hit the market in 1959, and it being a Tek I guess they would have used the best caps available at the time.  I would need to give it a closer look, but at first I didn't see any of these caps in this instrument. So, if there is a failed cap(s) in this scope, I don't think it would cause a catastrophic failure. I am not too worried about that.

Then there is the problem with the trace intensity  getting gradually dimmer and dimmer. Again, the progressive nature of the fault points to a capacitive issue, in the HV/CRT section this time.

I have ordered an HV probe so I can poke around that area, but while I am awaiting delivery, I can start probing all the LV rails with my DMM. They range from -150V up to +420V, which a DMM should be able to manage/measure safely, I would think.


Service manual : too big to attach here, 60MB in all. tried to pull the schematics out of it but that 'still too big to attach (4MB or so). I might attach single/individual schematic pages though, as I go over the various sections one by one. Manual can be downloaded on this page, from a Tek related website. The page also gives some info about that scope :

http://w140.com/tekwiki/wiki/317

Direct link to the PDF :   http://w140.com/mmm/tek-317.pdf




RESTORATION
-----------------------
Overall it's in great nick, no major issues.  I mean, the case is not damaged, and no parts are missing, and there is no corrosion inside, and only a very minimal amount of dust inside. Would still like to clean its guts though, to make it all nice and shiny.

At the back, the air filter has seen better days. It looks dark yellow, like it was filled with honey or something... brrr....
Should be an easy fix though : the manual says that the filter is made of a metallic "wool", and is designed to be washable. So, let's wash it then...

At the front, looking good too. The front panel is in great condition, the lettering is in top shape, no buttons are missing nor damaged. It just needs a good clean. There is however a large/long scratch at the bottom left part of the panel, in the vertical section, but it is very light/superficial, not really a show stopper. It could probably buff out just fine but I fear this might cause slight change in the surface finish, which would be noticeable, and overall do more "damage" than anything else. So for now, I think I will just leave this scratch as it is...

There are a few bits and bobs that need some attention too :

The 3 coax connectors, which are "UHF" type rather than BNC, are rather crusty and one of them is even missing its dielectric piece ! So I ordered new ones. Looks however like changing them won't be fun, access to two of them looks somewhat challenging so to speak...

Most UHF connectors I see for sale, have a square shaped/4 hole base plate, but the connectors in the Tek are 2 holes things. 4 hole connectors would not fit, it would not clear the surrounding metal work.  Luckily I came across on Ebay, some 2 hole connectors, which therefore should be a direct fit, I am hoping. But they are cheap stuff from china, so I have yet to receive them but when I do, if I am not happy with the quality of them, I would have no choice but to buy quality stuff from known manufacturers like Amphenol or TE connectivity, but they only come in 4 hole versions. So I would have to saw/cut two of the corners to trim them so they can fit the chassis.. but no big deal, and actually, looking closely, it seems to me that it's exactly what Tek did back then ! So if they did it, why couldn't I...

The UHF-BNC adapter (required to hook the probe), is starting to show its age, nearly 60 years now. They are still made these days, so I ordered one of those too.

Then the neon indicator bulbs (for the UNCAL positions of the knobs) look tired to me, though I have no experience of them to be honest so can't really say for sure. Still, these appear to be standard items, called "NE2"  bulbs, which are still available today for little money, so I bought a few of them, will see if that makes any noticeable difference in brightness terms.

Then last : the cooling fan. it doesn't start right away. When you  power up the scope, it takes a minute or two before the fan decides it wants to start spinning. When it does, it is very quiet and with no parasitic noises. It's a silk smooth ride, quite impressed I must say ! Quite worried too : it's "too" quiet I think... I would expect such an old fan, in such a power hungry instrument, to be quite noisy. So I fear that it might be "tired", somehow.
I checked the schematics just in case there somehow was a timing mechanism somewhere, for some reason that escapes me, to purposefully delay the spinning of the fan. But no, no such thing : the fan is connected directly across the primary windings of the transformer, so it ought to start right away.
Well, it's not STRICTLY connected directly to the transformer... the schematics show a resistor in series with the fan. So maybe that has drifted over the many decades, and gone up in value, restricting current flow, which would explain things. Will check for that.


I think that's about it for the initial rambling/introduction. Sorry to all those who fell asleep before reaching the end,  :=\ and congratulation to all the others, you deserve a pat in the back   8)


I will now start working on the thing and post as I go, as always. Hopefully some Tube Guru will come by and offer guidance whenever I am stuck, out of ignorance.  I do have a clear path of investigation though, to get me started, and plenty of things that I can check, so hoping to make some progress by myself and bother the tube gurus only when I really run out of ideas.


PICS
--------
I opened the cabinet to expose its guts, from all 4 sides. As for the original accessories, you can see the probe and the inside of its body (a nice die cast aluminium construction !), the UHF-BNC adapter that goes with it, and.... and.... this huge eye piece, no idea how it's called exactly, which I think is used to see dim traces when the ambient light is strong. Especially useful I guess, because this scope as I understand it is designed as a "portable" unit to be carried around and used outside in the field, rather than in a comfortable and well controlled lab environment, which is more where the more popular 500 series scopes would be used, typically.... I think !









« Last Edit: October 29, 2017, 08:29:20 pm by Vince »
 

Online tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #1 on: October 29, 2017, 07:24:31 pm »
Do you know of member Martin.M ?
https://www.eevblog.com/forum/testgear/vintage-tek-restoration-pictures-by-martin/

Have fun with that glowing Tek Vince.  :)
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #2 on: October 29, 2017, 08:45:35 pm »
Hi Tautech !

Yep I know that page, you linked me to it a few weeks back when I mentioned my 317 in another of thread of mine. Sure is interesting, Martin has LOTS of these old scopes, impressed !

Actually I still have this page opened in my browser, still not finished reading it, I am on page 7 at the moment !

In these first few pages he explains how he proceeds to clean the internals of these scopes, just need to the same !  ;D 
Unfortunately I don't have a compressed air supply at my disposal just yet, so I am reluctant to use any liquid no matter how little, inside the scope, as I would have no way to force it out all efficiently and easily. Can't afford a decent air compressor at the moment. So I don't want to rush the cleaning part, end up doing a crap job that will ruin the scope, and make me really angry at myself ! So, I prefer to first fix it, because I can do that right now, then clean the outside, but I will leave the cleaning of the inside for later, once I have the appropriate equipment. Might take while though, as I have higher priorities at the moment... saving almost every penny as we speak, as I am very soon to start building my first house. You can only chase that many prays at a given time...
As soon as the house is built and the garage/workshop is ready, I will be able to get some more tools, and get more involved into restoring or building stuff.  8)

« Last Edit: October 29, 2017, 08:49:43 pm by Vince »
 

Online bd139

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #3 on: October 29, 2017, 09:56:59 pm »
I use paper towels, kitchen cleaner and my wife's hair dryer (when she's out  :-DD )
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #4 on: October 29, 2017, 11:09:06 pm »
LOL ! No wife/hair dryer here, but a hot air gun, might do !  ;D

I just shot a quick video as mentioned earlier. Don't worry it('s REALLY quick... just over a minute : when I said it took one minute or so for the trace to disappear, I was not lying !  :-DD

On that video it's not so obvious that the trace drifts upwards, but at least you can see that , from a cold start, it takes kinda exactly one minute for the brightness to go down to zero, even once I have turned it all the way to the max/CW.

You can also see that during all the video, no matter what happens, trigger and horizontal sweep are rock solid. So I am not going to spend to much time, at first at least, investigating this area. Played some more with it off camera, and indeed the vertical section is pretty sick, more than initially thought :

- indeed the trace shofts up and up, but it's hard to see because the trace disappears due to lack of brightness, before it disappears due to its vertical position.

- I now notice, that the trace expands vertically progressively, as if someone were playing with the red variable button...

- I also realize that said red button acts weirdly : it spins round and round ! There is no stop to it. I can feel the detent, and the associated "UNCAL" neon bulbnl lights up when expected, but there is nothing that keeps the know from going past that detent point. It can be spun CW or CCW indefinitely ! Maybe just a mechanical failure... needs investigating. Might also explain why the trace expands.
So I definitely need to have a real close look at this variable red knob.

BUT, I think something is clear now : the very, very thing problem I must tackle, is this brightness problem ! I mean, as I saw, from a cold start I get only 60 seconds to work with the scope, before the trace disappears for lack of brightness. Then I have to let it turned off for while, while the faulty caps discharge, then I can have another 60 seconds of play. Not very practical is it...

So I will work on this right away, and only once I can count on actually SEEING a trace on the screen... will I have the luxury of worrying about trouble-shooting the vertical section. First things first, eh .....

So there you go, a plan is shaping up !  ;D

So, what do I need for schematics then... here is attached : block diagram for the instruments, just because. Then the LV power supply, because well, that's the first thing to check. And then, the CRT circuit, to more specifically investigate the brightness problem.

As for the HV, as the manual explains, it's not just 7700 volts, as we can see on the anode on the schematic sheet, but rather 9kV because the Cathode is already sitting at minus 1300Volts or so.  The HV is generated from 420V unregulated, so at least I can check for that using my DMM, don't need to wait for my HV probe to arrive in the mail.



https://youtu.be/ZssrbExKi5M
« Last Edit: October 29, 2017, 11:10:37 pm by Vince »
 

Offline Cyberdragon

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #5 on: October 30, 2017, 05:45:46 pm »
Quote
- I also realize that said red button acts weirdly : it spins round and round ! There is no stop to it. I can feel the detent, and the associated "UNCAL" neon bulbnl lights up when expected, but there is nothing that keeps the know from going past that detent point. It can be spun CW or CCW indefinitely ! Maybe just a mechanical failure... needs investigating. Might also explain why the trace expands.
So I definitely need to have a real close look at this variable red knob.

no, that's exactly what it's supposed to do.

EDIT: unless someone broke both of the knobs in my 561 too
« Last Edit: October 30, 2017, 05:49:28 pm by Cyberdragon »
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #6 on: October 30, 2017, 10:01:54 pm »
no, that's exactly what it's supposed to do.
EDIT: unless someone broke both of the knobs in my 561 too

Oh, thanks for that !  Great then, one less thing to worry about  :)

OK, worked a bit on it last night.

Checked the LV power supply. Refer to the schematic I posted above....

So basically we have 3 secondary windings, and as many regulated rails: -150V  ,  +100V and +300V .

-150V is the one that's regulated, the other two are based on it.

I quite like this -150V regulated rail.... it's cute, it's nothing like I have ever seen in solid-state of course, yet you can instantly see that it works exactly in the same way : a reference voltage, here with V609, a neon bulb, instead of a Zener, then a resistive voltage divider tapping the output so you can adjust the voltage level to your desire, then apply both these things to the inputs of an error amplifier... no op-amp back then no worries, they just do it by hand, with a pair of triodes contained in V606, arranged as a differential amplifier, then drive the pass transistor... here  another triode,  V617B... and bob's your uncle !  :)

And let me tell you that this works really well indeed : I measured this -150 V reference at....   -149,95V !  and only 20mVpp ripple !
I am well impressed. The other two rail logically are very accurate as well : + 100,06V (10mVpp ripple) and +298V (couldn't measure the ripple, was beyond the rating of the scope probe... ).
Only thing that puzzles me, is the ripple... as you can see, its frequency is.... 17kHz ?! Where the heck is that coming from...


Before measuring this, I measured the outputs straight at the transformer taps.  I saw 182, 120 and 165V respectively for the 195 / 130 / 175 mentioned in the schematics. A tad low, but should be alright eh...   Checked also all the low voltage 6V windings that power the heater filaments for all the tubes... they are all good. Only thing as you can see (wave forms below), is the distorted signal, not exactly a pretty sine wave. All windings look  exactly the same, including the HV ones.  So I guess this must be inherent to the construction/quality of the transformer ??   I don't think I should worry about it...

Then I measured the UNregulated 420V (or just 400V, depending on where you look in the manual...) : 350V ! Measured again this evening, a minute ago : 390V better ! then powered it off and tied again a few minutes later : 370V !  So it a bit low eh, and not quite stable... this unregulated voltage (measured 4.6Vrms of ripple with the DMM) is used solely for the HV section, for the CRT, and nothing else.

What I also tried, once the trace had disappeared from the screen and the scope looked dead as a result... I wanted to see if there was still a sign of life: the front panel conveniently gives you access to the sawtooth output from the time base, so I looked at that. Indeed the scope is well and alive, the time base works just fine, I tried all ranges. Nice straight ramp, even on the fastest range (though the negative part of the ramp is not so linear, but that's irrelevant anyway).
You better watch where you put your fingers, though ! High voltages are not only present all around INside the scope... the output signals on the 4mm jacks are scary too : between 160V and 190V for the sawtooth, depending on the range. I also tried the "calibrator" (probe compensation signal), and boy the knob is not lying : forget about your traditional 500mV signal on most scopes... this thing can greet you with a 100V square wave !!!  Goes down to 20mV or so, pretty low !  I looked at it on the scope too, square wave is beautiful, a really nice square. Checked all ranges, voltage level is pretty darn accurate on all the ranges.
Only the frequency is a bit off : 870Hz not 1kHz.  Well, not that it matters does it.  It's not adjustable anyway, only the signal amplitude is, and it's already spot on, so I am not touching this.

So, in short : be it from cold/power up, or after an hour or two of running, long after the trace has vanished from the CRT... it's clear that all the voltage rails are well, and that the time base and calibrator are running just fine...  If nothing else, that means that the 60 year old electrolytic cans are not causing any problem, somehow !!! I guess they made good stuff back then ! LOL

The issues I have are then most likely not due to system wide circumstances, but rather confined to the CRT circuit and Vertical section.
So, as I said yesterday... I should logically first worry about the CRT problem, for I need to see sometihng on the screen if I want to get a chance  to trouble-shoot the vertical section ! LOL

So, CRT it is then...

Obviously there is a problem with the 400V it uses, as it's going all over the place, and is a bit low... so maybe the CRT circuit is loading this supply too much ?
Still, at power up, brightness is fine, the dim appears only gradually. So again must be related to some crappy cap somewhere.

This CRT circuit is actually much more complex/sophisticated than I imagined, especially for a 60 year old gear, especially when it used precious tubes which use up valuable space in this portable instrument and must therefore be used sparingly !
Just look at it... modern scope just use a HV tap from the main transformer... but this old grand-pa plays it SMPS style ! It takes the 400V DC then feeds it to an inverter ! The thing oscillates at 60kHz according to the manual, uh forgive me, 60 KC !  ;D
And the sophistication doesn't end there... this things is implementing a closed loop regulation ?!  :o    Boy if I had known such an olds tube instrument, with only 30 tubes or so in it, would feature such things !  Next person that tells me these old things are crap, will get a punch in the nose  !  ;D

So let's see that circuit. Where do find capacitors ? We have C825 which locally decouples the +100V rail... but I guess this is fine, otherwise it would have showed up when I measured that rail upstream, I think.
What's next ? We have C807 which locally decouples the 400V rail, which is tied to just one point : the primary of the invert transformer. So... I think this hold water what do you say ? A faulty cap at this location could well mistreat that poor 400V supply and explain the variations it exhibits. So I must check this cap... if I can find it that is !!!! Yes... that's something I have not yet mentioned but.. how the hell are you supposed to locate/identify components ion these scopes ?!  :o
The tubes and the few big cap cans are identified, yes, but the myriad of passives populating the ceramic strips, or just hanging in the air, going from point to point.. are not !  So it's not going to be fun...

I can also check that the frequency of the inverter is correct and stable, doesn't hurt. It contains two caps ( C805 and C806 ), so maybe these are faulty too, and cause the frequency to drop, hence the HV at the anode, hence the brightness going to null. That would hold water too I think ?

The feedback loop can be at fault too I think : there are two caps in there (C814 and C815 ) across the voltage divider tapping the HV.   So if tehy go bad, they might affect/ corrupt the feedback signal and therefore cause the inverter to vary its frequency / HV output in unwanted ways.  So I must check these caps too.

Then we have the caps in the secondary / HV part of the circuit.
At this point I think I may have an opinion on whether the brightness going to zero is due to the CRT anode voltage dropping, or the brightness control going bad : I think it's most likely the HV dropping... because the trace on the screen, expands vertically as it gets dimmer. I guess a dropping HV could cause this.

So, the HV multiplier is made of 3 diodes and 3 caps... again if these are leaky, sure enough that would not be good. So I can check these caps too... well again I don't have an ESR meter nor a leakage tester... but well, I will do what I can for now....

anyway, at any rate I am now moving all my attention to the CRT circuit. will remove the shield protecting it, and start probing around....

I am really impatient to receive my HV probe, though ! It's coming form the UK rather than China, so I am hoping to receive it soon ! Crossing fingers...

 

Offline Cyberdragon

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #7 on: October 31, 2017, 02:00:20 am »
Why assume C807 and not C660? If C807 leaked I'd expect R807 to be a goner! Also, C805 is a cermic and should be fine, suspect C806 though (paper).
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #8 on: October 31, 2017, 03:21:14 am »
Why assume C807 and not C660?

Simple ! ... because I didn't think of it !  :-// I was focused on the CRT section/schgematic page... I had moved on from teh LV power supply...   

Quote
If C807 leaked I'd expect R807 to be a goner!

More on that further down....  you might be right !  LOL


Quote
suspect C806 though (paper).

Is it ? You sure know better than I do. The only paper caps I saw on YT videos were the kind that looks like a miniature loo paper roll, with prints on it, then filled at both ends with wax.   I don't see anything like this in my scope. Can these paper caps come in more "modern" looking packages as well ? Look the pictures below : C806 is one of those black plastic molded thing, with red prints on it. Looks a lot more "modern" than the vintage wax caps...
But if you confirm they can be paper too, then that might explain why the oscillator doens't oscillate at the frequency stated in the manual. Manual syas that C806 is used to "tune " the oscillator.... maybe it's not tuned right then... assuming that by "tuning" they mean adjusting the frequency ?

Alright. So here is tonight report, spent a few hours on the thing, I think I am making some progress...

- At some point, had a Eureka moment, please dont laugh, young players mistake.... I had plugged the scope to my isolation transformer, as I always do, so I can probe around carelessly with my (other) scopes.  But my transformer is only rated at 100VA, enough for most purposes but obviously a tube scope is not "most purposes"... the thing is rated at 300W IIRC, 3 times the rating of my transformer ! I now understand why I saw 240V at one end of tghe power cord... and only 200 at the other end, by the scope, hum hum...
So I plugged the scope to a regular socket, and hey presto things are improving... cooling fan now starts reliably and sounds much healthier... now manages to make the workbench vibrate.... I think the rubber mounts onthe fan must be worn out, hopefully I can find something suitable to replace them.

Then rushed to check all voltages again, much better : all the secondary winding of the transformer now output exactly what the label on the transformer, states ! That's about 10V more than what's indicated on the LV power supply schematic.

The LV power supply still regulates just fine : zall 3 rails still dead acurate and stable, no worries. The change, welcome, is on that uinreglated 400V foro the CRT circuit !  Was low and wandering all over the shop between 350 and 390 or something, and now it's a much healthier 450V ! And it's stable now, as well.

So I am a happy camper. OK 450V is a bit more than the 400 or 420V you find int he manual, but can hey...

So I started poking around the HV/CRT section of the scope. removed the shield.. and the bar holding the leather strap of the handle, because it was in the way.

I looked at all the caps and C807 was the only electrolytic in sight, so that was my first target. It's the one for the RC decoupling network for the 400V supply, as discussed ab it earlier.  Could get  reading in situ, so pulled one terminal so I could measure it reliably. It read 12uF instead 8uF, 50% more.. well at least it's not shorted nor open circuit.   So I put it back in,power up the scope, measured the voltage across R807 whch goes with it.   15 volts or there about. I was calculating in  my head what that meant in term of the current begin drawn by the oscillator circuit, and how much power that resistor was having to dissipate....  then I heard a little noise, turned my head, then a brief puff of smoke... yep, that was coming from tht very resistor ! Nothing dramatic though, no explosion. Pulled it off to inspect it. The body is dark brown by design, so hard to see on the picture, but the body has indeed overheated, and one can just about sense a hair crack in the body of said resistor.  So definitely gone...  it measured at 220 ohms rather than the nominal 330ohms.

Rating wise.. 15 volts drop across 330 ohms (assuming it was still at nominal value before it died in, front of me), then that's what.. say 45mA going through it. No idea how much current the oscillator is supposed to draw, but 45mA sure sounds to be within reason, eh ?
As for power dissipation, 15Volts saqured, divided by 330, that's about 0,7W  ..which looks a bit on the edge for that resistor tha tlooks nothing particular in terms of package size. Must be 0.5W at best I guess. Also, 0.5W is givne for an ambient temp of 20 or 25 degress IIRC, but in this scope it would likely be much hotter, so the resistor would not be able to dissipate as much. So basically, I think that resistor was under rated power wise, what do you think ?
I replaced it with what I had at hand... lucky me I found one of the exact same value, but beefier. Power rating is not printed on it, but judging by the size of the package, I would say it's good for 2W at the very least, maybe 3Watts.  So, put that in place.  No change... but still, had to be done, so one less thing to worry about !

Then I measured what I could, armed only with my DMM, because no HV probe just yet, and voltages are too high for the scope even with the x10 probe.

I tried measuring the frequency of the oscillator. Manual says 60kHz, and my DMM can measure up to 500kHz so its good enough for the task at hand. I measure across C806, the tuning cap across the primary of the transformer, got about 42,5kHz.  Then powered up the scope, let it rest for a few seconds... powered it back on, hey presto, the trace is back on the screen.. only for a few seconds before it vanished again, but a few seconds is all I needed to make a second, comparative measurement. Result ? Teh frequency is exactly the same when the trace shows up, hence when the HV is good.
So, it's not 60kHz but it's clearly not where the problem lies.

Then I looked at the feed back  loop : measured the image of the HV, at the output of the voltage divider. Schematics says it should be -158V but I got -150V. So, again cycled the scope to bring the HV/trace back up to normal... and.... it still gives the exact same reading, -150V.

So.... at this point I would say that the entire inverter works just fine :   400V supply, oscillator, transformer/HV secondaries, and the feedback loop.

So now it's all happening on the HV side of the CRT circuit. The multiplier needs to be checked stage by stage... it uses what looks like special purpose/heavy duty high-voltage caps, so I would assume for now that they are good, not being able to test them properly at this stage.
But what about the 3 vacuum diodes ?  Are these things reliable ? I noticed that one of them looked a bit odd inside, compared to the other 4, like it had over heated of worn out or something... will try to take a pic.  Maybe once its filament heats it up, it start going bad ?

I guess there is one thing I could do, though, in this multiplier, despite the lack of HV probe : the filaments I guess are low voltage like those in the triodes ? So even though the multiplier is producing scary high voltage, I should still be able to probe in there, and stuff the DMM probes across the two terminal of the filament.. without risking my life ?! ....  :o

Well, I gotta try t find ways to make progress despite only being able to use a DMM.

The HV probe will possibly arrive later this week, so no need to rush things, let's just wait for that probe to arrive and then I will be able to probe the HV parts at will and stop that guesswork...

Anyway, making progress I think !  :)



 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #9 on: October 31, 2017, 11:29:15 pm »
Couldn't do much today, really need to wait for the HV probe to arrive.

Had a quick go at measuring the voltage across the filaments of the 5 tubes.... no joy, it's all over the shop, couldn't make any sense of it. Never mind...

So instead I looked more closely at that rectifier that looked a bit weird. I am no tube expert that's for sure, but still, it really looks shot what do you think !  :o

It's V824, actually it's not part of the multiplier for the anode of the CRT, rather its the tube used in the intensity/brightness control circuitry... mmmmmm..... interesting.

I attached a close-up pic showing the weird looking part of that tube, next to a healthy one, or so it seems. It's located right at the top of the tube, where the anode is. It was hard enough to make a decently helpful picture of the problem, so I worked the pic a bit with some colour.

It seems that the delicate metallic structure inside the tube, is damaged, broken. There is what looks like a little annulus (in yellow), connected on two tiny posts (in blue), for want of a better word.  I guess it must be spot welded or something... a really tiny spot it must be ! 
One of the two joints broke off (metal fatigue due to repeated heat cycles maybe ??), and the annulus lifted up. The metallic  looking coating inside the tube in this area (in green),  followed the same path.  Then the entire area around that annulus, is like burned up/cooked, making it difficult / impossible to see the annulus on the picture. But in the flesh, I can see it... just.

Rectifier model is called a  " 5642 " . Just did a search, it's still available. 15 bucks or so, in specialist websites, but I found NOS items on ebay from the UK, for very cheap, only 3+ Euros !  So I bought 5 of them so that I have enough of them to rebuild the entire CRT section if need be, in the future (or right now as preventive maintenance ?). 

http://www.ebay.fr/itm/Mullard-5642-DY70-subminiature-halfwave-rectifier/390863449752

UK means it should arrive in just a few days !  :)  So hopefully I can fix this quickly.



« Last Edit: October 31, 2017, 11:36:50 pm by Vince »
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #10 on: November 01, 2017, 01:47:10 am »
Oops forgot.... I must find and buy a bit of that 3% silver solder that's required when working on these ceramic strips !

Where the hell am I going to find this stuff in 2017... and will it be sold for an arm and a leg... hmmm....


 

Offline Cyberdragon

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #11 on: November 01, 2017, 02:28:37 am »
LOL, that tube was assembled lopsided at the factory. That darkening is probably just a bit of the getter (matallic stuff) that splattered during the firing (metalizing) due to the ring being crooked. It's probably harmless if the tube worked in the first place, just don't jar it enough for the other weld to break.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #12 on: November 01, 2017, 02:37:24 am »
Ah well.... too late, new ones are ordered already. Will change it anyway then, can"t hurt... at least I won't be disappointed /surprised if it doesn't improve things at all...
 

Offline richnormand

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #13 on: November 01, 2017, 08:56:36 pm »
Oops forgot.... I must find and buy a bit of that 3% silver solder that's required when working on these ceramic strips !

Where the hell am I going to find this stuff in 2017... and will it be sold for an arm and a leg... hmmm....

Found the Tek manual for my 545. There is a page on how to deal with the ceramic strips. Can scan it for you if you wish. A low power (75W) iron with a bit of normal lead solder is acceptable in low quantities.

Also some scope of that era had a bit of silver solder included on a post inside for repairs. Might be worthwhile to look over in yours.

Finally some of the lead-free solder these days might contain enough solder to be compatible?





« Last Edit: November 01, 2017, 09:23:20 pm by richnormand »
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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #14 on: November 01, 2017, 09:26:01 pm »
Oops forgot.... I must find and buy a bit of that 3% silver solder that's required when working on these ceramic strips !

Where the hell am I going to find this stuff in 2017... and will it be sold for an arm and a leg... hmmm....

If I remember correctly years ago when I repaired my Tek 545 scope there was a about 10 to 15cm of silver solder on a storage post inside the scope to use on those ceramic strips.
Give me a day or so and I'll try to locate it.
Mmmm, I thought 3% silver would be easy....it is, but Pb free only ?   ???  :-//
http://nz.rs-online.com/web/c/tools/soldering-desoldering-tools/solders/?searchTerm=solder&applied-dimensions=4294768924&pn=1

More luck with 2%.  :phew:
http://nz.rs-online.com/web/c/tools/soldering-desoldering-tools/solders/?searchTerm=solder&applied-dimensions=4294834805

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Offline richnormand

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #15 on: November 01, 2017, 10:02:17 pm »
From W9GB on that site:
http://forums.qrz.com/index.php?threads/tektronix-style-terminal-strips.240478/

""""
Many older Tek scopes had a small loop of silver alloy solder inside for that specific purpose.

You can also use Kester Pocket PaksĀ® exclusively available through NTE Electronics dealers.
These are usually $ 2 to $ 3 USD per pocket tube
http://www.nteinc.com/kester/pocketpak.html

NTE # 83-7145-0415
SN62 / PB36 / AG02 Elec. Silver Solder Pocket PakĀ®, 0.020 dia.
""""

Post is quite a few years old so prices may have changed and as per my 545 manual a small amount of regular solder should not be an issue if used in small amounts not to leach the silver out.

Hope that helps. I have to dig up my 545 from the pile in the back of the garage to have a look now. Have a look in yours.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #16 on: November 01, 2017, 10:21:10 pm »
THanks guys !


Yes you are right, this stuff is still available today ! Great  :)

My "local"/European supplier has plenty to chose from, had to narrow the search criteria down quite a bit not be overwhelmed by the results.. Here it is :

https://www.tme.eu/fr/katalog/fils-a-souder_100482/#id_category=100482&s_field=niski_prog&s_order=ASC&visible_params=2%2C74%2C74%2C142%2C436%2C909%2C1624%2C1677%2C1682%2C1683%2C1683%2C2953%2C2953&used_params=74%3A24545%3B1683%3A11119%3B2953%3A398251%3B

So for 5+ Euros only, I can get 100G of the stuff, leaded + 2% silver, even get to chose the diameter.   Affordable yet gives plenty enough of the stuff not to run out any time soon.. so gonna order that.  I already placed an order with them last week, they shipped in 36 hours ! So I will have the stuff in time for when the new tube rectifiers arrive from the UK, probably next week.

One less problem  :)

Very kind of Tek at the time to give some of the stuff in the instrument itself !  How thoughtful. Companies aren't like this anymore, that's for sure...  :-/

Spent a couple hours going though all the tubes of the instrument, one by one, schematics at hand. The instrument is really well laid out, it's all organized logically, it "flows" nicely, very easy to follow. I now feel more a tone with the instrument, I know what tube (or small groups of ) does what, just by looking at where it is in the instrument. Granted, there are only 30 or so of them...

In the process I eventually found the voltage reference for the LV power supply. It's physical package doesn't look like like a neon bulb, instead it looks very much like a little tube, in a socket.. but with only two pins on it of course.


 

Offline Cyberdragon

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #17 on: November 01, 2017, 10:50:59 pm »
Oops forgot.... I must find and buy a bit of that 3% silver solder that's required when working on these ceramic strips !

Where the hell am I going to find this stuff in 2017... and will it be sold for an arm and a leg... hmmm....

Found the Tek manual for my 545. There is a page on how to deal with the ceramic strips. Can scan it for you if you wish. A low power (75W) iron with a bit of normal lead solder is acceptable in low quantities.

Also some scope of that era had a bit of silver solder included on a post inside for repairs. Might be worthwhile to look over in yours.

Finally some of the lead-free solder these days might contain enough solder to be compatible?

Unfortunately, I can tell from experience that the original solder has indeed expired and will cause nothing but smoke and sadness. :-- It might be usable with additional flux, but IDK.
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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #18 on: November 01, 2017, 10:59:22 pm »
You'll love that solder Vince, the first I bought ~20 years ago and was I think this one:
http://nz.rs-online.com/web/p/solders/0551665/
It's now reserved for special occasions as it's twice the price it was back then.  :o

The low temp feature was what I wanted to minimise risk of PCB traces lifting before I had a temp controlled soldering iron. These days I use it to dilute large thermal mass joints done with that Pb free muck like on PCB's with a large heat hungry Gnd plane.

Excepting for anything like above I use any Pb solder I can get my hands on. Still got ~1 kg of Pb solder so that should see me out.  :phew:
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #19 on: November 01, 2017, 11:12:24 pm »
You'll love that solder Vince, the first I bought ~20 years ago and was I think this one:
http://nz.rs-online.com/web/p/solders/0551665/
It's now reserved for special occasions as it's twice the price it was back then.  :o

Yes that's the one I just linked previously, 36/62/2 ... just not the same brand, but composition is the same so the physical properies/melting temp ought to be strictly the same  :P


Quote
Excepting for anything like above I use any Pb solder I can get my hands on. Still got ~1 kg of Pb solder so that should see me out.  :phew:

You mean 100% lead ?  The usual 60/40Pb is still available it seems, and quite cheap, 25 bucks for one kg.
 

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #20 on: November 01, 2017, 11:19:33 pm »
Excepting for anything like above I use any Pb solder I can get my hands on. Still got ~1 kg of Pb solder so that should see me out.  :phew:

You mean 100% lead ?
No, any solder with a significant lead content as opposed to that Pb free muck.  :--
60/40 or whatever. I don't much care. They all work.  :)
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #21 on: November 01, 2017, 11:22:44 pm »
Ah OK, well then no need to panic as it's still available... no risk of running out  :)
 

Offline flowib

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #22 on: November 04, 2017, 12:22:08 am »
Replace the black coupling capacitors, my bet is that your problems will go away. I replace those before even turning them on, Also replace them in the power supply. They are nearly allways leaky. Axial replacements are easy to find.

The HV rectifiers can be replaced by 15-20mA 5Kv high voltage rectifier diodes.
I got 5 Tek scopes, and all the spares from tearing down gutted chassis, if you need anything i think i can help out..

I had a 543 where the -150V ref kept drifting, turns out it was the 0.01 coupling cap on the 12AX7 grid.  Also fixed the high voltage on two of my scopes by replacing these.

Should give my 585A some TLC sometime.



« Last Edit: November 04, 2017, 12:28:27 am by flowib »
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Offline Cyberdragon

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #23 on: November 04, 2017, 12:52:23 am »
He is replacing the paper caps. The problem is identifying them, apparently there might be some square ones in here as the manual identifies paper caps but it has what looks like micas. There are such things as square paper caps, so I'll have to find the color code to identify them. Although, assuming the manual is correct, replace the ones in the parts list that are labeled "PT" (paper tube) even if they are square.

Also, don't just go replacing rectifiers unless they are bad it's pointless and looks tacky. :--
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #24 on: November 05, 2017, 01:33:38 am »
Thanks for the advice chaps.  As a result I spent a few hours working on the thing, investigating the capacitor side of things...

Went through the parts list and searched for all the paper caps. They are labeled " PTM ", Must be "Paper Tube Molded" or something ? Paper anyway...

Then marked all these on printed copies of the schematics, so I can better visualize where they are and what they do, to see which ones could be responsible for the symptoms I am seeing.

Then I went to the scope and searched for each of these paper caps, to see what they physically looked like.

I also wasted some time on some "caps".. that turned out to be resistors, but their package made them look more like caps so I got fooled ! You live and learn. These old instruments sure know who to make you look stupid and ignorant !  :-DD

Anyway, apart from the few paper caps which are used for AC coupling between stages, which come in metal cans, all other paper caps come in the same type of package : yes, they are indeed those axial black SPRAGUE caps with red markings.

I might replace them all eventually, to make sure the scope performs as it should, and is reliable in the long term, since I sure intend to keep it... but right now is not the time. I am not going to mass replace caps all over the scope until I know exactly where the problem comes from. Otherwise I might fix the scope more quickly but I will however not have learned as much as I could have... and I want to learn from this scope at least as much as I want to fix it. And it appears, looking at the schematics, that most of paper caps are places where they clearly can't cause any of the problems I am seeing, and/or have been ruled out by previous testing. Let's go over all the schematic sheets methodically :

- LV power supply : 3 of them in there, but all rails have been tested 100 healthy : dead accurate, low ripple and stable. This also rules out all the huge filter caps / electrolytic cans as well.

- Preamp section : one paper cap (can type) used for the AC coupling of the input signal. AC coupling works fine and has been ruled out : I tested using DC coupling, problem still there. There is another AC coupling paper cap , can type too, at the output of the preamp, feeding the vertical amplifier. But again this works just fine, and has been ruled out easily by using the appropriate vertical sensitivity setting : anything from 100mV/DIV upward, bypasses the entire preamp section altogether and it goes straight to the vertical amplifier directly.

- Vertical amplifier (which is half of my symptoms ->  the vertical drift of the trace) : zero paper cap in there.

- Delay line : obviously no paper caps in there

- Time-base Trigger : one paper cap for AC coupling of the trigger signal. I use DC coupling and works fine, and even if the cap were bad, it's not even used/in circuit, so can't possibly affect anything.

- Time-Base generator : no paper caps in there.

- Timing Switch : these are all the caps wire directly to the time base range rotary switch, to define the various sweep speeds (and the corresponding "hold-off" timing).   There are 4 paper caps in there. But again there easily ruled out because they affect only some sweep speeds (the slower ones, which requires larger caps/delays). The symptoms  I am seeing happen regardless of time base setting, and the time base appears to work just fine anyway. The worse that could happen is for the corresponding sweep speeds (1ms up) to not be accurate anymore, (I will check for that once the scope is fixed), but this is not going to cause the trace to drift upwards nor the trace on the screen to go dim and vanish.

- Horizontal Amplifier : zero paper caps in there.

- Heater Wiring Diagram : no paper caps there either.

- Calibrator (probe compensation signal) : no paper caps in there. Also, as reported earlier, this part of the scope works like a charm. The output frequency is a tad low at 870Hz , but nowhere does the manual claim that it is meant to be spot on 1kHz. Might tweak it later on though, to bring it back to 1kHz.

- CRT Circuit : last section, and the one that pertains the most to my dim trace problem of course. I attached a copy of it, see below. This is where most of the paper caps are !... no less than 12 of them !  I highlighted them in yellow on the schematic, see for yourself. However I failed to spot most of them in my scope !  turned out I am very lucky : as you can see in the parts list, only the early units (first hundreds ) had all these paper caps, but  my unit happens to be serial number 2369, and for these layer model Tek replaced most of these 12 paper caps, with ceramic ones ! Yeah !  8)   The entire HV side of things is covered : the multiplier, hooray, the intensity/brightness, and the CRT cathode side of things. How lovely. That will make my trouble shooting easier, I will assume that these caps are good, at least at first.
There are however two papers caps that they did not replace with ceramic ones. I circled them in red. They are both in the LV side of the circuitry, in the inverter.   AS you can see it's C806, which tunes the frequency of the inverter. I pulled one leg of this cap to measure it... came out at 3 times its nominal value. For a tuning caps, sure can't be good ! Might explain why the inverter runs well below its advertised frequency, 45kHz instead of 60kHz.  Will replace it and see if that brings the frequency back up.  However, again, I don't think it's the problem, due to my earlier trouble-shooting which ruled out the inverter : it runs at a constant 45kHz or so, doesn't change regardless how bright or dim the trace is on the screen.

Then the second and last paper cap : it's C815. It's part of the feed back loop, doing maybe some filtering at the control grid of the error amplifier, because the feedback signal is rectified by V820 but I assume it needs to be a fairly stable DC voltage for the error amplifier and inverter further down, to operate properly.

However, again, prior testing showed that the voltage level at that point of the circuit was perfectly stable and did not change regardless of how dim or bright the trace was on the screen. So it will replace it later probably, but for now I am not holding my breath : I doubt it's the cause of my problem. For the record, I did lift one of its legs and measured it : no short no open circuit, just reading a bit high like his brother in the inverter. Nominal is 47nF, measured at 71.

So, at this point, it still all seem to point in the same direction : that V824 rectifier in the intensity/brightness control circuitry, rather than the inverter or HV multiplier for example.

Hoping/expecting to get the HV probe early next week, and finally have a definitive answer on the matter !  :)

It begs the question, though : why did they go to the trouble of replacing all the caps in the CRT section....but left these two paper caps in place ?! Surely there has to be a good reason !  For C815 I think it may be because it's a bit on the high side : all those that were replaced were in the single digit nF range, 10nF max, but C806 is 47nF !  A corresponding disc cap probably would not have been practical (too big and/or too costly) or simply not available at all !
As for C806, it's only 1nF so the size can't be the problem. However it has a critical role of tuning the LC oscillator of the inverter. Maybe the big ceramic disc caps possess some intrinsic/particular properties, which the paper caps don't,  that might make them UNsuitable for this type of application ?


That's all for now !  ;D


Oh ! Forgot... I also did a bit of troubleshooting on the vertical drift issue, some very interesting/promising results, more on it soon !  :P  .... but let's first finish to trouble shoot this HV/dimming trace problem !  ;D



« Last Edit: November 05, 2017, 01:50:37 am by Vince »
 


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