Author Topic: Tektronix 317 classic scope - repair and restoration.  (Read 9111 times)

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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #50 on: November 14, 2017, 10:49:40 pm »
Worked on it some more.... I received my HV  DMM probe, as well as the 4 NOS 6AU6 tubes !  8)

- Cooling fan

When it does feel like it wants to spin at its normal speed (ie when it is in a good mood), it makes the whole scope vibrate, and the work bench with it ! I assumed that the 3 rubber mounts holding it, must be perished by now, and gone too soft. But then they didn't look, to me, sooo far gone as to allow for so much vibration.
I removed the air filter, and was overjoyed to see that the fan assembly can actually be pulled as a unit from the outside ! Great. At first I thought I would have to battle to work on the thing from the inside of the scope, which didn'l look like it would be much fun at all...
As I pulled the fan  (itself,) free from the shaft of the electric motor, I realized boy just how soft the blades were ! Look like it's made of aluminium, and a very soft one at that. SO soft that it might as well be pure aluminium rather than some alloy.  So it really doesn't take much at all for the blades to bend quite a lot, even if you are careful. So... checked all the blades one by one, found a couple that were noticeably bent, enough I though, that it might create an imbalance strong enough to make the motor vibrate. So I bent all the blades back to shape one by one, just by eye... put the fan back onto the shaft, turned it on... waited for the motor to eventually start spinning at its normal/full speed, and hey presto, no vibrations anymore !  :)
So that's one more little problem that's fixed, for zero penny !  I am particularly happy because I just coulnd't see how I could do to replace these rubber mounts... obviously NOS items even if I could find some, would be expensive and just as perished as mine, and fitting something non-original would required some work and look a bit out of place, inevitably.

So quite happy with the fix, simple yet effective !  8)

- Vertical amplifier : variable cal knob that makes the trace go up and down !

This weird problem is now fixed as well !  8)  It has nothing to do with the health or lack thereof, of the tubes. This strange behavior is absolutely known and talked about in the Tek service manual !!!  In the calibration procedure, when you go to calibrate the vertical amplifier, they tell you that, once obviously you have checked the power supplies, then the VERY FIRST thing you must do before attempting to calibrate the thing, is actually to fix this very problem !!!

And there is a control dedicated to fixing this issue, it's even right there on the FRONT panel ! Sure the control is recessed so you can't move it by accident, but it's there, readily accessible, and is meant to be adjusted as required, during normal operation !  All you have to do is tweak it with the tongue at the right angle, while fiddling with the red variable cal knob, until you can cancel any vertical offset.    The control is called "Variable Attenuation Balance".

I quote the service manual :

Quote
21 - Variable Attenuator

This adjustment is performed by the operator of the oscilloscope during the course of normal operation. However, the maintenance technician must also make the adjustment before he can proceed to calibrate the vertical amplifier.

Misadjustment of the control is indicated if the entire CRT display is positioned vertically as the variable attenuator control is rotated. To perform this adjustment, it is first necessary to get a horizontal reference trace on the CRT. This can be done most easily by turning the red TRIGGER SELECTOR control to AUTO, and the TIME/DIV switch to 1ms.

With the trace vertically centered on the CRT screen, adjust the VARIABLE ATTEN. BAL. control so that the trace remains stationary as the red VOLTS/DIV. control is turned back and forth through its range.




Did that, works just fine. This problem is now gone !   :-+ I notice though, that the little pot that's concerne dhere, is in bad mechanical shape : the 3 metal abs that wrap/secure the two plastic halves of its body together, have a bit of play. The thwo halves can be wiggled a littel bot. However in practive this does not appear to be so bad as to make the pot lose its mind. Once set, it stays there. Still, I might pull it out the scoep to try and see if I can tighten up the tabs a bit, should be possible.



- CRT HV

The new tool in the lab is just arrived !  ;D   At first I could not get a stable reading. I unscrewed the gold tip from the probe, only to find out that the thread on the probe (which is not gold plated) looked crusty. I cleaned it with a brush and some contact cleaner. Screwed the tip back on. Then cleaned the test point on the scope itself (a jumper solder between two slot in a ceramic strip), and yeah, I could not have a very stable voltage reading, lovely  8)

It read a fair bit low : - 1225V (cathode voltage) instead of the required -1300.  Then I measured the voltage straight at the output of the HV multiplier. Supposed to be 7700, was 7200 IIRC. That's consistent : the CRT voltage comes from a tap on the same winding that feed the multiplier, so the th cathode and anode voltage vary in unison by a fixed ratio.

Then I tried to adjust the HV pot. For safety, I was reluctant to use  metallic screwdriver... and I didn't have a plastic one. I mad ea compromise : I grabbed a piece of heat-shrink tubing and fitted that to my flat bladed screwdriver, leaving only the very tip of the blade, apparent.  Worked fine, I am still alive.
However I could not adjust the HV reliably : no matter which way I turned it it, it would be go all over the shop and do silly things. So.. power off the scope, some contact cleaner in the pot, exercised it a bit... yeah back in business, I could now adjust, and measure, the HV perfectly smoothly, a joy.  The pot gives you quite accurate control, I was able to set the tset point/cathode voltage to pretty much spot on -1300V, very happy with that  The anode/multiplier voltage when up as expected, but not quite 7700V. I think I got 7620V or something IIRC.   So a total PDA voltage of 8920V instead of the required 9000V. Still that's less than 1% off, and the service manual says to check the cathode voltage with a meter with a 3% accuracy ! So I though I was just fine and I let it at that !

I played with the scope for a few hours. I checked the cathode and anode voltage one more time before turning it off for good, and both voltage were still spot on, still exactly where I set them to hours earlier. So, perfectly stable. At least one part of the scope that I can now count on !  8)



So, nothing extraordinary I admit, but still, I am happy I fixed these 3 little things. 3 less problems to worry about, 3 things out of the way ! Getting there progressively, a little at a time...


As for the NOS 6AU6 tubes for the first two stages of the vertical amp, the seller sent me 2 Japanese tubes branded "Mazda" (hoping their tubes  are as reliable as their cars...), and too  local/French tubes branded "Radio tubes Paris".
Obviously it is impossible to prove whether they are actually NOS or not, but well, at the least they come in their original little box, and physically definitely look like new.

Will play with them and report back later...

« Last Edit: November 14, 2017, 11:14:17 pm by Vince »
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #51 on: November 19, 2017, 09:34:32 pm »
OK, spent many an hour on the thing again...

Played with the NOS trubes, swapping here, swapping there... I can't seem to get the exact DC levels indicated in the schematics. I can get very close, almost spot on.. (within a few tenths of a volt), but never spot on. .... in these circumstances, the best I can get is less than a 2 major divisions offset on the screen. Still a lot, but can be easily compensated with only a 1/8th of a turn on the vertical control knob.
So I started to think that probably I am chasing a red herring again (a trademark of mine for sure...   :-\ )and that the figures indicated on the schematics are merely given as a guidance, rather than accurate values that require to be achieve for the scope to work properly. Plus, people on TekScope said that these scopes were designed such that they would will work just fine even with pretty worn out tubes that would even fail in a tube tester, but work just fine in the scope !  A bit like the good practice design that dictactes that a good transistor circuit should be able to operate properly regardless of the huge variations of Beta from one transistor to another.

So, at least for a moment, I took the problem the other way around (<which I should have done from the start I guess, truth be told...) :  start from the last/fourth stage of the amp, and work my way backwards.

So, I grounded the input, then centered the vertical control knob on the front panel, that brought the trace one or two DIV below the center line. The I measured voltages. Across the vertical deflection plates, I get like 15V or so. Let's say 15,8V for the case /round of measures for which I did take notes on paper.
I get that voltage upstream : at the input of the delay line, ie at the output of the last/fourth stage of the amp (V244 / V254). The upper tube gives 158,7V and the lower tube outputs 174,5V.  Take the average of that and you get 166,6, not far from the 168V quiescent voltage indicated on the schematic. So, Looks like it works fine ? The output is symmetrical, with each tube pulling or pushing an equal amount form the 168V bias point.
Well... that would be fine if I had an a signal at the input that is ! But there isn't any of course. So how comes the last stage produces such a large signal in the absence of an input signal...
So I checked the input of that stage, the control grids. I do a get a voltage difference there, a small one :  gird of the upper tube sees 88,9V and the lower tube sees 88,15V. That's not much of a difference is it... 0,75V.
How can 0,75 turn into 15V... well when you do the math, it's a x20 amplification, spot on. Sound like a nice roudn number, not too big, not too small... maybe it is the actual amplification that this stage is supposed to provide... so that would mean it works just fine ? Probably.... no luck then, still no answer to my problem !  :-/

So, I move upstream one step : the third stage (cathode followers, not amplifiers). These two cathode followers (V233 A and B) appear to drop about 3Volts each. The upper one gives 88,9V as siad earlier, and at its control gird I measure 85,0V. The schematics suggest 84V , one volt off.
As for the lower cathode follower, 88,15 at tit's cathode/output, and at the input/control grid, I measure 85,5V. Schematics suggests 86V, so 0.5V off.

So, the upper cathode follow drops (88,9V - 85V ) = 3,9V.
the lower tube drops (88,15V - 85,5V) = 2,65V.

No idea if this is normal or a sign of a problem. Both triodes are in the same tube technically, so you would think they are decently "matched".  so either the mismatch comes from the external circuitry and is normal (since I measured all the resistors and they are fine), or from a defective tube. 

I pulled the datasheet (attached) for the 6CL6 tube (amplifier/fourth stage)... boy I can't believe it but it's actualyl available. First time I look at a tube data sheet... not exactly my era. Still, it looks surprisingly familiar... data sheets are all allike I guess, be ti tubes or integrated circuits ! LOL

Looks like the transconductance is of the thing is huge, 10,000+ at the least... so given that we have onlky a gain of 20, that means that must have built bags of negative feedback in, plenty enough I guess to indeed make the amp/scope immune to variations in transconductance from one tube to another, and for a given tube as it ages/degrades.  So, I guess this is one more clue that this stage is operating properly, I guess... so back to square one : WHAT is wrong in this amp....  :(

Looks like I will need to seek help from the tube expert on TekScope...

Was starting to make me a little depressed, so to keep moving, I decided to check the calibration of the vertical section. After all, the offset can be easily compensated for, using the verticak control knob on the front pane, and the calibration procedure doesn't state that it is mandatory to have zero offset in order to be able to calibrate the thing.... maube it was implied, oh well...   The calibration is pretty simple/quick to do anyway, so I didn't mind having to do it all over again, should fixing the offset problem happen to affect the calibration.  At least it would give me a first idea of where the scope stands on the vertical side of things.

So I did that. Followed the calibration procedure very closely, no skipping any step fo course, doing it all by the book.. while looking at the schematics to try and understand why the calibration procedure was sequenced in the way it was. This way you get to learn a thing or two on how the scope works/is designed, always interesting...

First thing yo need to do is calibrate... the "calibrator" (probe compensation signal), because that is what they then use a reference to perform the calibration.
If you turn the calibrator knob ot the "OFF" position, you get a 100BV DC voltage on a test point in side the scope. Because it's DC and not a square wave anymore, you can easily adjust accurately with a DMM, which I did. the cal pot allows for an accurate setting, took only seconds to bring it spot on to 100V.  Was only a tad too low, just a hair.

Then you can start to calibrate the vertical amp :

- First the "Variable Attenuation Balance" control, which I talked about in my previous post.

- Then the (main) amplifier gain.. was quite a bit off, 20% or so. Now fixed. It's trouble-less : once set on one range, it stays correct on all the other ranges as well.

- Then we adjust the gain of the pre-amp (for the AC only, high sensitivity ranges). Easy enough again.

- Then the attenuator high-frequency compensation : you can't use the internal calibrator signal anymore, since its edges are not fast enough. So I used an external signal generator. Looked pretty decent to me so I didn't touch anything... I left all those little ceramic trimmer caps alone...

- Then the attenuator Input capacitance : goal is to compensate the scope to work with the bespoke x10 probe.  Yes you compensate the scope itself.. not the probe, as you would do with more modern scopes.
I do have that probe, somehow still works fine 55 years later... a tad under copensated it seems, buit nothing dramatic... I chose to play it safe and not mess with those ceramic trimmers...

- Pre-amp low frequency compensation : was impossible to do because there is too much "humming" in the way. In this case the manual suggests to put the cabinet covers back on, but I won't do that until the scope is fully restored/cleaned, and ready to be put back together.  So, I might come back to play to this particular step, later on. From the looks of it though, it again seems a little under compensated.

- Then there is ..... there is.... oh my god.... there the delay line !!!  I guess you need a PhD in astrophysics to master the calibration of such a thing, so I didn't even consider, not even for a split second, messing with this thing ! LOL   Especially since the scope works just fine as it is...


Then... I went to check the horizontal side of things, the time base.

Overall it was half a major division too short. Not a big deal, but still plenty noticeable, so I adjusted that.

Manual tell you to use a "Time Marker" generator... never seen on of these, so instead I used my programmable pulse genrator. allows to prodcued calibrated pulses, and can run in "continuous" mode, making it kinda like a signal generator.
Only problem is that said pulse generator is defective : the fequency is very unstable, jumps all over the place. So in order to keep an eye on it, I added a frequency counter to the mix : I thought my old 8 digits Nixie tube counter would be fitting with that old tube scope. The pulse generator was kind enough to at least give me the first 4 digits stable. Plenty good enough for the 1% accuracy we can about dream of, for the scope. So I used this setup, while going through the long and tedious calibration procedure...

The first few steps are pretty straight forward : adjust the gain in "Magnified" position, the adjust the gain in "normal" mode. this takes seconds and allowed me to get an accurate time base easily. Then comes the more strange things, like this "Magnifier registration" setting... boy if you don't know about it, no way you could figure out how to calibrate it.  The the less fun stuff : going though the fast sweep speed settings, and fine adjusting the sweep speed AND linearity, via ceramic trimmer caps. The sweep speed and linearity interact with each other, and no matter how long you play with them, you just can't get set it 100% right. you have to come up with the best compromise and accept it as it is. The manual doesn't even pretend otherwise, they just tell you basically : "good luck with it !!! " LOL   I tried it for "fun", to get a feel of it, on the 1us/DIV setting... saw how much of a headaches I was getting, and promised myself not to mess with the other sweep settings as a consequence ! LOL   Well unlike I had too... fortunately I didn't have too, they looked good enough to my taste.

So, it's only a 10MHz little scope yet there are quite a bunch of trimmers to play with and it's a nightmare to get everything just right, as you get into the high-frequency settings. Which leads me to : in the same era, I gather the 500 series scope could go 10 times faster, 100MHz BW ?! Now I am scared just to think of how much more of a nightmare these beats must be to calibrate at high frequencies !!   ..... or maybe not ? I mean, unlike the compact/portable 317, the 500 scopes were very large, which means Tek could stuff a lot more tubes into their cabinet, which means they could afford to come up with more complex/sophisticated/refined circuitry... maybe that meant they could produce designs they were easier to calibrate ?  Don't know, but I hope so.. for the sanity of any 500 owner ! ....


Sooooo.... looks like this scope is getting and better. Vertical is calibrated, horizontal is calibrated and working fine too, so is triggering.  CRT HV is stable and strong...

The only problem is this vertical offset which I would like to get to the bottom of, if just out of pride... but the truth is that it can now be compensated easily with the vertical position knob, and that the scope is actually perfectly usable now. 

So I guess I could now start to clean/refurbish it.... I can always keep working on this offset problem after it's been cleaned. Cleaning was postponed at first, because I was in the dark and didn't even know if I could diagnose the various problems, let alone fix them. But now the scope is clearly working and just need minor debugging for this offset problem. The thing is basically operational now.

So, let's now start making this thing shiny again....  8)

 

Offline flowib

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #52 on: November 19, 2017, 09:37:14 pm »
Check the manual, most voltages are measured with a 10Kohm/volt meter.

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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #53 on: November 19, 2017, 09:58:28 pm »
Yes good point.  The manual states to use a voltmeter  of at least 5,000 ohms per volt.  So my 10Mohms modern DMM would load the circuit much less and probably explain why I am always a bit off compared to the annotations on the schematic.  I might put a resistor across my DMM to simulate the impedance of a 5k/V VOM of the day, just to see what that would get me.

So when I measured the 1,5V at the grids of the first stage, let's imagine I am using a VOM on say a 3V range, hence a 15K input resistance, so I would put that across my DMM. Then when I am measuring the 85V or so at the grid/input of the third stage, let's imagine my hypothetical VOM would be on a 100V range, hence 500kohms.  And when I measure the 168V at the output of the fourth/last stage, say the VOM would be on a 300V range, so it would have a 1,5Mohm resistance... well that's not that far from the 10M of a DMM so I probably would not see much of a difference there, I guess.

 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #54 on: November 22, 2017, 10:21:37 pm »
Hi gang,

Starting work on the refurbishment of the thing. Trying to it one thing a t a time... at present, the cooling fan.  As I already said, the vibration problem was easy enough ot fix.. just bending a few of the blads back into shape.  Now trying to fix the second (and last) problem this fan exhibits : it's very lazy.. so to speak. In case someone has been there done that 'n all, and might be able to help... as I have zero experience with these old fans.

I mean, it does start reliably : as soon as you flick the power switch on the front panel, it starts spinning.

The problem is that it spins very slowly : just about fast enough for the human eye not to be able to distinguish individual blades turning, you do see a solid "disc" .. but nowhere near fast enough to move any significant amount of air. Only "benefit" of this is that... it's totally silent, you have to actually witness it turning, to convince yourself that is alive.

AFTER A WHILE, it eventually, all of a sudden, decides to switch second gear and speeds up to something that feels( airflow) and sounds (noise level) adequate.  The problem is that the definition of " a while " can vary greatly : can be anywhere between 2 minutes (when the fan feels in an extremely good mood ! ) up to... 2 hours.. or never, ie I get bored waiting and eventually power off the scope...

sometimes, often, before speeding up, you can hear the fan "complain" : it emits some kind of weird and horror movie like sound, very low pitched, sounds like the scope is haunted...  a bit weird.


What I have done so far :

- checked power supply : the fan is powered from a 115Vac tap from the main transformer. I get 110V so that's fine.

- according to the schematics, the fan is in series with a beefy 20+ Watts wire wound resistor, 125 ohms. I found it (strangely, there is no silk screen for it on the chassis, unlike any and all other WW resistors and components around it ?!  Go figure.  Anyway, it looks fine and measures fine, 120 ohms.

- checked voltage across the fan while it's running : I see a bit under 90V.

- pulled the fan out the scope, wired it back to the scope using long leads (a spare power cord...), so I can "play" with the fan with ease. I removed the fan piece from the motor shaft, to reduce mechanical load, just in case : no improvement whatsoever.

- tried wiggling the motor while powered up, to see if the might help it decide to spin faster : no luck either.

- put some fine machine oil in the two bronze bearings at both ends, exercised the motor a bit, power it up... no luck, still  doesn't help it go faster. The rotor appeared to me to be running smoothly and effortlessly (enough). Bronze bearings look good, no significant radial play that I can detect by hand, turns smoothly.

- Measured the resistance of the windings : 50ohms.  I don't have any data on this fan, but 50ohm sounds plausible, given that the wire is quite thin and there are quite a few turns. Also, the two windings measure exactly the same, 25 ohms each, spot on. So let's that one of the coils were shorted somewhere internally... what are the odds that the other coil would fail too, at the exact same physical location ? Yeah, none. So, I think these two coils are just fine as they are.

- So, next I dismantled the motor altogether. Quite straightforward  actually !  Just 6 bolts, and 2 minutes later the motor is in bits.  See pictures below.
This allowed me to get a closer look at the bearings. As I said already, teh bronze bearing, which actually hold the rotor shaft, are in good nick as far as I can see : turns freely, smoothly, no sign of any scoring or any damage. However... this bronze bearings are themselves part of a large assembly : they are in a "cage", in which they can move like it were a spherical joint. You can stick a screw driver in there and move the bronze bearing back and forth at will.  I assume this is meant to help align the bronze bearings. Back in the '50's I guess they could not achieve machining tolerances tight enough to do away with such an arrangement. Anyway, point is, this spherical joint is shot.  It is very stiff : I can see what I think must have been a rubber O-ring, sealing the cage around the bronze bearing. but that O-ring is lonnnnnnnnnnng gone, and this cage is now dry as a bone. It sucked every drop of oil I would throw at it ! I exercised it for quite a while. Helped only marginally, the thing is too far gone for any maintenance to be helpful/meaningful, I am afraid...  The manual says to put a drop of oil in that thing every 3 months or so, IIRC..... I bet the only oil this scope has seen in its entire 55 year long life... is what the guys at the Tek factory put in it ! So it's a bit overdue what do you think ?!  :-DD

I don't see how on earth I could refurbish these bearings ?! I would need to be able to remove the "cap" flange so I cna open up the assembly... but how to do that without damaging this thin and fragile little cap... then put it back on  ? Humm.... hopeless I think.
I guess the best one could do is replace the entire assembly with a modern/new ball bearing, assuming there is a standard sized one that can fit just right in there.

Anyway, stiff or not, once the motor is assembled, it's aligned well enough for the shft to turn smoothly? so ultimately it should not b ea problem should it ?
Still, that's my only lead... so if I could replace or refurbish these dead bearings, for a reasonable price..  I would.

Last idea : the fan is a US unit : made to run at 60Hz, sorry, 60 "cycles per second" as it says on the tin ;D  .   The fact that here in Europe it actually runs on 50Hz, a lower frequency, might explain why it's lazy ??  If so, I fear there is not much I can do about that...

So, any comments welcome...


Bye for now...

 

Offline richnormand

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #55 on: November 23, 2017, 12:04:44 am »
Look at the two copper windings in the armature. This looks like a shaded pole motor:
http://www.wow.com/wiki/Shaded-pole_motor

Might be dependent on the proper phase lag/line frequency to start and then lock?
Wonder if a motor capacitor along with the resistor might help to start for the initial lock?

But, at 50Hz,  it should spin a bit slower, the mains transformer should have slightly higher losses and the DC ripple a bit higher at the same power draw overall I would think. :-//

At the end though from a mechanical point of view, make sure the bearings are smooth with no resistance before doing anything. Then go hunting.


« Last Edit: November 23, 2017, 12:20:36 am by richnormand »
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Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #56 on: November 23, 2017, 12:16:55 am »
If the bearings are sintered bronze they can soak up oils to remain lubricated for some time.
This can be extended by immersing them into heated oil and letting it all cool before removal.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #57 on: November 27, 2017, 06:08:39 pm »
Hi chaps,

Thanks for your input, interesting article.

These copper bars in the armature are indeed the culprit I think. On my motor, there are 4 of them, and they do not all look the same... I tried to illustrate that by taking close-up pictures of them. Did as best I could with the camera, not easy but I think one can see what needs to be seen here... which is : I think the copper bars are supposed to be recessed into the armature, so as not to come into contact with the rotor. One of the 4 bars in my motor is like that : you can see it is intact, and still has its "patina" all over it.    However, the 3 other copper bars as you can see, have obvious signs of wear.. more than that, looks like they somehow ventured outside of the armature, into the space where the rotor spins and.. well, the rotor kinda "machined" these copper bars ! See how their edge are not rounded and painted anymore, but rather they are shiny and scored all over, and their surfaces is flush with the armature surface.

Then I started to look at the rotor and though hmm... looks strange, like one big solid piece of copper, shiny orange all over its surface... doesn't look normal. So I grabbed some fine sand paper (started with 400 grit then 600) and as "expected", this copper "lining" easily came off to reveal the true, grey/shiny iron underneath !  So indeed, the stator machine the 3 copper bar and got covered by copper all over its surface as a result.

I reassembled the motor and put the fan back on the motor shaft, and the shroud, just to see if this cleaning would make any difference, before going any further. Obviously this copper lining was only a consequence not a root cause, so I was not holding my breath. But I go figure, it appeared it helped ?  First time I powered it up, as usual it started right away but at slow speed.... but after only a few seconds ((under 5), it sped up ti normal speed !  OK maybe I just got lucky, maybe it was just because it was laying flat on the bench so the mechanical load might not be the same as when the fan is mounted vertically as it should. So I held the assembly vertically, powered it up again.... and again it was willing to switch second gear after only 3 seconds or so.  Cycled the power a dozen times... got the same response every time.

So !  Looks like an acceptable fix what do you say ? I say it is... I don't care if takes a few seconds to speed up, and I just don't see how I could fix these copper bars, not that they have been machined./damaged, it's too late for them I am afraid...  :(

The bearings have no radial play to them, so I don't think the stator moved and "caught" the coper bars. Plus, the bars are supposed to be recessed into the armature anyway, so even if the rotor ventured outside its envelope, there is no way it could reach the bars... well that's my understanding.
So indeed the only explanation is that the copper bars, somehow, moved out of their slot, and had a peek into rotor land... which I am sure they now regret !  :-DD   Oh dear...

So I will leave it at that. If the problem comes back again, at least I will know what's wrong with it and I will probably need to find a replacement motor.

Oh, and while I was reading Martin's post on his classic Tek restorations, I stumbled an interesting comment from VK5RC :

https://www.eevblog.com/forum/testgear/vintage-tek-restoration-pictures-by-martin/msg1012201/#msg1012201

He found replacements for the rubber mounts for the fan assembly !   Farnell sells a brand new rubber mount that looks an exact match for the old OEM one !  :D   So I might be tempted... mine still work (the vibrations were due to the fan being unbalanced, rather than soft/perished mounts), but they lokk very sorry so would be a nice touch to refresh that old scope  8)

I won't put the fan assembly back into the scope.. gives better access to the guts of  scope, will help me in cleaning it, which I think I will be doing any time now.


 

Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #58 on: November 27, 2017, 06:30:16 pm »
Is it possible when the fan was unbalanced the shaft could've flexed enough to allow the rotor to contact the stator and rub on the copper links ?

Can you move the copper links some, marginally further back into the groove ?
I'd be tempted to clean the burring off with some sharp tool then blow the groove clean and continuity test for isolation and finally cement them in place with shellac or epoxy.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #59 on: November 27, 2017, 07:02:21 pm »
Is it possible when the fan was unbalanced the shaft could've flexed enough to allow the rotor to contact the stator and rub on the copper links ?

That's an idea !  Didn't think of that, why not....


Quote
Can you move the copper links some, marginally further back into the groove ?

Nope but been thinking of it AFTER I had re-assembled it.. typical  :-//

Will disassemble it again....

Quote
I'd be tempted to clean the burring off with some sharp tool]I'd be tempted to clean the burring off with some sharp tool

Yeah that did tempt me too. Might give it a try.. if I can find a suitable tool somewhere...

 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #60 on: November 27, 2017, 10:57:30 pm »
OK, dismantled it again.   The copper bars are well secured, no way I could get them to move.

I tried a sharp blade/small box cutter, and it worked quite well to remove the burrs. The 3 machined bars now have clean edges and clear the armature.

Reassembled the motor, turned it on... oh joy, it starts on full speed right away ! Yes !  Cycled the power a few times,  it's a win every time.

Left it alone for a couple hours while attending other business, tried again... oh noooo, back to slow speed again, and this time it won't switch second gear any more... I waited a couple minutes but no joy.

Alright, it's time to surrender I think, I did more than most would have bothered doing I dare to believe... still, I hate to surrender.  But well, pragmatism ought to be a key trait of every technical person's personality, I guess.

So, I will leave it at that and start cleaning the scope at last. Point is : there is no need to do a 100,00% restoration right now. I have done enough by now that it makes sense to clean it up and put all back together. I can always come back to it in some time, to sort out this or that detail of the scope. For now, it basically works. It was unusable before, now it is working quite well and properly calibrated. Might not be brand new, but it's plenty good enough to be enjoyed and useful. So..... it pains me, but I will give up on that fan for now, and start cleaning the machine.
If someone hears of a working/tested replacement fan for cheap, count me in...

OK, so now I guess it's time to buy a tiny brush, a bag of Q-tips, a few gallons of distilled water, and a few pounds of patience....
« Last Edit: November 27, 2017, 11:18:12 pm by Vince »
 

Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #61 on: November 27, 2017, 11:06:05 pm »
Do you think the fan winding isolation might be breaking down ?
Dip it in shellac and try again ?
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #62 on: November 27, 2017, 11:30:52 pm »
I am not sure I understand ? You want to dip the motor (armature + its two windings ) into something ?! 

Searched for "Shellac" on Google... all it refers to, is some kind of manicure nail varnish or something ?!  :o

Anyway, isolation wise, you mean that the yellow cloth-y "tape" covering the windings, could leak into the armature ?

Who knows... just would find it unlikely, that yellow insulation "tape" looks in good nick and waaay thick enough/over -engineered given what a low voltage this motor is running at : 115V, and only sees 85V or so in practice ( I measured), due to the series 25Watt resistor probably...
I am not even sure why they needed to wrap the windings into this yellow stuff ? I mean, the windings are obviously made of insulated/enameled wire to begin with...

OK, I will give it one last shot : I will call Martin to the rescue ! See if he has anything to say about this motor problem... who knows.
 

Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #63 on: November 27, 2017, 11:40:19 pm »
Transformer shellac or varnish is used to provide additional insulation/isolation between turns and/or layers and core/bobbin.
Where the winding passes in the slot and is damaged, the clearances are small and that might be OK if the wiring enamel is undamaged but not when it is.

I did mention epoxy as that would provide the insulation but as you know you only get one chance with it.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #64 on: November 27, 2017, 11:44:56 pm »
Thanks for the clarification.

Well in this case I guess it's worth a try... if only I knew where to buy the appropriate stuff. I assume "shellac" is not the name it goes by in the trade... there must be some more technical term to refer to it, when one is looking for the stuff on-line ?

I just posted my S.O.S on Martin's thread....   :-//
 

Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #65 on: November 27, 2017, 11:58:48 pm »
Thanks for the clarification.

Well in this case I guess it's worth a try... if only I knew where to buy the appropriate stuff. I assume "shellac" is not the name it goes by in the trade... there must be some more technical term to refer to it, when one is looking for the stuff on-line ?

I just posted my S.O.S on Martin's thread....   :-//
Google 'transformer varnish' and have some study time.  :)
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #66 on: November 27, 2017, 11:59:44 pm »
Will do...
 

Offline bd139

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #67 on: November 28, 2017, 12:03:39 am »
I get mine from here: http://www.brocott.co.uk/electrical-varnish/

Clear stuff is pretty good.

Not sure where they ship to.
 

Offline richnormand

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #68 on: November 28, 2017, 03:53:58 am »
OK, dismantled it again.   The copper bars are well secured, no way I could get them to move.

I think from your pictures that not only the copper phase shorts were an issue but the rotor also contacted the stator.
That might indicate the bearing (bronze crap, not real bearings) might have worn in an elliptical shape allowing the contact.
An unbalanced fan could do that over the years.

Look at it with a high magnification. Try to feel the slop in that direction compared to 90 degrees. If so, a new drilling with a sleeve will solve the issue for a while.
Another solution is to sand away the contact zone, but it will come back as the sleeve wears down.

Also to keep in mind that a rotor/stator contact will look like a shorted turn and induce a lot of heat in the winding.
Check how fast it heat up with the rotor out. Be careful there, the amount of heating is low with a normal motor running properly, a bit more for a few minutes with no rotor and a lot with a shorted winding turn or a shorted rotor ( ie: contact from the rotor to the stator) or a shorted winding.




« Last Edit: November 28, 2017, 04:15:11 am by richnormand »
REPAIR, RENEW, REUSE, RECYCLE, REDUCE, REPURPOSE....
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #69 on: November 28, 2017, 06:19:21 pm »
I get mine from here: http://www.brocott.co.uk/electrical-varnish/

Clear stuff is pretty good.

Not sure where they ship to.

Thanks for the link BD.   Well the UK is not that far from France is it... would be a shame if they didn't ship. Might change once the UK have actually parted with Europe, but for now nothing is changed.  From what I understand it will take 2 years at least for the split to be effective.

One thing I am wondering about : the shellac thing is a varnish... it's viscosity must be relatively high I suspect ? Will it manage to seep between the yell insulating foil of the winding, and the armature ?!
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #70 on: November 28, 2017, 06:23:19 pm »
I think from your pictures that not only the copper phase shorts were an issue but the rotor also contacted the stator.
That might indicate the bearing (bronze crap, not real bearings) might have worn in an elliptical shape allowing the contact.
An unbalanced fan could do that over the years.

Look at it with a high magnification. Try to feel the slop in that direction compared to 90 degrees. If so, a new drilling with a sleeve will solve the issue for a while.
Another solution is to sand away the contact zone, but it will come back as the sleeve wears down.

Also to keep in mind that a rotor/stator contact will look like a shorted turn and induce a lot of heat in the winding.
Check how fast it heat up with the rotor out. Be careful there, the amount of heating is low with a normal motor running properly, a bit more for a few minutes with no rotor and a lot with a shorted winding turn or a shorted rotor ( ie: contact from the rotor to the stator) or a shorted winding.

Thanks for that. Looks like my motor is well f****d up then... I don't have any money to throw at a simple fan motor right now, unfortunately, so I will have to leave at that. I will keep an eye out for a replacement fan, if these things can be found that is, no idea. Will check Qservice ans the Sphere...

 

Offline tautech

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #71 on: November 28, 2017, 06:32:30 pm »
Well you know that rotor you nicely sanded up, this one:



The original what looked like aged patina was in fact some form of shellac or varnish to seal it from rust.
Transformer shellac/varnish is generally quite low viscosity to allow it to flow into windings. It serves more purpose than just insulation by adding environment protection and physically fixing mechanical parts that might oscillate in use and cause noise.
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Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #72 on: November 28, 2017, 06:48:47 pm »
Well you know that rotor you nicely sanded up, this one:

The original what looked like aged patina was in fact some form of shellac or varnish to seal it from rust.

Oh... shellac that looks and feels like copper ?!  When I sanded it, it didn't look like any kind of varnish or anything liquid... just an ultra thin layer of copper deposited on it. There was no ambuigity to me, when I looked at it in the flesh and when I sanded it down. But well...I can always shellac the thing if need be, I guess...


Quote
Transformer shellac/varnish is generally quite low viscosity to allow it to flow into windings.[..] 

Sounds good  :)  Will give it at try... it's about affordable... in small quantities.

 

Offline richnormand

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #73 on: November 28, 2017, 10:20:32 pm »

Thanks for that. Looks like my motor is well f****d up then... I don't have any money to throw at a simple fan motor right now, unfortunately, so I will have to leave at that. I will keep an eye out for a replacement fan, if these things can be found that is, no idea. Will check Qservice ans the Sphere...

Usually only one of the bearing is worn out depending on how the vibration pressure and its rotational harmonics are. Many quality motors will have easy to replace bearing sleeves that are just pressed-fit and of standard size. No need to replace the whole motor. I cannot say from your photos but can you reassemble the motor front piece 180 degrees from the original and see if it still has an issue. Also have you tried to put the rotor/fan assembly on two knife edge to check the assembly balance?

REPAIR, RENEW, REUSE, RECYCLE, REDUCE, REPURPOSE....
 

Offline Vince

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Re: Tektronix 317 classic scope - repair and restoration.
« Reply #74 on: December 16, 2017, 03:01:58 am »
Back to the scope... been lazy on that front recently, I must admit...  :-//

Thanks for the help on the fan... I give up for now, it's way beyond the quick/easy fix I anticipated. Doesn't mean I won't come back to it later obviously, as access to it is easy, no worries.

So I started to clean it.  Bought a few small brushes, a tooth brush, 5 liters of distilled water, to stat with.

First thing I just tried this evening... let's start small, no liquid involved... tried to breathe second life into the UHF-BNC adapter that came with the original probe... just to see if it would at all be possible, and to what extent it could be improved. Used what I had at hand : an old bottle of chrome polish I bought years ago for my old car. Might not be the ideal/best suited grain of abrasive in it, but well it's polish for metal, so couldn't be that far off I thought.

I attached some before/after pics of course !  ;D

As you can see the adapter was pretty much... black. After 30 minutes of scrubbing with the chrome polish and the toothbrush, it looked much better !  not concourse perfect of course, but such was not my intention anyway. It looks much better. Clean and bright enough to look good, but with enough imperfection/"patina" so one can see it's the original part and has history behind it. I prefer that. I did buy replacement adapters, but I think I will not use them. They are physically larger/bulkier, and the finish is brighter, a bit too "in your face", looks out of place I feel, against the otherwise overall mat finish of the front panel of these era of instruments. So for now I am happy with the cleaned old adapter.  Might have another go at polishing it once I am equipped with a polishing wheel, which might be more efficient and less exhausting than the toothbrush method...

« Last Edit: December 16, 2017, 04:33:22 am by Vince »
 


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