Hi there,
Now that I have finally completed the restoration of my first tube scope, that little Tektronix 317 (
https://www.eevblog.com/forum/repair/vintage-tektronix-317-repair-and-restoration/ ) first tube anything really, which was a rough road, knowing nothing about vintage scopes or tubes at large... it's now time to get my hands on another one of these beasts !
Yes, when I bought the 317 a year ago, I "promised" myself it would be my first and last piece of tube equipment.. because they are large and heavy, mostly.
Problem is : once restored, my 317 is such a joy to look at, and play with... that now all I can think of, is getting more and more of these old Tek scopes ! I got infected by the virus, it's in my blood now, too late to go back !
So, just in the recent weeks / months, I managed to source a few more of these : 3 more 317, so that's now 4 of them . Then a cute little 310A, then my first my '"big" 500 series scope : a 531A... normally you can't buy these models because they exceed the weight and size limits of transporters, not by much, but still.
But luckily this 531A was "reasonably" close to me (under 100 miles away), so I could, just about, consider driving to go pick it up myself... so I did.
Then I bought a 502, then a 502A, then 515A which is on its way currently, and I am in contact with a seller for yet another 502A !
And I don't plan on stopping there... For 50 to 75 Euros a piece, a modest spending, you get quite a nice piece of engineering, a beauty to look at, a piece of history, and a good few hours of fun troubleshooting and fixing the stuff, then restoring it, then a lot of satisfaction playing with it, once it's all shiny and working like a champ. A lot of satisfaction indeed in many regards, and for very little money ! That's why I don't think I will stop buying them
Anyway. They will eventually all get fixed and restored, at some point. For my second restoration, I will lay my hands on.... on.... on the 502A !
You can read about this scope (502 and 502A) on Tekwiki, including service manuals and pics :
http://w140.com/tekwiki/wiki/502Basically it's a special purpose scope : monolithic, high-sensitivity (0.1mV) / low bandwidth (1MHz max), dual channel, differential inputs, and a dual beam CRT, and an XY mode as well ! So not your general purpose 500 scope indeed. That's mostly what attracted me to this scope, along with its looks.. I quite like this scope.
Actually this is a scope I bought for spares only (or so I thought ! ) to help me restore the 502 (non 'A'). The 502 was tested good by the seller, and scope had a good life, a nice scope all around.
The 502A however, had "donor unit" written all over it : sold "as is", untested, seller knew nothing about the scope, didn't even have a power cord to test it anyway, looked real crusty on the outside, and blatant signs of physical damage at the rear of the left side panel of the cabinet. Pictures taken on a concrete floor outside... looked like it had sat rusting in some hangar or barn for the past 30 years...
Really, I bought this scope purely for parts to assist me in restoring the 502, no other thoughts in my mind when I posted my cheque to the seller...
Then when I received the scope, I was comforted in my plans, as I discovered that the physical damage was even much worse than what I noticed on the seller's picture : as you can see, there is severe damage to the rear top left corner of the cabinet !!!
Looks like the poor scope fell from the bench on its arse, this corner of the cabinet took most of the impact, and the damaged side panel now looks looks like the least of the damage, in comparison with the state of the chassis/framework...
The air filter frame/holder has a big dent in it as well. Also, there was a problem with the CRT : even without opening the scope, you could figure there was something wrong with it : looking at the front of the CRT, you could see that the face of the CRT was not "right behind" the graticule... it was like half an inch further away, like it retracted or something... not good ! Then I opened the cabinet to look at the CRT, and well yes, the CRT took a hit as well indeed : the area around its neck looked like a total mess : the nylon clamp mechanism that's supposed to hold the neck firmly in place, was all over the shop, shattered. Also, there was lots of unidentified debris all around the neck (see pics),... I thought oh no, the neck must have broken/snapped during the impact, the CRT is dead, for sure ! Still, when I wiggled the connector at the back of the CRT, it seemed like it was still firmly attached to the CRT, so maybe the neck was not broken after all.... just maybe.
Add to that, that the seller doesn't understand what "packaging" means... he got confused with the word "wrapping" I think ! There was ZERO packing material in the box ! The scope was literally shoe horned into a box that happened to be just the right size ! So this combined with the initial condition of the scope... doesn't make for high hopes, doesn't it...
Still, I decided to power the scope up anyway, just to confirm that the CRT was dead and the scope a write off, and I would not have been surprised if the scope smoked, caught fire or exploded at me. Really, I couldn't possibly think of a worse scope, so I had to expect the worst at power up, in return, logically, eh...
Powered it up, with the calibrator signal feeding both inputs (set to single ended for convenience).... fan comes on, rattling... and no trace on the CRT... "of course". No trace whether I set the scope to XY mode, or "normal"/swept mode. There is a switch accessible through an opening in the right side panel, that lets you select either mode of operation.
However, fiddling with that switch back and forth, I eventually noticed that if I left the switch in between its two stable positions, in mid air so to speak, then I would get something on the screen, somehow !
A couple dots, whose exact shape and focus and position would vary in funny and unexpected ways, depending how I fiddled with the various controls...
WOW ! So that meant that the CRT was not dead, at least not completely ! It meant at the least that the neck was not broken.. .for if it had been, there would be no vacuum anymore in the CRT and it would not work. It also means that the two cathodes/guns work, since they can produce enough electrons to shoot a spot on the screen... Also means that the CRT HV is working (though there is no PDA on this scope, unnecessary given its very low bandwidth).
Still, it was acting very funny, and the dots were immobile. so there could still well be internal damage to the CRT electrodes, which would explain the bizarre behavior.
Then I had a look around all the tubes, and as I marveled at how nice all these glowing tubes looked, one of these numerous tubes caught my attention : "HEY, looks like this one is NOT glowing, it's completely dark " That can't be right... no matter what happens, when the scope is powered up, the filaments of all the tubes are powered in unison, so all tubes should glow !
The one faulty tube was in the horizontal section of the scope, close to the CRT HV section (see pic). Hmm.... a faulty tube in that area would possibly keep sweep from working, hence no trace ! So the CRT maybe is not at fault here...
That's getting exciting, we are on to something !!
Looked that tube up in the schematic. It's V324, it's the upper tube in the horizontal amplifier : gets the ramping voltage in, and is supposed to amplify it to drive the horizontal deflection plates. Sure enough, sweep can't work without this tube ! It was a 6AU6 tube... by some miracle I DID have some of these tubes, the only ones I have, because I bought a few during the restoration of the 317 scope. It had a problem in the vertical amplifier, and it used a 6AU6 as well. Lucky me !!!
So I popped a new 6AU6 in the 502A, crossing fingers.... and hey presto, scope now WORKS !!!!
The square wave from the calibrator output, displays just fine on both channels, nice traces, sharp and bright, stable, like I bought one !
See pics below.
Couldn't believe it ! A scope in this bad a condition, severe physical damage/shock, a CRT that took a big hit, and shipped with zero packing material..... was somehow still perfectly functional, just had a bad tube ?! Really ?!
So, once I got two nice traces on the screen, of course I played a bit with all the controls to get a quick idea of what works and doesn't... and well, everything works ! Triggering works fine, on both channels, including external triggering input. Time base works. Time base magnification knob also works (x2 up to x20). Horizontal and vertical position knobs work. "beam finder" buttons work. Vertical amplifiers/attenuators work, focus and intensity controls work...
After a good hour playing with it, the traces were still bright and sharp and stable, which means that the CRT HV section is also very healthy : doesn't drop like it often does due to bad caps and / or leaky HV transformer ! The HV is rock solid !!
This scope is a winner, it's back from the dead, can't believe it !!!
So... there is now NO WAY I am using this scope for parts !! How could one resort to taking apart a scope that works just fine !!! With all the damage that this scope suffered, it's amazing that it's still in this good a working condition !! This scope clearly WANTS TO LIVE !! And I am not going to be the one sentencing it to death !
So, yes... I now fully intend to restore this scope ! The big damage to the chassis can be sorted I think. The rear panel can be removed from the chassis, just a few screws I believe. Then it will be possible to work on it, and hopefully bend it back into shape. Then repaint the panel, and that will do it
However this will need to wait until my garage/workshop is built... in a year or two maybe, not more I hope. I am also not starting the cosmetic side of the restoration just now either, because I will soon be moving home, so I am trying to tidy things up in the lab, rather than covering the bench with a scope in a million bits every where... However I can do the electrical diagnosis, and trouble-shooting, as this doesn't require to take the scope apart...
So, once the decision had been made to rescue/restore this scope, I played with it again, for a couple hours, for a more detailed/thorough test drive, taking notes and measurements. Yes, work has officially begun on this scope... hence I decided it was time to create a topic for it, so here I am again !
As I said, the scope basically works, no real show stopper. So I will only list the things that don't work, or need some attention :
HORIZONTAL- Sweep length :
As can been seen on the pics, the trace(s) don't span the entire width of the screen, far from that. They span about 7.2 DIV.
Manual says it should span 10.6 DIV. I tried adjusting that with the appropriate calibration pot, to no available : I can make it much shorter/narrower, but I can't make it any wider. So clearly there is something wrong there. The sweep length cal pot is located in the " Time Base Generator" schematic page, so will look into that...
I assume the ramping voltage doesn't reach the amplitude it's supposed to.
- Sweep calibration :
It's way, way off ! When I tried to measure the period of the 1kHz calibrator signal, I was in for a surprise ! LOL The calibrator is not far off its 1kHz target frequency, at about 1083Hz. So that's a period of 923us . What do I measure on the screen ? almost half that : 540us ! LOL
Again, I fiddled with the corresponding Cal pot, and the best it could manage, when turned fully CW, is 620us, which is logical as it's meant for fine tuning only of course... no way it was going to get me from 540 to 923us ...
So here again, there is something wrong. I doubt the precision timing caps in the time base, would be so far off. So most likely it's again a problem in the horizontal amplifier, where we just replaced a tube to get the scope working. Maybe the matching/symmetrical tube of that stage is weak, typical, or just as typical, some carbon composition resistor that drifted far away into a distant galaxy ! LOL
Anyway, these two problems don't worry me that much, shouldn't be too difficult to trouble-shoot I think...
VERTICAL- The vertical position knob for the lower beam, is not feeling too well : when you turn it CCW, you can make the trace go waaay off screen, downwards. However when you turn the knob CW to shift the trace upwards, you often can't even get it off screen completely (depending on the signal amplitude of course). So there is an imbalance there, behavior is not symmetrical.
Again not too worried : had the same issue on my 317, and others had it as well.. looks like a typical problem. Issue was a weak tube in the vertical amplifier. So again will be swapping 6AU6's LOL ! I am glad I bought a few of them back when I was fixing the 317, they are quite handy now !
- (calibrated) Gain : slight mismatch between the two channels : when I feed the calibrator signal to both channels, and try to superimpose both traces, then one is slightly taller than the other, but only by a small division, for a 5 DIV trace on the screen. So no worries there, it's the kind of minor variation that will get ironed out once the scope is restored and I go calibrate the whole thing.
- "DC Balance" control (one for each channel) : looks like the pots have given up the ghost ! It's a regular single turn pot, not a multi-turn pot, yet I can sense no stop in them... I can turn them freely 360°, round and round and round !
They still do their job, mind you !
So since they are not of the sealed type, I should have no problem opening them up to see if it's fixable. It not, will replace them. New ones might not have the exact same shape and fitting, as there are so many variations possible, sadly. But still, new one I can open/dismantle as well, take its guts and stuff them into the body of the old pots ! Make a good working pot out of the two...
So this issue is not worrying me all that much either.
- Frequency compensation (I think ?! ) : if I look real closely at the leading edge of the calibrator signal, I can see that the upper beam/channel displays a tiny bit of "overshoot", juuuust a hair, whereas the lower beam on the contrary, displays a tiny bit of undershoot/damping, corner is ever so slightly rounded off. So I might be able to correct this during the final calibration, will try anyway !
MISCELLANEOUS- Cooling fan : starts on the button and blows healthily, and minimal axial play.. however for some reason it rattles ! At first I thought the blades were hitting the air filter mesh/grill... but no, they aren't. The noise does indeed emanate from the motor itself, somehow !
Here it is :
The motor is also very rusty, not very good looking eh... but I shall take care of that in the final stages of the restoration. For now it's about fixing it from a functional point of view, ie get rid of that rattling...
- mains/power socket : somehow it's missing the earth pin ! However it's not due to physical damage. Looking up close, the socket is 100% free of damage. The earth pin looks like it's designed to screw into the chassis, through the socket. And it's just not there. Why someone would remove the earth pin, I am not quite sure... so he could make floating measurements perhaps ?... but this scope precisely has differential inputs, come on !
- Scale illumination : doesn't quite work --> one of the two light bulbs is kaput...
CONCLUSIONWell that's it for this introductory post, you know as much as I do about this scope now
So I now know what needs to be done on this scope. First fix the electrical issues, then once I am moved into the new house and lab is operational again, will work on the cosmetic side of things, and minor refurbishing. Then much later once the garage is built and I can do some mechanical work, I will look into straightening the rear panel, repairing the chassis damage. But that will take a year at least (money doesn't grow on trees just yet... ), so in the meantime I will start work on all my other Tek scopes of course !
A few pics and then off to bed...
Not gonna all fit in the present message so will have to spread them over several posts.
First some pics of the exterior, and then panels off, of the inside, all 4 sides, to get a good look under its skin, and at the same time see just how crusty and dusty it is...
Then pics of the physical damage
Then pics of the trouble-shooting / repair.
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