Tektronix 317 classic scope - repair and restoration.

[flowib]

C815 is likely bad. if the voltage becomes positive the grid current of the ECC82 keeps it from rising above -150V. and unless youre measuring with a 6.5 digit DVM the voltage will indeed be "stable" the gain of the ECC82 stage is huge, and if the voltage on the grid of the ECC82 is -150V its pulling an awful lot of current, or the ECC82 itself is bad.  (Or this is an measurement error)

C806 idem dito, if this cap becomes lossy the pentode has to put in more energy to keep the output in check.

I would advice you to replace these before going further.

[Vince]

Thanks for that, explains a lot indeed 

So the tube itself is "clamping" the voltage to -150V, hence as you say it's never gonna move... Yes, it's indeed stuck at -150V spot on, the same exact voltage as the -150V rail, no headroom whatsoever, nothing like the -158V mentioned in the schematic that's for sure.

OK then I will replace those two caps and see what happens...

I hope the tubes have "survived" , we shall see.

Thanks for the help....

[flowib]

You did measure the grid voltage between kathode and grid? and not between ground and grid? cause 10 Meg input impedance of your multimeter is gonna clamp the grid aswell

[Vince]

I measured between grid and ground/chassis....

But no worries, because there are good news to be told.... it's FIXED !!!  YES !!! 

You were right, these caps were the culprit. I couldn't wait to find and source those yellow axial high voltage caps everyone uses, I was too impatient so see the result.  I happen to have salvaged a few mains rated film caps recently, as I salvaged a few old CRT television sets. I managed to find suitable replacements, so gave it a try 

First, I replaced C815  ( HV feedback ).   47nF 400V....    didn't have one, but I put a 150nF cap in series with a 68nF one, got 47nF spot on !  How happy I was... though I doubt that the value is super critical here.. but still, if you can put the same value, why not.

Voltage rating wise, these caps only specify AC rating of course, 250 or 400V in the case of these two caps. But from what I gather, the equivalent DC rating is about 2 to 3 times the AC rating. So even the 250V rated cap can probably handle 600 or 700V DC.. well above the 400V DC rating of the original paper cap at any rate, and waaaaay more than the 150V DC that this cap is actually experiencing in the scope ! So, I was not too worried.

Result ? Did not chance a thing. No better, no worse... status quo.   Voltage at the grid was still 150V, no change there either.

So this cvap was not the main culprit.

Then I replaced the other one, C806, the LC oscillator of the inverter. This one is only 1nF but rated at 1kV ...  my first reaction was to say : "no worries, I will just put two 250 or 400V caps in series, that will make for higher overall voltage rating "... then realized I was lucky and did not even need to do that : what were the odd, in the pile of caps I found a few that featured an unusually high voltage rating : no 250V, no 400V... but 1800V , and guess what.. they are the right value as well, or almost : 1,5nF instead of 1nF !!!      Had 6 of these 1800V caps, all were 1,5nF caps. I measured them all and hand picked the one which read closer to 1nF.  I think it was 1,25nF or something, close enough to at least giv e ti a try... and the old paper cap was reading over 3nF yet it was still working ! So, I thought it was well worth a try.... 

Glad I tried : works like a charm !  When I power the scope up, I immediately noticed a change : the trace was MUCH brighter (even though it was perfectly fine before), I had to turn the brightness knob way down in order to get a normal/adequate brightness level. So that was very encouraging indeed !
Then I waited.. I waited, to see if the trace would dim or not.... I waited 10 seconds, still no dimming... 20 seconds, still no dimming... 30 seconds... still there... 60 seconds... at this point it normally wold have turned black/invisible already, but no, trace was still visible, still going strong, no dimming whatsoever.
I let the scope run for a couple hours, and still super super bright, no dimming a all. It is absolutely rock solid ! 

So that's it, the CRT is fixed ! 

Which means that indeed that V824 rectifier is working just fine, despite it being damaged and looking horrible !  The replacement rectifier tubes have already been ordered, oh well... I will keep them  anyway, might be useful in a few years if one of them fails, you never know.


So great, the first step of the repair is completed : I have a visible and lasting trace on the screen... which now allows me to further trouble shoot the scope.

I can now turn to the vertical related issue.

Played with the scope while I let it warm up... people on Tekscope told that the scope had a really nice trace, sharp, and very bright.... I can confirm ! The trace is at least as sharp as my Tek 2232 scope which is obviously much younger !  As for brightness, it's good as it get : even at the fastest sweeping sweep PLUS with the x10 magnification turned on... you still get a perfectly visible trace !  I don't mean "barely visible".. no, it's a really good trace !
By comparison, on my 2232... ahem.... at the fastest sweep sweep it get really dim, and if I turn the x10 mag, then the trace become pretty much invisible and the scope therefore pretty useless 
Maybe the 2232 has a tired voltage multiplier....

The HV probe should arrive very soon, so I will be able to play with it/get the hang of it, by practicing on the Tek 317, then poke around the 2232 maybe.... from memory it's supposed to put out 12kV...

Alright, so now let's work on the vertical issue...

[Vince]

Oops forgot to say, about the -150V at the grid of the feedback tube : it dropped to -160V once the second cap was replaced !  It's now much closer to the -158V mentioned in the schematic !  ...and we now have 10 Volts of negative drive at the grid, so the tube is "safe" , if I understand...

Ah, and also forgot about the frequency of oscillation of the inverter : replacing its cap didn't make ti climb back up to the 60kHz mentioned in the service manual. It did increase a little bit but barely.  It's now around 48,5kHz. Before, it was more like 42 or 45kHz....   So let's say it's about 45kHz give or take a few kHz.

[flowib]

Dont forget about the black capacitors in the regulated power supplies on the underside of the chassis..

these spragues are junk. nothing more nothing less.

I've thought about building a FET replacement for the 12AX7 error amp to get higher DC stability.

Late tektronix scopes didnt use these at all, the coupling caps were replaced by other types such as GOOD ALL capacitors.

I mostly replace these by old stock Philips axial caps

[Vince]

Yep no worries, I will replace them all, I started looking into it that last night, hoping to place an order tonight.

The ones which are going to be a nightmare to replace are the 4 caps in the time base switch, the "hold off" capacitors. The 4 papers caps are impossible to replace in situ, access is too bad./.. so I must remove the entire rotary switch assembly so I can work on it outside of the scope. It looks a little bit involved...  my spanners can't even remove the necessary nuts : looks like I will have to buy a set of small wrenches .. Imperial sizes !      That's US gear for you...

Yesterday I came across this site, interesting... I wish I had seen it sooner... they clearly tell you that these black caps are molded paper and need to be replaced no matter what.

https://antiqueradio.org/recap.htm


While I am at, and to correct myself : in my previous message I said I turned on the x10 time base magnification.... actually I looked at the manual and double checked on the scope front panel... on this scope the magnification is not the usual x10.. it's only x5.

Still, the trace is really bright indeed...

[flowib]

If the DC bias on the timing caps isn't too high they wont leak enough to significantly affect timing.  you'd have to figure out what the DC ramp voltage is.

if you have a imperial hex driver you can remove the knob and access the screw under the timebase knob and take out the entire assembly.

[richnormand]

if you have a imperial hex driver you can remove the knob and access the screw under the timebase knob and take out the entire assembly.

That is how I proceeded with my 545. But, even with a proper schematics, I take a nice set of photos and closeups from various angles of the insides and surroundings.
The idea is, if I come back a month later, all the wire colours, layout and hardware details will be there.

This habit has saved my butt many many times both in car repairs (engine and transmissions) and electronics.

[Vince]

OK so it's definitely possible to remove that assembly... will give a try then.

I do have the hex screwdriver handy.. had to buy it when I restoredmy old Tek 5111 scope.  1/16" size IIRC.

Glad to hear that the old tube scopes used that size too.. one less tool to buy.


DC bias I guess I can just stick my DMM in DC mode and see what it says...

According to the schematic, which show s a waveform, it looks pretty triangular in shape, with about 50Vpp across the caps.

Good news is that these 4 paper caps are used for the Hold-off timing, not for the sweep, so they would not be able to affect the accuracy of the time base sweeps as such.  Also, these caps are only involved with very slow time base settings, at which the scope is not really usable anyway, because the trace on the screen is just a slowly moving spot...   So overall, I am not too worried about these 4 particular caps... but I will try to replace them anyway, if I can manage to get the switch assembly out without losing my sanity...

[Vince]

OK, so now that the CRT is fixed, I started testing the vertical amplifier now, more seriously.

The goods news : as I assumed might be possible, the trace shifting upwards very quickly at the same time that it was getting dimmer and dimmer... must indeed have been a direct consequence of the HV dropping.... because it doesn't do it anymore !    So one less problem on the list.

So now I can safely assume that all the symptoms I see on the vertical side of things... are indeed related to the vertical amplifier, and not a consequence of some other part of the scope misbehaving.

So, what symptoms does the vertical amp really have, then ?

Well, to start with... when you power the scope up, with the vertical position control knob  centered as it should be... you do NOT see any trace on the screen ! ... it's there... but way, waaaay up, out side of the visible area.   If you turn the vertical knob allll the way CCW, you manage to lower the trace enough to bring it in sight, but the best you can lower it to, is the center line (that was yesterday) or +1 DIV (that's today), and as the scope warms up, it progressively moves up a bit more, one more full division or so.  So basically, once warmed up, the best you can do is bring it 2 DIV above "ground" level. Not great eh ?!....

As luck would have it, I remembered a YT video I saw a few weeks back, of a guy troubleshooting his Tek 317, the very scope I have, what were the odds....
Did not think too much of it at the time, but then realized the chap had the exact same problem I am having !
So watched his video again to refresh my memory :



His scope has a also a severe positive offset, though not quite as severe as my scope : at least he can see a trace at power up... I can't.

To rule out any problem with the CRT, said a simple test is to simply short the two vertical deflection plates... did that, worked fine : the trace is centered... well almost, maybe a bit less than a small division off, but I won't cry, and it proved the point.

Then I looked at the tubes in the vertical amplifier. I don't have a tube tester of course, nor do I intend to buy one as I don't plan on buying more tube gear.

However I noticed that one of the tubes in the second stage of the amplifier (V224, lower part of the stage), was glowing very noticeably dimmer than all the others around him. Guy on YT had also a problem on this particular stage of the amp !   Unlike I don't have a tube tester. However,  though again I am  no expert but my reasoning was that if the glow comes from the heater, if it's dimmer it's colder, hence the cathode is less energized hence will "produce"/free less electrons, therefore less current can flow through the tube, therefore the amplification will be less on that side/half of that stage, hence create an imbalance, which I guess could well cause a big offset.  I tried to take a picture of the tubes glowing, not easy to capture this kind of thing properly using a camera, but I think we can easily see the difference in brightness none the less.

Had nobody around to tell me if that cheap newbie theory was worth anything, but at least I knew that if I was wrong, then swapping the upper and lower tubes should not induce any major change in the offset I am seeing. However if I was right, then logically the offset should still be there, and also going downward not upward.  So, I swapped V224 and its counterpart V214.. and hey presto, now the trace is way... DOWN, not up anymore  ! 

I posted on TekScope to get some good advice about tubes, they were most helpful, great bunch over there. I ordered a few of them (6AU6 type) NOS items of course, not used, for a very reasonable price and as luck would have it, local to me/in France, so postage was cheap as well. The guy sent them the same day, so hoping to receive them pretty soon ! 

Then I tested the vertical amp some more : tried to check if the gain/attenuators/calibration was good or not.

Result : it's not good at all : it reads 25/30% too low in amplitude. A 1kHz sine wave that's 4 DIV on my main / working scope, shows up as 2.5 to 3 DIV on the Tek 317...
Then I tried the variable gain red knob : the world collapsed at this point ! LOL  Its does all kinds of weird and wonderful things !  Most of the time, it will shrink the signal down to less than one DIV .. then if you fiddle with the knob with enough persuasion.. it will display the signal again, thank you very much. So it probably needs a lot of contact cleaner in there. Not a problem I know.

What's more weird however, is that when you turn the knob, instead of decreasing the amplitude more and more.. it will move the trace vertically, just like it were the vertical control knob !    Also, it you turn it to the left, hence the trace goes downward, at the same time the amplitude of the signal will decrease a little bit, the more you go down, the more it will "compress" the signal.

Really weird indeed ! 

However I am not overly worried about that.. because in the schematics it shows the the variable gain knob is part of the first amplifier stage, that very stage with a dying tube !  So my take is that all the weird things that the variable gain does, are mostly due to the interaction with that dead tube.

So I will first replace the tube, and once this offset problem is fixed and that stage is back in business, I am pretty sure the variable gain will behave more normally... it will probably, then, just need a good cleaning... though it feels so crap that I may have to replace the pot altogether. I think I will try and pull the entire attenuator assembly out of the scope, so I can work more easily on this pot, and get better access to clean all the contacts in the switch assembly.


So.. I am waiting  for the NOS tubes to arrive, then will report back...

[flowib]

If adjusting the gain in uncalibrated mode moves the sweep there is likely a dead resistor or potentiometer in this part of the scope.

I'd unmount the pot assembly and check its resistors, and the pot contact. If its dead i might have a replacement.

If the gain is off by that much, suspect a tube or out of tollerance resistor.

[Vince]

Thanks for the kind offer on the pot ! 

Yeah I will pull the switch assembly off the scope at some point, but I first want to do as much troubleshooting as possible with it in place, because it doesn't look like fun to remove, so I want to do it only once, not 10 times in trial /error sessions...

Did some more testing with the tubes. Ebay notified me that the NOS tubes may take up to bloody TWO WEEKS to arrive ?! What's the point of buying locally then ! Grrr....  Hopefully I will receive them much sooner than this ! Crossing fingers...

Anyway, I am too impatient so... I found a "spare" 6AU6 tube to try out in the vertical amp !    Yes, Tek kindly put some spare tubes in a little compartment hidden in the scope.... naaahhh... I just pulled the 6AU6 tube from the trigger circuit !!  See top-right of the amplifier schematic, (posted yesterday) : there is an 6AU6 which picks up the signal to be used as the internal trigger source.... no worries, I don't need to trigger on the signal, I can just leave it on "auto", will do just fine for testing purposes.

So, I replaced the tired tube in the amplifier, with the tube from the trigger section... works much better !  Not quite there yet, but much better, it definitely proved the point !     The trigger tube looks tired as well (confirmed with the brightness level of the filament ).  Now, with the vertical position knob centered, the trace is visible on the screen. Way up, still, (top of the graticule), but well within sight. Pretty much what the chap in the YT video experienced.  And the vertical position knob now is able to position the trace anywhere on the screen, right down to the bottom of the graticule (but not more).

Also, gain wise, it's better too : applied exact same signal as yesterday, and now instead of 2.5 to 3 DIV (ought to be 4 DIV), I now get 3.5 DIV.

So, improved gain, improved offset.... it's definitely a tired tube, so I am hoping the NOS ones, when they arrive, should center the trace for good, and bring the gain back to normal.

HOWEVER.... what did NOT improve : the weird problem with the red variable gain knob : it does reduce gain as you turn the knob, fine, but it also insists on shifting the trace vertically as you do so ! 

So yeah, looks like this problem might be a separate issue, a problem of its own, nothing to do with the tubes.  So I need to investigate/treat this issue as a new problem.

As you said, will start by checking all the resistors in that stage of the amplifier...

[Vince]

Worked some more on the thing...

I received the silver solder, as well as a bunch of film caps, enough to recap th entire scope plus some spares, you never know.
Installed all that : 8 caps.  6 on the bottom side (4 for the LV power supply  + 2 for decoupling caps in the input signal path) and 2 at the top (for the CRT inverter).
The remaining 5 (4 "hold-off" caps in the time base switch assembly + one decoupling cap in the trigger switch assembly) I will replace later... because they require to remove the assemblies which is a PITA by the looks of it, plus there is no problem I can see, related to the time base of trigger circuitry... so as the adage says... "if it ain't broke, don't fix it " !  I do have the film caps handy, so I can replace them on the spot anytime, should I ever need to.

The replacement/new caps are MUCH smaller than the old paper caps ! Even though some of them are rated 630V instead of 350 or 400V.. just out of part availability.

Had to remove both beams from the bottom of the scope, to get better access (access at all) to the transformer taps. This also helped a lot in replacing the two big decoupling caps.


I fixed the "problem" with the CRT circuit supply voltage being 450V instead of the advertised (unregulated) 400/420 given in the manual. The electrolytic filter cap on this rail is rated at 450 Volts, so ZERO margin for error !  Made me a little uncomfortable I must say. I eventually sussed it... yes you guessed it, it was an easy one : the transformer primary allows for several adjustments, and was not quite suited to the voltage we have in France these days. 
the transformer has 3 possible configurations :220V, 234V and 248V.

In my neck of the woods, although the voltage varies all the time, in average it's more like 230/235V.   So the 234V setting would be much better than 220V. Modified the jumper...and hey presto, back to normal : I don't get 450V anymore now, but more like 415/420 ! Just fine !    So, one more little thing that is sorted 


Then I did some more testing on the vertical amplifier.

- Checked all (fixed) resistors (highlighted in yellow) throughout the amp... all were good.

- Checked  the variable gain pot : it's fine. Read 755 ohms for a nominal 780ohms, and the resistance can vary smoothly from side to side.

- Checked the vertical position pot(s, they are twins) : it's fine as well. Was not practical (because of access) to measure its resistance, so instead I accessed the wiper(s) (at the amplifier side) and measured the voltage on it : as expected, each of the pots can vary smoothly from ground up to 300V.  Only minor (I think !?!) issue is that the two pots are not tracking each other perfectly. For example when I center the knob, I get 148V on one pot, and a bit less, 137V, on the other pot.


Looks like the passives are not at fault then..so I checked the DC voltages indicated in the schematics (highlighted in blue): the voltage at the cathode of the FIRST stage this time, feeding the grids of the second stage.

Supposed to be 1,6V (bottom tube) and 1.5V (upper tube) ... and I measured, respectively, 1,44V or so, not far, and.... and... 1,90+ V !  Ah.. that is a bit far off isn't it...
So in order to figure out if the problem came from the tubes of the first stage, or if they were just being forced in to this, by faulty tubes on the second stage... I proceeded to swap the two tubes from that first stage.  Result ?   The 1,6V is still around 1,45V, however the 1,90V+  has improved, it's now 1,75 V.

This immediately translated into a modification of the behavior of the trace on the screen :

- now the trace is alllllmost centered, great ! Only half a DIV above ground level, and it only takes a tiny turn of the knob to bring it to the center...

- The variable gain knob now behaves a little differently : the gain still varies as it should, but now instead of shifting the trace downward, this is what happens now : as you reduce the gain, the bottom part of the waveform does not move at all, it's the upper part of the signal that moves downward.


So, to summarize... looks like :

1) this weird behavior might indeed be due to faulty tubes, rather than passives.

2) looks like all 4 6AU6 tubes are getting old/tired.... both the second AND the first stage.   So I am glad I bought 4 NOS tubes, rather than just one... I will replace them all.   Maybe I should have bought a whole bag of them...


So at this point I think I have done as much trouble shooting as I could... now I must patiently wait for the NOS tubes to arrive, and see what that does.

Anyway, it's laborious indeed, but I am getting there progressively, and learning as I go.. so all is well. 

[bd139]

Good capacitor choice. Those Vishay 1813's are really nice caps. I use them for timing caps. Expensive but worth it!

Looking good 

[flowib]

Marvelous work!


Those are indeed really high quality. For timing applications i use styroflex or silver mica if an old skool look is required.

Be aware that its best to run the scope 10+ hours before doing the final calibration, the "NOS" 6AU6 will change their characteristics slightly during the first 10h of use.

I once wrecked an 543 by rinsing it whit tap water like tektronix suggests, only to find out our water has some of the highest salt content in the country..
Atleast i managed to help out a couple of people, one needed a CRT and another needed a HV transformer.

As for the electrolytics, these are some of the most reliable caps from the 60s and are still good in 98% of cases, if they ever blow they take out a 10R carbon comp with not much damage to the rest of the scope..

[Vince]

Glad to hear that these Vishay 1813 are good... I picked them because ... actually they were the CHEAPEST Farnell offered, and although I have (had..) no idea whether Vishay caps were any good, I assumed that given how reputable for resistors, I hoped that although tehy were cheap, tehy would not take the risk of selling crappy caps. So I took my chance...

As alwyas with Farnell, they only sell a subset of what the manufacturer offers. So although the 1813 series offered everything I needed, I could not get exactly what I wanted. So some 400V caps got upgraded 630V, and some caps got upgraded to 10% tolerance rather than 20%... can't hurt I guess.

Although, one of the caps I had to buy from another manufacturer : " LCR component", whoever that is. Crossing fingers it's not too crap... was a bit more expensive than the Vishay one if anything.  The Vishay one was out of stock, and Farnel was only to get them bakc in stock.. in February !!! No kidding. So, had to switch to this LCR thing.

Point noted for the tubes needing some "exercise" before being put into actual duty.  I gather some manufacturer called this "aging", and aged their tubes at the factory so the customers get a tube that can be sued right out of the box, without further thinking...

As for water... yeah always safer to just use distilled water, peace of mind...
Didn't know that Tektronix actually instructed their customers to rinse the scope with water !!!  If Tek themselves are OK with that, then great, I won't be afraid when the time comes to wash its guts ! 
Come to think of it, I now remember my electronic teacher 20 years ago, told me about this... he used to wash them with  hose and let them dry outside in the sun, and the scope worked just fine after this treatment !

[Vince]

As for the electrolytics, these are some of the most reliable caps from the 60s and are still good in 98% of cases, if they ever blow they take out a 10R carbon comp with not much damage to the rest of the scope..

Oups, missed that part you just added.

Yep, I think I wil leave the electrolytics alone for now.... I mean, tehy might be 55 year old, but the fact is : the votlages are super accurate, stable, and ripple is very low, only 20mV or so IIRC, for 300V ?!  Sure can't be bad !!!  20mV is what requires for their 5V digital rail on more modern scopes, so 20mV on 300V... surely there is nothing wrong with that...

I don't see the point of replacing them "just because".   I mean, " restuffing " them is not trivial... you need to do a very clean, job of it, especially when putting the can back onto its base... and I don't yet how to be that in a clean way.

So, for now I will just leave these cans alone, they work just fine. If they ever need replacing.. well I will do it then, but I don' tsee the point of replacing good caps...
New caps can be ordered and delivered in just 48 H anyway !  Plus, it's hardly my main scope of course, so I don't care if it's unusable for days or weeks or even months, waiting for some part or some job to be done to it.

replacing crappy paper caps and tired tubes and applying lots of contact cleaner, seems to be what these old scopes mostly need ! 

[bd139]

Glad to hear that these Vishay 1813 are good... I picked them because ... actually they were the CHEAPEST Farnell offered, and although I have (had..) no idea whether Vishay caps were any good, I assumed that given how reputable for resistors, I hoped that although tehy were cheap, tehy would not take the risk of selling crappy caps. So I took my chance...

As alwyas with Farnell, they only sell a subset of what the manufacturer offers. So although the 1813 series offered everything I needed, I could not get exactly what I wanted. So some 400V caps got upgraded 630V, and some caps got upgraded to 10% tolerance rather than 20%... can't hurt I guess.

Although, one of the caps I had to buy from another manufacturer : " LCR component", whoever that is. Crossing fingers it's not too crap... was a bit more expensive than the Vishay one if anything.  The Vishay one was out of stock, and Farnel was only to get them bakc in stock.. in February !!! No kidding. So, had to switch to this LCR thing.

Point noted for the tubes needing some "exercise" before being put into actual duty.  I gather some manufacturer called this "aging", and aged their tubes at the factory so the customers get a tube that can be sued right out of the box, without further thinking...

As for water... yeah always safer to just use distilled water, peace of mind...
Didn't know that Tektronix actually instructed their customers to rinse the scope with water !!!  If Tek themselves are OK with that, then great, I won't be afraid when the time comes to wash its guts ! 
Come to think of it, I now remember my electronic teacher 20 years ago, told me about this... he used to wash them with  hose and let them dry outside in the sun, and the scope worked just fine after this treatment !

Vishay bought out Dale, Sprague, Siliconix, Draloric and BC (Philips) years ago. They make some of the highest quality parts on the market and military spec stuff galore (from Dale mainly). I'm a big fan of their MRS25 resistors, 1813 caps and the RN55D resistors.

LCR are a UK based capacitor manufacturer. They're also really good stuff. Used in military kit a lot.

None of that Wun Hung Lo stuff you there - just the best stuff on the market 

I have a picture somewhere of someone washing a Tektronix scope with a jet wash at a Tektronix facility. They bake them afterwards to stop water ingress. Apparently if they shouldn't be washed there's a big label somewhere that says so (usually).

usual Sprague and Dubilier electrolytics are good up to 50-60 years in some circumstances. Pretty good.

[Vince]

Thanks for the info BD139, I am a bit more clued now !  

[Cyberdragon]

I was about to find and post a video I saw about why you shouldn't wash electronics. But if the manual says so and they did it, well, ok then.

so the customers get a tube that can be sued right out of the box, without further thinking...

Oh my...poor defenseless electronic components...someone draw the tube man from the ads as a lawyer.

EDIT: wash as in hose down, it's ok to carefully wash any electronics if you use clean water or alcohol.

[flowib]

The characteristics of electron tubes are indeed quite stable after burn in, Burn in on recieving tubes was performed at elevated heater voltages. nowadays old stock tubes that havent been used in 50 years will still benefit somewhat from burn in as the cathode activity has decreased somewhat over time, this means that these tubes will stabilize after 10-20H.

the process itself is quite complex, during manufacture barium strontium and some other earth metals are applied in spray form to the cathode with a nitroceluose binder that allows handling of the cathodes. these carbonates disintegrate into oxides when heated producing Co2 and some other gasses in the process which are sucked into the vacuum pumps.

[bd139]

I’d hate to work in a factory that made them. Hazardous business.

[flowib]

That depends, if it accumulates in the body.
Barium oxide has a LD50 of ~400mg per Kg. Barium nitrate about 300mg/Kg

Unless your feeding thousands of tubes a day into a shredder you'l be alright.

i'd be more worried about the long term effects of exposure to Xylene used as a dilutant.

[Vince]

I was about to find and post a video I saw about why you shouldn't wash electronics. But if the manual says so and they did it, well, ok then.

so the customers get a tube that can be sued right out of the box, without further thinking...

Oh my...poor defenseless electronic components...someone draw the tube man from the ads as a lawyer.

   That was a good one ! Now fixed.. as well as half a dozen typos. The text now makes more sense here and there...