Test Equipment Anonymous (TEA) group therapy thread

[AVGresponding]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Not this again...

Now, I know that you know that energy/mass is not the most important factor here. Polypropylene for example has the same energy density as petroleum.

[tggzzz]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Diesel is surprisingly inert; I've sprayed it across hot engines when bleeding injectors but I wouldn't do that with petrol (and probably not LNG).

[Cerebus]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Not this again...

Now, I know that you know that energy/mass is not the most important factor here. Polypropylene for example has the same energy density as petroleum.

It's just an obviously sarcastic remark precisely because "Not this again".  CNG is no more or less dangerous than any other source of chemical energy currently used on the roads, I get a little tired of the reactionary "This will never be safe" rhetoric directed at any alternative fuel by people who nevertheless happily accept driving around with vehicles full of highly energetic chemical fuels (some of which contain non-trivial quantities of compounds that are directly harmful to health e.g. benzine and many other 'aromatics'). The one clear advantage of methane as a fuel is that you can literally make it from shit, which has to be an improvement on using dinosaur squeezings.

[Cerebus]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Diesel is surprisingly inert; I've sprayed it across hot engines when bleeding injectors but I wouldn't do that with petrol (and probably not LNG).

Inert and "has a relatively high flash point" are not the same thing. I was around then someone foolishly incinerated a can (metal 5L type) that had a very small amount of red diesel left in the bottom (~50ml). The ensuing explosion was non-trivial, but fortunately did no harm.

The individual responsible, Nick, also once tried to 'pep up' a damp and lack-lustre camp fire by tipping petrol on it. Obviously there's something to owning a Norton Commando that rots your brains because both the Commando owners I've known [Nick and one other] were prone to bright ideas of this ilk.

[Specmaster]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Not this again...

Now, I know that you know that energy/mass is not the most important factor here. Polypropylene for example has the same energy density as petroleum.

Yep, sorry but that is just C being C, ignoring the obvious reference there being the way that the certain fuels / compounds give up their energy extremely easily, such as CNG and Petroleum, but others, although containing higher energy density such as Diesel and as you rightly said Polypropylene need considerably more than a spark for example.    as this video very clearly and adequately demonstrates.



The BMW and Skoda cars burning is just a joke, any vehicle can catch fire for a number of reasons and when that happens, some fuels are noticeably safer to be around than others regardless of the energy content of their fuel.

[tggzzz]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Diesel is surprisingly inert; I've sprayed it across hot engines when bleeding injectors but I wouldn't do that with petrol (and probably not LNG).

Inert and "has a relatively high flash point" are not the same thing. I was around then someone foolishly incinerated a can (metal 5L type) that had a very small amount of red diesel left in the bottom (~50ml). The ensuing explosion was non-trivial, but fortunately did no harm.

The individual responsible, Nick, also once tried to 'pep up' a damp and lack-lustre camp fire by tipping petrol on it. Obviously there's something to owning a Norton Commando that rots your brains because both the Commando owners I've known [Nick and one other] were prone to bright ideas of this ilk.

I completely agree that diesel isn't actually inert, but the high flash point does make it much less "entertaining" than petrol. That's the limit of my "surprise".

But, as we both known, you can't assume anything is foolproof because fools are so damn ingenious.

[Specmaster]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Diesel is surprisingly inert; I've sprayed it across hot engines when bleeding injectors but I wouldn't do that with petrol (and probably not LNG).

Inert and "has a relatively high flash point" are not the same thing. I was around then someone foolishly incinerated a can (metal 5L type) that had a very small amount of red diesel left in the bottom (~50ml). The ensuing explosion was non-trivial, but fortunately did no harm.

The individual responsible, Nick, also once tried to 'pep up' a damp and lack-lustre camp fire by tipping petrol on it. Obviously there's something to owning a Norton Commando that rots your brains because both the Commando owners I've known [Nick and one other] were prone to bright ideas of this ilk.

This explosion you refer would have been the same with or without the red diesel which is only small amount of dye added to enable law officers to distinguish between diesel used for road use and therefore has had the proper added taxation paid, and diesel used for a purely off-road use such as farming, and this diesel has the red dye added.

https://www.utilitysmarts.com/automobile/diesel/whats-the-difference-between-red-diesel-and-regular-diesel/

[Ice-Tea]

Misbehaving display.

https://replay.dropbox.com/share/809IfayZpUGiQXgh?video_version_id=pid_rvv%3AAAAAAFIY4T4bRuYGxZbrET-8NZ8VUOkEEURo9BhV4eolUahI


Tips? Tricks?

[25 CPS]

No whoosh here. Fell with a thud. I know posi not phillips not JIS.   

And then the Canucks are fond of their Robertson.   

We are!  I showed one of my American friends the Sears Kenmore dryer that came with my house and he coudn't get over the Robertsons on it.  It would've been made here by Camco or Inglis, probably Inglis.

Then there's the story of the Canadian Westinghouse Ltd. motor generator set that nobody was expecting to see down in Baltimore...

Did it come with a hoser to fix it? Eh?

Not that time, but the same trolley museum always likes it whenever the right imported Canadian stops by and helps out with other stuff.

My portable test kit's actually gotten quite a few questions and prompted a couple of other guys to upgrade their gear recently.

[mnementh]

I think CNG is not a suitable fuel for motor vehicles, in the event of crash or a fire, it has the power to either demolish a whole block a become flamethrower if fitted with PRV.



Or even want a BMW, sorry C 

Awww, that'll buff right out.   

mnem
"Oh, yeah, we gotchoo covered; gimme feefy dollah, take joo alla way, mang."

[mnementh]

Misbehaving display.

https://replay.dropbox.com/share/809IfayZpUGiQXgh?video_version_id=pid_rvv%3AAAAAAFIY4T4bRuYGxZbrET-8NZ8VUOkEEURo9BhV4eolUahI


Tips? Tricks?

Alcohol, scrap of chamois held between tweezers. Get both sides of the Zebra strip, as well as the contact areas of the PCB and LCD.

In the case of this vid, look for loose clamping screws on that housing and possibly chipped/cracked contact area on LCD.

mnem
Give her bath; she love you long time!!!

[mnementh]

No whoosh here. Fell with a thud. I know posi not phillips not JIS.   

And then the Canucks are fond of their Robertson.   

We are!  I showed one of my American friends the Sears Kenmore dryer that came with my house and he coudn't get over the Robertsons on it.  It would've been made here by Camco or Inglis, probably Inglis.

Then there's the story of the Canadian Westinghouse Ltd. motor generator set that nobody was expecting to see down in Baltimore...

Did it come with a hoser to fix it? Eh?

Not that time, but the same trolley museum always likes it whenever the right imported Canadian stops by and helps out with other stuff.

My portable test kit's actually gotten quite a few questions and prompted a couple of other guys to upgrade their gear recently.

Contrary to popular belief south of the border; not all Canucks are hosers, nor do they have a Monopoly on hoser-dom.

We have plenty natural-born right here in Connecticut, for example. 

mnem
"You gonn' eat that funnel cake, ehhh,!?!"

[Cerebus]

Yeah, the only crazier thing would be to replace CNG (~39GJ/tonne) with something even more energetic like Diesel (~45 GJ/tonne) or Petroleum (~47 GJ/tonne).

Diesel is surprisingly inert; I've sprayed it across hot engines when bleeding injectors but I wouldn't do that with petrol (and probably not LNG).

Inert and "has a relatively high flash point" are not the same thing. I was around then someone foolishly incinerated a can (metal 5L type) that had a very small amount of red diesel left in the bottom (~50ml). The ensuing explosion was non-trivial, but fortunately did no harm.

The individual responsible, Nick, also once tried to 'pep up' a damp and lack-lustre camp fire by tipping petrol on it. Obviously there's something to owning a Norton Commando that rots your brains because both the Commando owners I've known [Nick and one other] were prone to bright ideas of this ilk.

This explosion you refer would have been the same with or without the red diesel which is only small amount of dye added to enable law officers to distinguish between diesel used for road use and therefore has had the proper added taxation paid, and diesel used for a purely off-road use such as farming, and this diesel has the red dye added.

https://www.utilitysmarts.com/automobile/diesel/whats-the-difference-between-red-diesel-and-regular-diesel/

Yes, we know what the red in red diesel is. The explosion would most defiantly have been very different without the diesel, it would have just been a can popping, not a superheated mass of diesel vapour igniting. Woke up the next farm over from the one we were camping on...

[factory]

CRT saga update


Thanks everyone for your help and comments.

1) HV too high. Yes did cross my mind last night, thinking yeah maybe the electrodes in the CRT are fine it's just the HV too high causing the arcing.

2) Plastic base in bits : it's not hopeless ! It's only in 4 big chunks easily put back together, see below. I could superglue them back. As for the two hacksaw slits I made, 180° apart, I could reconstitute those as well : I kept the plastic saw dust in a little ziplock bag. I could mix it with glue to reconstitute / refill the slits. Common technique in wood working for small repairs.

3) Hollow pins : ah ! so that's how they do it !!!  I could not get how they could manage to crimp these pins with the plastic base in place ! 
So the wire sticks out of the pin, they cut it flush and solder the end.... yes looking closely at those pins, I could see some have their ends still hollow !
Boy I learned something, one less mystery in the universe !   OK, so I touched up all 14 pin ends.


Then I plugged the CRT temporarily as Dragon suggested.

Result ?  Well for one there was no sparking in between the pins at base of the neck, so that means even without the base in place, it doesn't spark.. so the base is not vital. I am sure it's better with it, but not vital.

The CRT still does exactly the same shit as before ! no more no less.
Also, if I tap gently here and there on the neck (GENTLY I said, not with a sledge hammer !  ), I notice that... I don't notice anything !  It does not appear to affect the behavior of the CRT in any way shape or form. I find that a bit strange... if the problem was internal damage, a bent electrode in there, or a broken or intermittent weld.. you would thing that taping would have SOME effect... but no.

ALSO, of great importance  : I played with the CRT for like I don't know, 20 minutes, and it does NOT make any sound any more, nothing ! Yet the CRT still does the exact same shit on the screen !
And I can't see any arcing inside the neck, nothing.

So... I am now at the point where I think maybe all that crazy shit on the scree, really crazy shit, might not be created by the CRT... just a tiny hope.

So I am now thinking, if just as a sanity check, that it's worth spending a little time probing the scope to see what signals are sent to the CRT. H and V deflection, Cathode voltage, brightness etc.... everything.


I am not used to HP scopes and the CRT is not as easy to probe as a Tek, so it will take me some time to get around that scope that's new to me....


Stay tuned 

Do not use superglue, I repeat do not use superglue to reattach loose bases to valves/tubes of any type, unless you want the glass to crack, see the warning below from this link. https://www.vintage-radio.net/forum/showthread.php?t=400

Superglue Warning
Andy Emmerson posted the following warning on the Vintage Wireless email list:

    The January 2003 issue of the excellent American magazine 'Antique Radio Classified' contains two letters warning about the dangers of using cyanoacrylate glue, with trade names such as Super Glue, for repairing valves. I think the message deserves repeating here.

    Restorers often need a fast-curing glue for cementing loose glass into valve bases but danger lurks here. The problem is the difference in the coefficient of expansion of glass and of the cyanoacrylate will cause the glue to act like a hot wire glass cutter - cleanly cutting the glass off at the base when the valve heats up to full operating temperature.

    The bottom line is never use this glue on any valve or tube that runs too hot (when up to full temperature) to hold your hand on the tube. This means no power output valves and no rectifiers. However, this glue is considered safe, with some reservations, for battery valves and most small signal valves, if they don't get too hot.

    Cyanoacrylate glue also makes a conductive path. When applied with the tube held in a vertical position, the glue can leak down into the pin area - not a good thing. It is suggested you use the gel-type version of this glue, which does not flow as easily. Or else apply the normal fluid glue with the valve in a horizontal position. For diaply and non-functioning exhibits there's no problem at all.


    Credits go to Don Harrill in the USA and Gordon Wilson in Canada for these words to the wise.

Stever Ostler posted a message confirming that he had seen this effect with a PX4 in an RGD radiogram.
__________________

Some suggestions for different ways to reattach the bases in this thread. https://www.vintage-radio.net/forum/showthread.php?t=155355

David

[Vince]

HP 120B update

My old Safrane has LPG and never exploded ! 
But I am noty chancing it, the LPG tnak is empty and I only run on E85 instead ! 


This evening in the news we learned that this morning not one but two electric buses caught fire and burned to the ground. Two the same day... and completely unrelated. One in Paris the other one down south in Carcassone IIRC. No electric for me thank you ! 


Oh wait, that's an electronic forum, almost forgot !! 


Quick update on the scope, from a time where batteries didn't explode just because of the phase of the moon.

So I tried poking the scope itself rather the CRT.

Checked the power rails. Like in Tek scopes there is a -150V rail that's regulated and the other rails follow.

It was at -152+ volts. I adjusted it to spot on -150.0V because the trimmer is not touchy, it was easy to do.
Then measured the other rails, and they read a bit high. 305 for 300V, 106 for 100V.
Had a quick probing around, the grid of the tubes get a bit too much voltage so I guess it's just some resistors that drifted high.
Will fix that later if the scope proves to be fixable, but for now it's good enough, no need to waste time on this.

Then I went to do some cleaning of the little HV board, because so much black dust and fluff, not good for HV eh ?
So poured some IPA, gave it a quick brush... but access to the board is not great with those two tubes on it, like the World Trade Center.
So I removed them to make it easier to brush the board. Put them on the bench, they quickly rolled onto my lap, then fell on the ground, and shattered in a million pieces.

Problem is that I don't have many of this type of tubes !!  IT's a 12AU7.  Every Tek scope has one or two, but that's it, and I have already used up 5 of them to populate that HP scope !
Worse, much worse... the other tube is a 6AQ5 . It's the oscillator tube. This one resided in only TWO of my Tek scopes, and they only have one inside, because they too use it as the oscillator tube.

- Type 310A. This one is NIB condition, a museum piece, I am NOT touching it. I even doubt it has ever been powered up at all, it's that good.

- Type 515A. I have two of them, so two tubes I could use. I want to restore one of them, the other one is there to help with that.

So I stole a 6AQ5 and 12AU7 from on of the type 515.

Result : no HV ! I used my HV probe and probed directly at the connector that plugs at the back of the CRT. I get ZERO. No HV-ing going on, not a sausage 

Bad tubes ? As I said it's not like I have a zillion to try out ! 

So that's were I am now, trying to probe on that HV board to see if I can troubleshoot it, hoping my tubes are good and there is something else wrong with it... hopefully not a burned HV transformer 
Problem is that access to that board is not ideal. I can get to its bottom because of the way it's installed in the chassis. I can only probe the top, and that's not easy as most interesting components are by the read panel. Hard to get probes down there, or see what you are doing really. Also, the caps mostly can't be probed as they sit flush on the board, their terminals are not exposed 

So I am not happy. Lots of time and cursing will be involved in diagnosing this HV board I think   

You can now resume your normal off-topic discussions, I apologize for the interruption !   

[Vince]

CRT saga update



2) Plastic base in bits : it's not hopeless ! It's only in 4 big chunks easily put back together, see below. I could superglue them back. As for the two hacksaw slits I made, 180° apart, I could reconstitute those as well : I kept the plastic saw dust in a little ziplock bag. I could mix it with glue to reconstitute / refill the slits. Common technique in wood working for small repairs.

Do not use superglue, I repeat do not use superglue to reattach loose bases to valves/tubes of any type, unless you want the glass to crack, see the warning below from this link. https://www.vintage-radio.net/forum/showthread.php?t=400

Superglue Warning
Andy Emmerson posted the following warning on the Vintage Wireless email list:

    The January 2003 issue of the excellent American magazine 'Antique Radio Classified' contains two letters warning about the dangers of using cyanoacrylate glue, with trade names such as Super Glue, for repairing valves. I think the message deserves repeating here.

    Restorers often need a fast-curing glue for cementing loose glass into valve bases but danger lurks here. The problem is the difference in the coefficient of expansion of glass and of the cyanoacrylate will cause the glue to act like a hot wire glass cutter - cleanly cutting the glass off at the base when the valve heats up to full operating temperature.

    The bottom line is never use this glue on any valve or tube that runs too hot (when up to full temperature) to hold your hand on the tube. This means no power output valves and no rectifiers. However, this glue is considered safe, with some reservations, for battery valves and most small signal valves, if they don't get too hot.

    Cyanoacrylate glue also makes a conductive path. When applied with the tube held in a vertical position, the glue can leak down into the pin area - not a good thing. It is suggested you use the gel-type version of this glue, which does not flow as easily. Or else apply the normal fluid glue with the valve in a horizontal position. For diaply and non-functioning exhibits there's no problem at all.


    Credits go to Don Harrill in the USA and Gordon Wilson in Canada for these words to the wise.

Stever Ostler posted a message confirming that he had seen this effect with a PX4 in an RGD radiogram.
__________________

David

Thanks for the warning !

I only intended to use super glue to glue the plastic bits of the base back together again. So plastic on plastic only. I would not use it to glue the base to the glass of the neck. It didn't even cross my mind, because the old glue was clearly a very thick and wide layer, super glue is thin. I just don't know what glue would be suitable for that... I have other more immediate worries at the moment, I am in the diagnosing phase for now, far from the putting it together again phase ! 

[factory]

See the link I added to the last post for some suggestions, for reattaching the base to the tube. https://www.vintage-radio.net/forum/showthread.php?t=155355 there are probably many similar threads if you search that forum.

P.S. the manual gives tolerances for the supplies, only the 100V is out of tolerance according to the manual.


David

[Saskia]

what's wrong with CNG ? we got 3 cng cars in the family and they are doing great. cheapest way to drive, anyway

[mnementh]

Mostly, the fact you have to be smarter than lint to handle it safely as a motor fuel, something one cannot be sure of with the average motorist.

That, and the fact you can't just ignore the fuel delivery system for the life of the vehicle like most folk do with gasoline powered vehicles; I for one wouldn't use a 50-year-old CNG tank in a restored vehicle, yet have done so several times with no more than a careful visual inspection of a gasoline tank.

mnem
"Smoke 'em if you got 'em!"

[Specmaster]

what's wrong with CNG ? we got 3 cng cars in the family and they are doing great. cheapest way to drive, anyway

Agreed, but in the event of vehicle fire, the tank can become superheated and then raptures and turns into a devastating bleve

If the pressure release valve is mounted sideways as in the video of the bus, then it becomes a very dangerous flamethrower until the tank raptures and a bleve ensures.

This does not normally happen in the case of petrol or diesel as they already have vents designed into the fuel tanks and rubber hoses etc attached to the tanks. Thankfully, this kind of event is quite rare but if vehicle fuelled by LNG / CNG catches fire, you need to get plenty of cooling water squirted over the tank and the fire knocked back quickly to prevent a bleve from happening.

[Neper]

That, and the fact you can't just ignore the fuel delivery system for the life of the vehicle like most folk do with gasoline powered vehicles;

Your situation may differ but in most of the civilised world, motorcars have to undergo regular inspections, usually at 1 or 2 year intervals, by authorised organisations. This includes a thorough check of LPG or CNG systems and tanks.

Thankfully, this kind of event is quite rare but if vehicle fuelled by LNG / CNG catches fire, you need to get plenty of cooling water squirted over the tank and the fire knocked back quickly to prevent a bleve from happening.

Belgium and the Netherlands have large numbers of privately owned LPG vehicles on their roads. Many French towns have been operating fleets of busses running on natrual gas for decades. I've never heard or read of any BLEVE over there.

[factory]

HP 120B update

My old Safrane has LPG and never exploded ! 
But I am noty chancing it, the LPG tnak is empty and I only run on E85 instead ! 


This evening in the news we learned that this morning not one but two electric buses caught fire and burned to the ground. Two the same day... and completely unrelated. One in Paris the other one down south in Carcassone IIRC. No electric for me thank you ! 


Oh wait, that's an electronic forum, almost forgot !! 


Quick update on the scope, from a time where batteries didn't explode just because of the phase of the moon.

So I tried poking the scope itself rather the CRT.

Checked the power rails. Like in Tek scopes there is a -150V rail that's regulated and the other rails follow.

It was at -152+ volts. I adjusted it to spot on -150.0V because the trimmer is not touchy, it was easy to do.
Then measured the other rails, and they read a bit high. 305 for 300V, 106 for 100V.
Had a quick probing around, the grid of the tubes get a bit too much voltage so I guess it's just some resistors that drifted high.
Will fix that later if the scope proves to be fixable, but for now it's good enough, no need to waste time on this.

Then I went to do some cleaning of the little HV board, because so much black dust and fluff, not good for HV eh ?
So poured some IPA, gave it a quick brush... but access to the board is not great with those two tubes on it, like the World Trade Center.
So I removed them to make it easier to brush the board. Put them on the bench, they quickly rolled onto my lap, then fell on the ground, and shattered in a million pieces.

Problem is that I don't have many of this type of tubes !!  IT's a 12AU7.  Every Tek scope has one or two, but that's it, and I have already used up 5 of them to populate that HP scope !
Worse, much worse... the other tube is a 6AQ5 . It's the oscillator tube. This one resided in only TWO of my Tek scopes, and they only have one inside, because they too use it as the oscillator tube.

- Type 310A. This one is NIB condition, a museum piece, I am NOT touching it. I even doubt it has ever been powered up at all, it's that good.

- Type 515A. I have two of them, so two tubes I could use. I want to restore one of them, the other one is there to help with that.

So I stole a 6AQ5 and 12AU7 from on of the type 515.

Result : no HV ! I used my HV probe and probed directly at the connector that plugs at the back of the CRT. I get ZERO. No HV-ing going on, not a sausage 

Bad tubes ? As I said it's not like I have a zillion to try out ! 

So that's were I am now, trying to probe on that HV board to see if I can troubleshoot it, hoping my tubes are good and there is something else wrong with it... hopefully not a burned HV transformer 
Problem is that access to that board is not ideal. I can get to its bottom because of the way it's installed in the chassis. I can only probe the top, and that's not easy as most interesting components are by the read panel. Hard to get probes down there, or see what you are doing really. Also, the caps mostly can't be probed as they sit flush on the board, their terminals are not exposed 

So I am not happy. Lots of time and cursing will be involved in diagnosing this HV board I think   

You can now resume your normal off-topic discussions, I apologize for the interruption !   

You could swap the HV control tube with one of the others in the 120B, the other might be a problem as it's the oscillator tube. Also check the solder joints on the tube sockets, had many bad joints from thermal cycling, more of a problem with the higher power tubes.

David

[Specmaster]

<snip>

Thankfully, this kind of event is quite rare but if vehicle fuelled by LNG / CNG catches fire, you need to get plenty of cooling water squirted over the tank and the fire knocked back quickly to prevent a bleve from happening.

Belgium and the Netherlands have large numbers of privately owned LPG vehicles on their roads. Many French towns have been operating fleets of busses running on natrual gas for decades. I've never heard or read of any BLEVE over there.

Neither have I, but that does not diminish the risk of one occurring if the fire services are unable to get to the vehicle fire in time to knock back the fire and apply cooling jets of water to the fuel tank.

These videos give an indication of the power of a Bleve, but none of these as far as I know was from CNG.

[Saskia]

if none of them were from CNG then why do you quote them ? CNG tanks have a service life of 15 years which may be extended with proper inspection to 20.  biannual inspection is mandatory.
I had a petrol car ( Audi)) explode from under me, barely made it out. no LPG or CNG car has done this to me.

[Vince]

HP 120B update

So I stole a 6AQ5 and 12AU7 from on of the type 515.

Result : no HV ! I used my HV probe and probed directly at the connector that plugs at the back of the CRT. I get ZERO. No HV-ing going on, not a sausage 

Bad tubes ? As I said it's not like I have a zillion to try out ! 

So that's were I am now, trying to probe on that HV board to see if I can troubleshoot it, hoping my tubes are good and there is something else wrong with it... hopefully not a burned HV transformer 
Problem is that access to that board is not ideal. I can get to its bottom because of the way it's installed in the chassis. I can only probe the top, and that's not easy as most interesting components are by the read panel. Hard to get probes down there, or see what you are doing really. Also, the caps mostly can't be probed as they sit flush on the board, their terminals are not exposed 

You could swap the HV control tube with one of the others in the 120B, the other might be a problem as it's the oscillator tube. Also check the solder joints on the tube sockets, had many bad joints from thermal cycling, more of a problem with the higher power tubes.

David

Swapped the contorl tube / 12AU7 no luck.

Swapped the oscillator tube / 6AQ5 ... well no I can't. Both the one in my second type 515 and my type 310A which I resorted to opening up out of desperation, are cooked... 100% black inside. I tried them anyway, no luck.

So basically the 6AQ5 I pulled from my first 515 is the ONLY such tube I have here that's not black... it's crystal clear and I can see that at the very least, it lights up. Better than nothing I guess.

So I will or a new tube or two very soon but of course that will take time to get to me. So in the mean time I will pretend that my tube works and try trouble shooting the board.

- I checked all power supply going to that board, board does get all the supplies it needs, so that's good. That includes the unregulated 420V that goes to the primary of the HV tranny. Yes tranny again, transformer this time not transistor 

- Zero oscillation going on : HV probe says zero volts on the secondary side, and when I scope the board, I see only DC voltages, zero sign of activity.

- I measured DC voltages as indicated on the schematic below, around the 2 tubes, i.e 3 elements : pentode and x2 triodes.  DC levels are 100% completely wrong. See measured levels indicated in red. So if we look at the two triodes : both are fed with 300V, and both are supposed to have about 90V on their anode. But, in my case not at all. Get near zero (2.8V) and the other near B+ (300V and the rail is at 304V).
So one Triode is conducting like mad, and the other is not conducting at all, and it sits like that, stable.
As for the Pentode / oscillator tube, it's control grid sees 0.8V instead of negative 123V !
so no wonder it does not work... now the problem of course is that it's a close-loop system, so no way to know what element in the loop is causing the entire loop to fail.


So now what to do...

Checking solder joints yes but not easy as pulling the board from the chassis looks like it will be a major tear down, looks quite involved....

Short of having another oscillator tube to swap, I will make the hypothesis that it's either bad joint as you said, or some passive component that just died.
One thing that might point in that direction is that now I think of it, the brightness control used to have bugger all effect on bruightness. It had as I said effect on the pitch and loudness of the arcing sound if that's what it was, but it did not have much effect on brightness.. it was always very bright.
So right there, we can say that some component or joint in the HV board was not good...

Wish me luck.....