Hello and welcome back.
If you follow the channel, you know that we just restored a moderately evil HP 711A tube
power supply in the previous episode, with the help of my tube savvy friends.
I say moderately evil because it’s only 500V, and also you can tell from the smile.
It ended up working quite well, but 500V was not enough to light up our mystery CRT tube.
So on we go with the decidedly more evil ones that should go up to 3 kV.
I got a whole bunch of them a long time ago from an auction lot, which also contained
a Xenon lamp sparker.
That one was another step of evil, going up to 40 kV, good enough to make a terrific spark
radio.
So here the first of our HP 6516As, up to 3kV, but just 6 mA, so perfect for setting
CRT tube bias voltages.
It’s an all solid state and rather simple linear supply, so nothing much to do except
for capacitor checking.
If the high voltage, oil-filled caps read good on low voltage testing and don’t leak,
we should be OK.
All the caps passed low voltage testing with flying colors, the oil filled ones being 1µF
right on the dot, so we are good for power up.
But there is a catch, don’t yet have a good high voltage probe for measuring high voltages.
I have one on order, but I did not want to wait.
So in the meantime, I made myself one.
It’s nothing fancy, just a resistor divider.
Since you want to maintain the current very low even at 3 kV, you need a very large main
resistor, in my case 100 MΩ.
I did not have a such a high voltage, high value resistor, so I made mine out of of 10
10MΩ resistors in series.
Splitting it into many resistors also reduces the voltage across the resistors.
Run of the mill metal resistors can’t take much more than a few 100V so spreading the
voltage across many resistors ensures that they all work safely without arcing.
A smaller value resistor completes other leg of the divider, in my case a 1:1000 divider.
Wrap the whole thing in 3 layers of shrink wrap, and here you go, a temporary makeshift
high voltage probe that you should not make nor use at home.
So this one turns out to be a linear supply, so we can bring it up slowly.
Okay, we have a millivolts, so this is 50 volts.
That's not right, it tells me 75 volts.
100 volts...
There we go.
Now it regulates.
Yeah.
So okay, if I'm too low, I don't even regulate.
Let me have it soak at 50 volts.
Okay, nothing's blown up.
Oh, and it's 10 volt exactly!
Right, this is reading in millivolts what's in volts, it's a one to one thousand.
Right on the money!
20 volts, 30 volts, 40 volts, 50 volts, 60, 70, 80, 900.
OK and we have to go back to zero.
100 volts, 200, 300, 400, 500, 600, 700, 800, 900.
Pretty close we want to try the kilo volts.
1 kV, 2kV.
And oh yeah, so you have to go to 2.9.
2.9 kV, okay!
And we're trying to bring on a few more of these, I have like five or six.
They came in a pallet.
And a lot of them have the meter that's fallen off.
And we just discovered why: it has this very strong springs, that are on four pockets
here - and there's only one left here.
And you see on the one where it's still standing, it has this very thin plastic frame
which we try to reproduce by printing one.
Do you have one of yours?
[Eric] I'll grab it.
[Marc] But I gave Eric the wrong STL file from the Interweb.
It's for a different instrument!
I bet you, you could shrink that one.
But it's so strongly put in there that I'm worried that we would break this one.
[Eric] Yeah, this isn't going to work very well, because the 3D print, the Z-axis is
in this direction, and that's very fragile.
So if we put a lot of force on that edge there, then it'll just simply snap.
[Marc] And obviously a problem that people had in other instruments, if they remade that.
[Eric] Right, right.
[Marc] But just shrink that, I bet you that's the same thing, but smaller.
Eric just made an extra one, reprinted it.
I has a super thin arm, so I'm amazed he could do it.
So here's the replacement.
I hope it works, it's super fragile.
It didn't make it!
It broke.
[Eric] It broke?
[Marc] Yeah, it broke.
It broke exactly where you'd think it would break.
[Eric] Oh.
So did it break when you were putting the meter in?
[Marc] Yeah.
[Eric] Ah, yes.
[Marc] I'm going to use a hybrid technique here,
where I use hot glue to hold the meter.
That should do.
So I am revisiting one of the HP high voltage supplies, because it was slightly oily.
And here I got the back off, and I see some oil.
So my worry is of course that one of the oil capacitors is leaking.
But it's on the wrong side.
Anyhow I want to get to the bottom of it, make sure it's not an oil leak from the capacitor.
And it's a royal pain to get in there, but definitely oil here.
So I'm glad I looked.
But then, what I don't quite understand, is that the caps are on the other side and there's
no oil on the side of the caps!
And I made my way into there, and mystery of mysteries, it's dry as a bone around the
caps.
And those those caps tested good.
So was the oil from outside?
That is all so bizarre.
Let me remove that transformer.
Transformer removed, and a little bit of oil there.
Lots here, some on the side out here.
But there's no oil in the transformer.
So, I do not understand this.
If it came from the cap, the cap should be oily.
It's not, it's completely dry!
So I remounted the whole thing, couldn't find any evidence that the oil came from inside,
some faint evidence that it came from outside.
But I cleaned it so well, you don't see it.
There was oil over here.
The screws are oily on the outside.
And it's on the opposite side of the cap!
So I think it came from the - you can even see it here - it came from the outside in.
I wish I could be sure, but I really see no evidence of anything leaking out of the capacitors
at all.
So we're going to close it back and be semi-comfortable with it.
Okay, and I repaired the switch while I was at it.
There we go, three kilovolts.
So back to the task at hand, trying to get some light out of our mystery CRT, to check
the color.
We wanted to try a minimum setup with just one supply.
Normally this tube needs 17.5 kV acceleration voltage.
We tried it with 500V in the previous video but that was not do enough.
So, new improved but dodgy setup, approved by TubeTime.
[Eric] Yes!
[Marc] So, for now we're just going to try to do just filament voltage and acceleration
voltage with more acceleration.
And back there we have filament - actually you wrote it down here right?
[Eric] Well, we're not building it exactly to that diagram, but...
[Marc] But we have filament here, we have high voltage over here, and we grounded the
grid.
And what else did we ground?
We didn't ground any of these right?
[Eric] We grounded the cathode too.
[Marc] We grounded the cathode, we grounded the grid, we grounded the outside of the tube.
And that's it.
And all those are floating at the moment, which is not ideal.
[Eric] Not ideal.
[Marc] We'll see if that will work, and then if it doesn't we'll do some more, we have
more voltages.
[Eric] Let's see how our filament lights up.
[Marc] It's hard to see actually, it's very well protected.
You can only see through the hole at the back of the connector over there.
[Eric] Yeah, I can see it.
So we have the filament voltage.
[Marc] So let's turn the lights off.
Yeah!
Did I see it?
Nope, I saw nothing.
[Eric] Yes!
No, that's just the...
Well, you can go up to a higher voltage.
Yeah, there it is!
[Marc] Oh, it's orange!
It is orange.
[Eric] Ah, darn!
[Marc] Okay, I can put some more.
And that's what we thought it was.
So that's what Al told us.
[Eric] So Al was right.
[Marc] It's just the wrong color.
Let's get Master Ken.
Master Ken, color test!
[Ken] Yeah, that looks orange.
[Marc] Al was afraid that this was an orange tube.
[Ken] So how many volts did you give it?
[Marc] It started to glow at 2k, 2 kilovolts.
We have not hooked up the acceleration?
[Eric] Right, we could hook up 500 volts to that first anode, and then we should see maybe
a little bit more brightness.
[Marc] Let's try to do that, because we have the power supply here.
[Eric] Sounds good.
[Marc] Okay, so we have now connected the anodes to some positive voltage.
500 should be enough?
[Eric] Sounds good to me.
[Marc] It should extract more electrons and give us the brightness we're missing, hopefully.
[Eric] And we will let focus float so that we don't get a really small spot, because
that could burn a hole through the phosphor at the front of the screen.
[Marc] Yeah.
So that’s our final test setup.
Filament, cathode, first grid and tube rim to ground, first anode to 400V, focus electrodes
floating and final acceleration to 2 kV.
All right, so right now that's the same as before.
Let me put this one on.
So we have it but it's...
Oh there!
Oh, it's immediate.
All right, this is what we were missing.
And dang, it's orange.
So, tube works, and as Al thought it would be, it is orange.
So if we put that in the Alto, we'll have an orange screen.
Which would look really bizarre.
[Eric] Yeah that would be a little bit strange.
[Marc] If you put a magnet over there you should be able to move it, right?
[Eric] Oh yeah, let's try it!
[Marc] Alright, we got our answer, and CRT tubes are a lot of scary fun.
But as a bonus I now have a whole bunch of high voltage bias sources.
So why don’t we try to light up a few of the CRT tubes I have in my collection, but
correctly this time, with the correct voltages for each electrode, so we can explain what
they each do and have a bit more CRT fun.
See you in the next episode!