[Marc] So on the bench today, we have the HP 10638A degausser, courtesy of Marcel. And
it's a piece of kit that goes with the Cesium clock to get the real performance out of the
Cesium clock.
[Marcel] Extra fine resolution. Lets you help tune that C-Field for your Zeeman field splitting.
[Marc] So, hello and welcome back. If you follow the channel, you might have seen the
video where I explain how an atomic clock actually works. We fired up an HP 5061B Cesium
clock from the 1960’s. It was famously used for the first direct confirmation of the dilation
of time effect, as predicted by Albert Einstein’s relativity theory.
As the explanation of the clock was getting more and more complicated, I left out the
C-Field and its prominent adjustment button, which looks like a miniature clock with a
black face and white hands in the middle of the control panel. But it’s not a clock,
it’s a precision adjustment knob.
The C-Field is a small magnetic field inside the Cesium tube, which adjustment is quite
delicate and quite important. It is needed to extract the ultimate precision from our
clock. We are planning to perform this adjustment procedure, which involves yet another quantum
experiment, in an upcoming video.
However, before we can do that, we need to revive this very special instrument, the HP
10638A degausser, which is called for in the procedure. It is super extra specific: it
is only used with the HP 5061B clocks with the high-performance tubes, which is the ones
we have. As you will see, it is the king of precision degaussers.
[Marcel] Since we're using the Option 004 High Performance Tube, it has the ability
to use this degausser to help remove that residual field.
[Marc] Right, right, that's something that's specific to the better option 004 high stability
- no, high...
[Marcel] ...pperformance.
[Marc] ...performance tube which is the one you have in your clock.
So we're going to try to make it work today. But the degausser is just there to remove
any field that might have accumulated in the tube while it was operating in a specific
location.
[Marcel] So anytime you move it, you're supposed to redo this procedure.
[Marc] Right. And what a degausser does is, it removes magnetic fields. How do you remove
a magnetic field? Well you might have seen that in my previous video. This is a demagnetizer,
that I use for tape heads. And what it does, it's a big magnets at 60 Hertz. And then you
flip the magnet back and forth. Well, it will do it because it's powered by 60 Hertz. And
then you reduce progressively the amount, and then at the end, you should be at zero
magnetic field.
So right now I have a a screwdriver that will catch screws. But I've never tried it with
a screwdriver. Whoa, it certainly pulls it! So you want to do this, and then whoop...
So, usually, for screwdrivers, you would have a much much bigger thing. So this is meant
for a head, a tape reader head. There we go, I did it. Partially. It's still a little bit
(magnetic) at the tip.
So, anyhow, what this guy is supposed to do, it does it, but the HP way.
Degaussers are usually pretty rough affairs, just a big magnet with alternating current,
and sometimes a way to pull the magnet away in a progressive manner. See this big tape
degausser from HP. All there is in it is a giant 60 Hz magnet, and a fan.
So, it's a super simple machine. You put your tape in.
Push it in into magnet. Turn it on. Start the degaussing.
And it just spins the tape, and takes it out of the magnet progressively.
Our HP 10638 degausser is a super refined version of this. Instead of the 60 Hz, it
has a very low frequency electronic oscillator, a calibrated current driver to precisely control
the amount of magnetization, a very slow, perfect exponential decay generator, and a
digital timer.
There you go, no more magnetic data on this tape!
It has a perfectly calibrated field. Actually, it shoots into a coil that's inside the tube.
But it has a perfectly calibrated intensity. And then it does the super duper exponential,
we'll explain how that works.
And, well, we don't know if it works! We're going to test it, and then, from there, do
the calibration procedure for the Zeeman line in the clock.
Right now, we need to figure out what our oscillator period is. So it's like a few seconds
in each direction. The field, it's not 60 Hertz. Very, very, slow.
So here is the schematic, with my nicely degaussed screwdriver. That is the 2.4 second oscillator.
Then you have a counter, and that's to count - it's a very long cycle.
[Marcel] Divide by 512.
[Marc] This is 20 minutes, right, 20 minutes timer. Here is our exponential current generator,
and it's done with a capacitor and an op-amp, it's an integration. And then, which controls
the current into the coil. Is it a relay that does the?... Yeah, and the relay does the
two directions of field, click, click, click, click. So we should hear it.
So usually it's connected to the Cesium clock, and that's what powers it. But to test it,
you can hook it up to two power supplies. You need +/- 15 Volts and 28 Volts. Instead
of the coil, we have a resistive load, 0.5 Ohms. They tell you everything in the manual.
And then we are looking at pin 6 of this guy. Which is this, which would be your 2.4 second
oscillator.
The HP manual goes sort of crazy on how to check this already over-engineered degausser.
It calls for a digital counter/timer, a DVM, and a chart recorder. I have pseudo modern
HP equivalents for the counter and the DVM, but for the chart recorder, I have the real
1960’s thing. The HP 7132A dual pen strip chart recorder. Oh boy, oh boy, what a good
excuse to geek out checking vintage HP gear with more vintage HP gear!
And then, they tell you to use a counter to do that. So, super precise measurement. And
then, in the manual, it tells you: well, you know, if it's between 2.4 and 2.8, it's good.
So you do all that complicated super precise stuff for absolutely no reason!
But this is the way. This is the HP way.
And... Well, you know what Marcel, good news, your box works! Actually, the oscillator.
So we see it: 2.75 seconds so it's...
[Marcel] ...just within spec!
[Marc] ...just within the revised 2.8 second specs.
[Marcel] It's a little leisurely. This is one of the the oldest units out there. This
is a very early serial number, so it's probably a little tired by now, but still working!
[Marc] We can make it perfect, we can adjust the resistor, if you want.
[Marcel] It's within specs, I think with we should leave it!
[Marc] Within specs! Okay, 2.75 seconds.
Now they want us to measure one of the pins of the divider that toggles every...
[Marcel] This one.
[Marc] ...17 to 22 seconds.
So here, you can get the other one, so zero, it's clicking...
Reset...
[Marcel] Yeah, you should reset first.
[Marc] Okay, and then I go click...
I don't see gating.
We want A plus, and B minus. Hmm.
[Marcel] There, it just lit!
[Marc] Yeah, it did! The gate is on.
[Marcel] So get another 12 seconds?
[Marc] Oh, because it might have...
[Marcel] It's a very long period.
[Marc] All right
[Marcel] There we go!
[Marc] 22, whoa! Yeah, we're right on the money, 22 seconds. Okay, well we could have
measured it with a stopwatch, but we did it with the counter. I didn't know it could count
such long intervals. So I learned something.
All right, so far, so good!
So now, there is a freaking old op-amp that does the exponential. And it has to be trimmed
to zero, like all the older op-amps, it has offset. So in order to do that, we need to
short the big cap. That's the one that does the exponential.
And that's to make sure that, at the end of the exponential, there is absolutely no field.
That's the op-amp that controls the current in the coil.
[Marcel] It's the end point.
[Marc] Okay, so we have to remove the side panel and access the freaking thing.
Okay, your are -14 microvolts, and you want to bring it back up.
Okay. Keep going, here you go. Excellent! So, zero point something. Okay! So this definitely
what out of spec. So, good that we took care of it.
Well, I think we're ready for the big exponential thingy!
[Marcel] Gaussian distribution.
[Marc] Remove the short from across C2, and we just checked that out, all right.
So that's the final setup. We have the box in operating condition. This is our load,
represents the degaussing coil into the unit. A little pickup resistor to bring it to the
chart recorder. And this is going to plot the current, which should go back and forth
and decrease exponentially.
I think it's all good to turn it on, and you press start, and... Oh, wait, wait, wait,
wait, I need to go down, and I need to go on.
Go for it!
[Marcel] I can already see our nice exponential decay function.
[Marc] Yeah, yeah, it's exponentially decaying! It's a very scientific degausser.
And then, you can see the maximum voltage - which is the maximum current actually - going
down. And I think they have a spec on how low it has to go after a certain number of
minutes.
[Marc] We call it a go?
[Marcel] That's good!
[Marc] Alright. Okay, we have that.
Okay, why don't we do the other curve right here.
Okay, so that would be our next curve.
And the next curve, is the low, so we have to recalibrate this a little bit, off camera.
Okay, so, are you on the low setting?
Hold on, I need to get my paper going, pen down, you go.
And that's the 22 minute test. And that's at much higher sensitivity, you can see there
is some noise in the thing now. We did have more than 10 Volts, 12 Volts.
And this is good!
Okay, so now I just have to hope that I have enough battery to last for 20 minutes!
Okay, I think I will just call it quits at this moment. It will turn off automatically
after its 20 minutes, but I think we have it tuned perfectly, where it goes to vanishingly
small voltage. So are tuning did (some) good.
So that's the high, and that's the low degaussing.
It's going to be degaussed to perfection!
Alright, we are very proud of our exponential plots that look like perfect California earthquakes.
Next step will be a lot more arduous, as we will embark on a quantum Zeeman experiment
to tune our clock C-Field to perfection. See you in the next episode!