Hello and welcome back. If you follow the channel, you might have seen part I of this repair,
where we revived an old analog X-Y recorder from HP. After replacing a faulty capacitor, it started
to work again. But close inspection revealed that the trace was wiggly. We also learned that
it uses a photochopper inside, implemented with neon tubes and photoresistors. Ah, the good old
1960’s technology. To summarize, a chopper amp eliminates the offset from a near-DC operational
amplifier by chopping the input signal into AC, and unchopping it back to DC again at the output.
But ours seems to have issues. One of the photo resistors has a weak signal,
and to make matters worse, one of the neon tubes quit while we were investigating it.
Maybe this has to do with our wiggle problem. Or maybe not. Let’s find out.
Okay, so we definitely found what the weak link is in the X amplifier. This is a good photo
chopper - you see how big the signal is. And this is a bad one. There is hardly any signal,
and that's the one that's aligned with the lazy edge.
And if you look at it, the tube appears clear, and there's light coming right through. There we go.
So that's not the neon, unlikely. It could be this, it could be that the JFET it shoots into
has some leakage, that could be true too. So this is actually a super simple construction:
it's a piece of transparent plastic, it has a hole at one end that goes over the neon tube, and
another place at the other end where you can put the photoresistor.
So what I'll do is, I'll swap the two photoresistors, and see if the fault moves with
it. That would confirm that is the photoresistor fault, and not one of the FETs. Okay,
so I swapped the two, and not only the fault has moved with the photoresistor, it's still
the weak signal, but I killed my neon bulb on the other one! So now I have two faults. Great!
So now I have to find what component that is.
The photoresistor is 1990-0072.
And it's a photoconductor. I could find no cross reference, but i have it in my micro-fiche.
It says HP01 5082-4606. So I guess that's an HP part, and the commercial number will be 5082-4606.
Except when I look for that, I am getting a seven segment display!
So we are not making much progress here.
And of course the neon bulb is special, and it's an 2140-0047,
neon lamp, 0.8 mA. And of course, guess what, I have it on another fiche!
It's right at the bottom: strike of 135V, 0.8 mA, 1/2 inch tall. And it's an A1C lamp.
To which I go in my neon
lamp book, which i found over the internet.
And it's on the table at the top. It's indeed a T2 bulb, half inch long. And there it is:
135V strike, here it says 1.2 mA instead of 0.8. Yes, 1.2 mA with 47k, but they put 100k,
so they have much less than that. It's the correct length, it's a wire bulb. And I looked at 4,
8, 6. It means it has a radioactive material in it, so it strikes in the dark,
and it's a special high brightness light. Great, now I have to find that thing!
All right, found them all on DigiKey. The A1C is available. I'll change all the
100µF caps caps since one died.
And the CdS detectors, they are rare, because you have to take one that's sensitive to orange.
So it's the 7910, it's the modern equivalent. And it's on back order, so God knows when I'll
get it. But I tried another one on eBay, we'll see if it has the the right sensitivity by any chance.
Okay, let's see if we can climb out of our hole thanks to new parts. So I've received
the correct capacitors, I'll replace all of them.
The neon bulbs, A1C, that's still made. I had trouble with the
photoresistor. And it looked good, but it's not the right diameter. And it's way too big.
And I have a 9mm one coming from eBay, but I can't guarantee that it will be the right wavelength.
Okay so we're doing better, I have the nice caps now, no more dodgy repair. My neon works fine,
and I'm back into the configuration where the good one is at its original place, with the original
neon. And the bad photoresistor is with the new neon.
And the original setup, that works fine, I have a nice big signal out of the photochopper.
And the other one is weak. This is about half the signal it should have.
So I finally found a photoresistor to replace what I think is my weak one in the plotter. But
when I replaced it, it didn't quite work. And I was puzzled a bit,
because as far as sensitivity was concerned, it was better than the original. So then I
decided to get to the bottom of it, and made that magnificent test setup.
And I can put the photoresistor and test it. So that's the good photoresistor, and you can see
over there, it's showing photoresistor action. So it's better if I put a cap
to block the light. And you see it's super good. It's in the same circuit that's in the plotter,
it drives 100 kΩ, and you can see it's pretty good. This is 10 Hz, good response. If I go to
60 Hz, it starts to be a little short, doesn't go all the way to zero. But still a good signal.
Let's take the weak one.
Okay, it's pretty weak. But if I put the cap on it, you see that the weak one gives me half the
signal, which is the problem. However it's not because it's that weak, it's because it's slow!
So, if I go to much lower speed, it's fine! So, it's just that it's somehow gotten too slow.
If I now try the new one, which is way more sensitive,
it's way higher in signal.
But when I put the cap on it, it doesn't go any down.
It will only go down if I go very very very slow.
So finally it gets there. So it's more sensitive, but
at the expense of speed. So when I put that one in the circuit at 60 hertz, it was way too slow.
If you look at this one, I think this is specialty made.
See how much smaller the detection zone is, If it wants to focus on it...
And I haven't found any device that's as small as this guy, and in a TO can package.
So I guess I'll remount this for now and just live with it,
until I find a detector that's both sensitive and small.
So Ken had the genius idea to swap the two amplifiers, and see if the fault would move
to the other side. So let me try to do this.
I have the X on the Y amplifier, and that's jerky. And I control the
X by the Y.
Yeah, so it seems to be not the amplifier. So it's mechanical. But man, I don't know what else to do!
So I think I finally got it. It's now running much smoother. And it turns out,
it was a combination of this gear backlash adjustment that was too tight, and
the adjustment of this which was too loose.
And both are really tricky.
You have to make three adjustments. One is over here. Loosen this, and then you
can turn this gear, which is eccentric. It will move it closer or farther from the next gear,
and that's the backlash. And that's the one that was the most off: it was way too tight.
And then you adjust the tension similarly by rotating
the motor. That's the one where you get it to 25 ounces. So that got me to where it started
to move smoothly without making noise. I basically adjusted it for minimum noise.
And this guy was still rattling a bit.
And the way to adjust it, is to move it all the way to the back.
And here, it's quite hidden, there's a slot.
And if you put your screwdriver in the slot, you can access a screw right here, which
vary the tension between the carrier and the rod. And that one was way too loose, so that was
causing it to vibrate. And then, once you get all of that, the X-axis started to be really smooth.
And I had to do the same thing for the Y-axis. There's also
in adjustment which is even more hidden. So you have to bring it over here,
you bring the carriage, and here it is: a screw that's hidden, that allows you to do the same
pinion adjustment for backlash as for this gear, but for the little gear that's for the Y-axis.
It had the same problem, it was too tight for some reason. Which is rare.
Usually when you get the old things it's too loose, but this one it's too tight.
And then similarly, you adjust it for minimum amount of noise.
And there was yet one more hidden adjustment for the Y-axis ball bearing play. This one
is adjustable with a super small hex wrench via two access holes at the very top and the
very bottom of the slide. Quite tricky, I would have never found it if i didn't have the manual.
And I think I have it pretty good when it's not vibrating at all, very smooth.
Pen down... There you go, no more wiggles!
And there is one more thing I wanted to show. The plotter came with a little add-on
time module, that provides a time base for the X axis. That’s something that was
part of the more expensive Moseley 135, but was optional on the 7035.
It was powered by this huge mercury battery, 12V mercury battery. So I made
a little adapter for a modern Lithium in it. So let's try to make it work.
5s per inch. On the Y, I have a nice sine wave. Let's just sweep, and get the pen down.
And there you go. You could get a nice trace of any signal you wanted!
And for the grand finale, I made a little box with two multi-turn pots and a switch and a
few resistors, and you guess what it's going to do. It is going to turn this into a giant
Etch-A-Sketch. I call it the Hewlett-A-Sektch. And it takes advantage of the port at the back.
Okay, should be able to etch the sketch. So fair warning, I'm not good at etching sketch at all. So
this one has one up on Etch-A-Sketch, is that you can put the pen up and down. Let's see... Down.
Tada! Hewlett-A-Sketch with the 7035B. And I eventually used as I originally wanted to,
to plot my filter curves, but that’s another story.
[In a very soft voice] And it can also be used in the ASMR mode.
And I will see you in the next episode...