Today on the bench, we have the HP 7035B X-Y Recorder. It's an X-Y
chart recorder that has been donated by a viewer, Kevin Counihan. Thanks Kevin! I've
been looking for one of those for a while. And I need it because I want to make some
measurements on the Apollo hardware. It's one back there that I need to
make a filter for. And it would be a good idea to plot the results on an X-Y plotter.
This cutie comes to us from the early 1960’s,
and was the budget model, which would have set you back only $800 in 1963.
It was made by HP’s first ever acquisition, the Moseley Company, which would later been known
as HP’s San Diego Division and would gives us all the HP-GL plotters you’ve seen in previous videos.
It is itself the cost reduced version of the more potent Moseley 135, designed even earlier.
It's a servo-controlled machine, with a pot which is on the slider over here,
and another one over there. They made it very easy to access, so here you go.
And here you can see, the slideware is just right there.
There is a leaf spring over here that you have to push. There we go, voila. And now it will
come apart, yep, all right. So this appears in good shape, I think it will clean fine. I'll
just use alcohol first, and then contact cleaner. I usually do my cleaning with mostly isopropanol.
It gets rid of most of the dirt. Okay, it needed to be cleaned!
So there is not much oxidation at all, so I think all I'll do, is do Deoxit on a piece of wipe.
Now on this side, to remove the plate, it's just four screws, so it's really easy.
I need to get in there so I can lubricate this and check for backlash. And there is no backlash,
okay. The only backlash I see is here: that should be tighter. And there's an adjustment
here at the back. We'll see if it works first, and then adjust the backlash second.
This is some 0W20 oil, I should probably do on the plastic gear over here.
Make sure I don't put it somewhere else.
OK, let’s take a peek inside now. And I am particularly excited to see an unusual component
called a photochopper. We’ll definitely need elevator music to explain this unusual animal.
So, let’s say we have to build a servo amplifier.
It’s a high sensitivity, instrumentation-grade operational amplifier, and it’s 1960,
so we can’t buy it in an IC. We have to build it ourselves out of discrete transistors.
It will have two important sections, the differential pre-amp,
which is the sensitive part that matters, followed by a power amplifier.
But there is a problem. If I set the input to zero volts,
the output will NOT be zero volts.
This is because our differential input stage is made of individual transistors,
resulting in a small input offset, usually on the order of 10 mV.
But our amplifier is super sensitive, in the case of our plotter, the lowest range is 1 mV
per inch. So the offset is much bigger than the signal and will send the pen up to the rails.
Now, we could deal with it by providing But that won’t work
either. The offset is very sensitive to minute temperature variations between the transistors,
and it will start to drift all over the place. You’d be zeroing your recorder all the time.
However, a solution to this problem was invented in 1949, in the tube era,
by a very clever chap named Edwin Goldberg. He added a switch at the entrance of the amplifier,
which was actually a fast relay. He actuated the relay at a few 10’s of Hertz,
which chopped the slow input signal and transformed it into AC.
So now, assuming the input signal is something else than 0 Volts,
one gets a square wave at the output.
Mind you, it is still on top of a large DC offset that has been amplified.
The next trick is to add a capacitor between the stages.
Only the AC goes through the cap, and the DC offset it blocked at
each stage and not amplified into a large error.
The first stage DC offset doesn’t matter anymore.
Now, all you need to complete the amp is an un-chopper stage at the end.
And voila, you have invented the chopper op-amp. Congratulations.
You can still buy their improved descendants today, the chopper stabilized op-amp,
like the AD8571 from Analog devices, with offsets down to a microvolt.
But we are not done. It’s 1960 after all, the era of solid state electronics,
and we don’t want to use a clunky relay for our chopper. That’s where our photochopper comes in.
It uses a photoresistor as the switch element.
You guessed it, a photoresistor is a resistance which value changes with illumination, from 10’s
of megohms in the dark, to a few 100 Ohms when illuminated. A light controlled switch, really.
We need two of them that close in alternance to complete our switch.
To activate them, we can’t use light bulbs as that would be too slow.
And we won’t have LEDs either for another 20 years.
So instead, we are going to use a miniature neon bulb.
We connect it to the mains through a diode and a resistor,
so it will light up every half-cycle at 60 Hz assuming we are in the States.
We use the same circuit for the other half of the switch,
this time with the diode in the other direction, so the bulbs are illuminated in alternance.
Alright, we can get rid of the mechanical switch altogether now.
You can use the same scheme at the output end to unchop things,
but in our case the plotter uses another trick that we don’t need to get into.
Alright, photochopper amp done!
Oh, I see, my photochopper is right there! I think that's what it is,
a photo resistor/transistor at each end, and a neon tube at the other end.
And I took mine out of its little tube, and indeed it looks like a Cadmium Sulfide photoresistor. So
let's put a little bit of contact cleaner on the rotary switches, and check the capacitors.
So from the schematic, I thought it was okay to test the few caps in circuit.
It's a 100 µF. Tests at 113 µF. 107 µF, 98 µF. So those appear that they
are not modern Chinese crap-acitors, they are good old capacitors that have retained
all their capacitance. This one, in circuit, it says 283 and should be 350 µF.
But all it does is the pen lift I think. It doesn't go to the electronics,
it goes to the solenoid that lifts the pen. So I think we are fine to leave it at that. And I
don't think I have to reform anything, I can just go straight and put the 110 Volts on it.
Some groans...
Pen down, pen up, that works. Servo on. Okay...
Zero. Whoops, that's not working. So it went - okay, this one goes.
So Y, the Y servo works. But X, we got nothing.
Okay, so do we have chopping action? Can we see? Oh yeah, I do! So that is fine, the neons are
striking. Okay, then we just have to figure out what's what's happening. So this are basically two
big op amps. Then we have to take it from one end and see if it comes at the other end as it should.
That's the whole schematics of this thing. There is the Y amplifier, and the X is the same,
so they don't repeat it. So we're going to start with just the power supply, it's just done with
power zeners. Two power zeners in series, one that drops to 15V and the other one that drops to 9V.
All right, and it gives you an order of the ripple. Let's go look at that.
And if we turn it on, it starts good, and then goes to zero.
Looks as if we were AC coupled, but I'm not.
Ah, okay, this is better! I just changed the ground. I grounded myself to the chassis
before, but apparently the chassis is not it. Over here, moved that from the chassis to
over here, and now it's a lot better. All right! Do the same for the negative supply.
It looks to same.
Oh, the two channels are isolated! That is why! Okay, that makes sense.
So X and Y are completely independent. They can be grounded differently,
which is why the chassis ground doesn't work. Okay, so that's the second channel, which is fine.
And then we need the negative.
Oh! No negative supply!
I have the plus voltages, but I have zippo zero on the minus voltages. And I have nothing over there.
So since I have the plus, I would suspect it's one of the
two diodes. Or the two diodes that don't work?
This diode is a short. Okay, I need to get in there and disolder a diode.
Okay, it fell right out.
Okay, this one. Oh this one is good!
Did I take the wrong one?
So now, wait a second, this diode, we removed it, it's still shorted!
This one is good, this one is good, this one is also shorted.
What? Is the negative cap shorting?
Okay, maybe that explains it.
This diode looks shorted, but it's not shorted! I removed the diode and it is
good. Is the capacitor shorted, or is the transformer shorted?
So here is the cap in question.
Zero volts across it, it failed short!
Okay. Right, so that would be better than a transformer short.
I didn't have the right kind, I can never remember which one is the
axial and which one is the radial, so I made this. I thought I measured all the caps before
and it tested good. So it might have failed when I repowered it.
And it's a 25V cap on 20V so it's pretty stressed, maybe I should replace them all.
Alright, try it...
Yeah, -23V! Did I recover X-Y? Oh, we certainly did something!
It was making this weird noise because it was at the
end of the travel. But I think we have recovered X servo action and Y servo action.
Okay, let's do a quick test. I have my precision voltage supply, from HP of course.
X axis, one volt per inch. So if I go 1V, it moved. One, two, three, four, five, six, seven,
eight, nine. Oops, went back to zero.
Zero, one - say one point one. Two, two point two. Yeah, this one is too high also.
So the gain makes it a little stiffer, and the calibration
is actually to adjust it. So we should be right on the dot. That's a zero,
one, two, three, four, five, six, seven, eight, nine. We got it!
One, two, three. And the bar, you can't probably see it, it's right there. Four, five,
six, seven. It's in the middle of it!
Okay, so it should be adjusted.
That brings us to the very annoying question of the pen. And I tried a few, I have them down here.
Unfortunately the original stubby plotter pens that fit in the 7035B are different
from the later plotter pens that we revived in another episode,
and I could not find any of them anywhere. So, not these ones.
But it turns out the simplest solution, the one that worked the best on normal paper, was our
sponsor's pen. It was almost meant to be, the PCBway pen! You can doodle all you want.
If you want something that's closer to the original result,
Pilot pens, of which I have a few, are a good solution.
But they work best on better paper, so I use the back of photo paper.
I think the best one was the Pilot Razor Point 2. And I just put some tape around
it to make it the right thickness. I think you just put it like that.
So these need a lot less pressure, so they work with the original setup. So I have the pen on-off,
and how does that work - there we go.
This one makes a super beautiful trace.
But you need a really good paper, if not it bleeds. So you can't do it on regular paper.
So we got it to a point where it kinda sorta of worked.
Good enough to trace a curve. However when you looked closely, something was off.
There was a lot of wiggle in the trace that shouldn’t have been there.
So I was noticing some wiggles in the curves. You can see them pretty good over here.
It was most obvious at very low speeds. It looked almost like a staircase.
I think it's the X. And if I make it work here, you can see that this wheel is kind of jerky.
And if I take it off, I turn that, there is no jerk at all into the mechanicals.
Then I was checking the tension with that fancy tool.
33, so it's almost in spec. I'm going to loosen it a little bit, to see if it changes anything.
But if it doesn't, then I'll bet you it's an electronic problem.
So I readjusted all the mechanics. The pulley was not quite straight, it was wobbly. And the tension
of the belt is now perfect the 25 oz, right in the middle. The motor is still not doing good!
And if I check the photochopper there is a little bit of a suspicious action.
Green is the Y-axis, which is doing fine. Yellow is the X-axis, and you can see it's a little lazy.
You see how it's not chopping quite right.
So one side of my amplifier, one resistor or one neon is lazy.
Alright, it looks like we’ve hit another snag and we’ll need a deeper dive into the
photochopper after all. Fret not, we will get it to work right eventually, and even make a
fancy Etch-A-Sketch out of it, but I’m afraid it will take another episode. See you then!