Hi Just a quick follow-up video on this classic Hulet Packard 35608 Dynamic Signal Analyzer or Fft analyzer as they're known in the trade. And if you haven't watched the previous uh repair video of this, um, it'll be linked in down below. So please watch that first now. Uh.

One thing that was noted in the previous video is we've got rounding on the CRT display here. Look at these dark round patches on there like that and uh, anyone in the uh, photography or video industry will know that as uh Vig netting uh effect around the outside now um, what is causing this now uh, you know there were quite a few people who said oh, there's a, you know, a circular mask on the front of the CRT like that and that's what causing it. You got to shrink the display and all that sort of stuff. Well, no, that's not the case.

Trust me. This is the proper uh size for this screen. I've used used it before. it is actually a rectangular front.

It's nothing to do with any sort of circular masking there whatsoever. Um, but the reason for this is, uh, very obvious. but to understand it I Think we should go to the Whiteboard but just a quick little uh note on why you might see this screen pulsing a little bit. Um, the variation in the brightness.

Well, that's to do with the update, the scan update rate of this uh, ancient CRT display and my camera. My camera is currently uh, shooting this at 25 uh frames per second. All right. I Set the camera to Now 1 12th of a second.

So 12 updates per second and you'll notice we don't get the flicker anymore. but look at that, you don't. You get blurring in my hand as I move it around. That's because.

well you know we're only updating that 12 times per second I can set it as low as six I'm using manual uh shutter priority here and that's one six of a second and you'll notice of course no more flickering of course cuz we get that Persistence of vision update but this is really slow and this is the maximum shutter speed of 12200 times per second. and look at that. I mean you know it's going to be really Qui now but sorry for anyone having Eptic fits or something. Should have put a warning on the video anyway.

I Just thought I'd throw that in there cuz that's interesting to know. if you're ever shooting uh video of any CR displays be the television, old computer, you know, retro computer monitors or something like that you've got to use. Uh, if you don't want that flicker to show up, it could be. It depends on the uh raid.

Of course you might just get lucky. but uh, generally you want a camera with shutter priority mode and here we go. How a CRT works and this is probably almost very familiar to most of you? I'm sure, but we'll just go through it now. Um, as you know, CR screen.

It's an evacuated uh tube like this. In this case, it's not a round tube on the front. A lot of the old oscilloscopes back in the old days. Yes, they actually used to be round front uh tubes on them.

If you've seen those real ancient valve based um, crows, cathod oscilloscopes, well this one's not. Of course, as you saw, it's like a square front on it, but then it tapers off into a round neck. This is the neck part of the tube down in here and basically how CRT works. It's got a heater in here.

it's got it heats up the cathode which generates electrons which are then accelerated in a beam via the high voltage anode here. and that's that uh, little strange plug you see attached to the side of the CRT And it's often as you saw inside the Uh DSA it's got a protective shield over the wiring. that's the real high voltage stuff. You know, that's the 5, 10, 15, 20 kilovolts stuff, you know, so you're really don't want to be touching that.

but then that inside generates the potential difference between the cathode and the anode to accelerate the electrons as a beam via this. and then we have a focusing coil here. In this case, it's an electronic uh focusing coil. It allows you to uh adjust as it says the focus of the beam, the narrowness of the beam in there.

and if you and if that's all you had, then you would see a very bright dot right in the center of the screen on the front. and of course the front of the screen inside has a phosphor coding. This is only a Mono CRT I won't of course go into color. Um, which is, uh, different? Well, it's similar.

It's got three different beams. Well, let's not go there. gets a bit complicated. let's stick with the mono one.

So if that's all we had the heater, the cathode, the anode, the high voltage potential difference, the phosphor on the front, and the even without a focusing coil, you would still get a DOT on the front of the screen there. But of course, what we need to form a raster image like this. with a horizontal and scan like this. As you know, a CRT is scanned in lines, horizontal and then a vertical number of lines like that.

this DSA Works No different whatsoever. It's scanning like that. Um, and the way they do that is with deflection coils because this Electron Beam can be deflected with a magnetic field and that's all they have. They have four coils here.

Please excuse the crudity of my Uh 3D model here. I'm not very good at drawing this sort of stuff, but eh. anyway, it'll do. Basically we have wide deflection plates on the top and bottom or deflection plates.

deflection coils. Um, they're implemented as a as coils inside the unit as you'll actually see, but they've got one on the top and bottom and one on the side for the X. So when you energize the X coils here, you can make the beam sweep across the front of the CRT like that. and likewise with Y, you can select which line you want.

And of course, you can turn the beam off and on very quickly so you can actually generate individual dots. and then you can build up a dot matrix rasted screen like that. So that's all there is to it. Pretty simple, but what's causing the Ving getting on the screen? As you saw we were getting dark patches, a dark corners on the screen like it was some circular.

You know, like there was some C cular mask on the front of the screen. And what is the only circular thing inside this thing? It's not the front of the screen cuz it's like rectangular. It's the neck. Here, the neck is circular.

So what we've got If these are uh now, I've got a side view of the CRT and if these are the deflection coils here, the X and Y, the Y's on top there and the x's on this side and The Far Side there. Now Normally this uh, deflection coil is put pushed right up against the neck of the CRT here. so let's actually draw that as if that was the case. Okay, if it was like that, please excuse the crudity of that.

it's right up against the neck like that. And of course, this is the point where the Electron Beam actually bends. So if we've got our Electron Beam coming through here like this, it's at this point that it's going to bend like that and go up and hit the screen up here like this. And if it's right up against the neck, then there's nothing in the way to display the full screen of this thing like this.

But if this deflection system is moved backwards along the neck like that, which will represent which we can show you right now like this. Let's say it's like that and it's moved back along there. Then what happens? The beam comes through here and then it tries to bend too early and it hits some of it. The outer part of it hits the corners like that.

and what does that turn into on the front of the screen? If this is the front of the screen like this, you get an image that is, you know, perfect in the center, but then has these rounded corners on them because it is getting, uh, the beam is hitting the internal edge of the Uh CRT there. So you're getting these rounded corners and that's what's Happening Here. The uh, deflection coils are too far back on the neck like that. Very simple.

There's no real other explanation for it. So yesterday when I was doing The preliminary repair on the scope, I did actually push that uh for for cuz that's the obvious reason for it. but it didn't budge so and I couldn't because it had all the shield in all over the CRT I couldn't see that it was directly up against the neck and I thought it was, but obviously it's too far back on the neck. I've got to use more Force use a bigger Hammer get a bit medieval and give this thing a bit of percussive Maintenance and there's lots more advanced stuff which goes into these CRTs as well.

You saw those little permanent those four little permanent magnets sort of, you know AR changed in various locations. There's various techniques. there's various patterns on that sort of thing for actually. uh, optimizing.

uh, the screen, the roundness of the screen, and the geometry of the image and all that sort of stuff. bit of black magic goes into that, but we won't try and explain that today cuz we don't have the full info. and yes, as it turns out, I just applied a bit more uh force on this thing and I was able to move it uh, back in there so that looks like it's the issue now that looks pretty much that feels like it's now all the way in. Uh, to really see that I'd have to take the metal shield off which I don't uh, particularly want to do, but uh I believe that that really feels like it's in there now.

so there was just, uh, something maybe stuck on the uh side of that. Which meant that when I pushed it Forward uh in the previous video, I thought it was in, but it wasn't So I'm absolutely certain that's what was causing the V head in that we saw on the front of the Uh CRT in there. but um, we can actually play around with that and experiment and uh, see that we'll get um, as we move this entire deflection assembly back and forth, we'll be able to see that uh, we can change the amount of uh vignetting on the front of the CRT and of course, we can uh, rotate this entire uh assembly as well. which uh, then gave us the rotation issue which we got before.

but uh yeah. basically we just want to move it back and forth on that neck and we should be able to see this. and by the way, yes, this is all powered off and it is uh, uh, safe. once it's all powered off and discharged, then to um, uh, go in there and sort of, you know, play with this assembly.

but you don't want to be around with this when the thing's live, that's for sure, unless you absolutely know what you're doing. Anyway, what we've got here is our four wires. They're our deflection coil assembly one for the X, one for the Y coil up in there, which it goes through all the Bobb and I won't. um I don't know.

I might take this Shield off. maybe if it's relatively easy, but you'll notice that there was no uh focusing coil which we saw on there. There actually is, but it's inside and it's coming up. You can see the focus pot down here.

there's the focus pot and it looks like we got the focus wires coming up down into the uh neck board here and then they've got extra pins on there which then go through. So they're going to have the the focus coil on the inside of the tube instead of on the outside like we uh, showed simplistically on the Whiteboard and Bingo. As we expected, once we pushed that deflection coil All the Way Forward right at the base of the neck. Tada there's our test pattern.

No problems whatsoever. Beauty Let's see if we can make it come back. I've just moved it a tad back here I Think the rotation slightly off so we might see a little bit of rotation there. but let's have a go.

And there we go. It wasn't uh, quite as far back as what it was originally and you can see it just starting to appear on the corners there because that Electron Beam is hitting the inside of that CRT tube there. Bingo fixed. I'll just push that back and uh, screw that back into place and Bobs your uncle.

and by the way, this front panel mesh here, as several people pointed out, is actually a screening mesh to stop any of the Uh CRT scanning frequencies escaping from this thing. And as we saw in the previous video, the uh, the shielding on this system, the way theyve designed the shielding is absolutely incredible. Belt and braces stuff, cuz this is a really, you know, a Precision bit of Kit down in the low frequency range. So um, the scan frequency of the CRT you know, however many tens of kohtz it is.

Um, it's going to be in that range is smack in the middle of the DC to 100 khz measurement range of this thing. So you really, you know, um, that's a really vital part of getting. uh, you know, the noise floor and the performance out of this instrument. Now you may actually be wondering, well, where is the ground connection for this? uh, metal mesh screen? By the way, it's just like your, uh, microwave oven.

uh, for example. except that's stopping, you know, 2.45 GHz This one's stopping low frequency stuff. but to do that, it's got to be grounded in some way. and there's no grounding connection on there.

Uh, really. But this, um, the plastic. What you might think is a plastic front panel in here? It's not. Check it out.

nickel screened look at that. theyve got. it's all conductive. So and of course, this goes back to this metal tab which is connected onto the Shazzy and everything's just fine.

So they're really gone to town on the shielding of this thing. but not surprising considering the Precision instrument that it is. It's really designed for low noise, low signal level, and they just can't tolerate any lack of shielding whatsoever. So of course they just went completely.

belt and braces on the thing, nickel screened all the plastic in the front, the shielding on the CRT which keeps it out all the you know, triple screening in here, all of the design of the shazzy this ridiculous, you know, like just even the front, the gold on the front Um plate there, and the Uh shielding, huge shielding box on the Uh input and Source terminals. Then they've got the Uh shielding inside these individual cans and then they' be shielded again and they're probably using Um screened read relays in there for the switching. and oh goodness, wo talking about guilding the Lily But that's what you get in a high performance, high price instrument like this. And can we actually measure that noise? Oh, you bet we can.

All you need is a scope probe, put it up to the front there, and uh, just the pickup on the probe is enough to get the noise there. And if you take that off, you don't get nearly as much because you've got some extend metal in there, um acting as an antenna. But there you go, That's a rather effective antenna. We've got the uh, vertical uh rate here.

56.1 Hertz There it is. So that's our vertical scan and then if we zoom in on the scope, you'll notice this uh, higher frequency stuff and that There is about four divisions there and we're 10 microc per division. 40 micros? That's 25 khz. That's that's the horizontal scan rate.

So there you go. We can easily pick up that and in, you know, High Precision measurement environment which these Dsas are specifically designed for. That sort of noise can really kill you. and of course we can make that go away with the magic screen inside here.

So here's our probe. There it is. It's picking all picking up all that junk. You put this in front of it and the tabs on there are connecting through to the metal case on the back, which is actually uh, screened.

As you can see very well, it's probably there, but it's probably going to be very low. It's going to be very effective. Look at that. Ah yeah.

I Can't even get that. That's just other crap in the air. Bloody Ripper What a Bobby Desler This one Really like it. Hope you like the video.

catch you next time.