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- Опубликовано: 20 сен 2013
- Dave explains what causes CRT vignetting problems and how to fix it.
A follow up to the HP 35660A Dynamic Signal Analyser video here:
• EEVblog #523 - REPAIR:...
Whiteboard explanation of how a Cathode Ray Tube works, and how it can cause interference to sensitive measurements and the shielding required in a precision instrument like the HP DSA.
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As Mal said, it's a lot more complicated in the actual implementation, but in general can be thought of as three separate beams, one for each colour, with three associated colour phosphor dots on the screen. There are several different technologies for the phosphor part, shadow mask is one.
By the way, I love your videos. You are very informative on fascinating topics in an entertaining way.
Well done Dave. I'm glad you figured out the shadowing. From the other damage, it was clear that this instrument has taken a tough shock from the back. Perhaps dropped on its back end. Enough to shock the neck board off and move the deflection coil back. That's a big shock! Even so, we can see it didn't take a lot of TLC to bring it back. If this were a plastic fantastic 21st-century instrument, you'd have been holding the thing in pieces.
Love the video Dave. Always great to see older analog technology! Especially vacuum tube based!
It is strangely satisfying to see stuff successfully repaired :)
At 2:17 Dave says the shutter rate is 1200 times per second. What he means is that the shutter setting is 1/1200 of a second. The camera captures 25 frames/sec regardless of the shutter speed.
That's a brand new working puppy !!
"Not a problem what so ever"
Love the perspective on the white board BTW :)
Awesome video dave!
Gotta love Dave's pronunciation of things. That's the problem when you're book-smart, know what it's spelled like but never heard it before :D
Thx dave, you made my day with this vid!!
I have to say even though this is one of the newest segments. It happens to be my personal favorite.
Are there any videos of that manufacturing process available?
Yes, I instinctively drew an electrostatic deflection system. It just didn't occur to me at the time. With hindsight it's probably less confusing that way to a beginner?
A vignette on vignetting? Great video Dave! Thanks.
I had initially suspected this was the problem, and mentioned I had already given it a push, but it didn't budge at the time. After the first video I figured (helped by all the confirming comments), that it must have been the yoke position, there was no other explanation, and indeed it was. It's not something I was unfamiliar with.
Very nice video as allways :)
Thanks Dave!
Very interesting thank you Dave
Just a note on what you said about filming a tv screen. You need to have the same (or matching) shutterspeed as the screen, but you also need the same vsync, or you'll likely get screen tearing.
God I almost crap myself laughing, "gilding the lily" "bloody ripper! what a bobby dazzler, this one" oh god it gets me every time!
A few additions to that video... There is not only magnetic deflection but also electrostatic deflection. So if your CRT hat plates inside (like most old oscilloscopes) then the beam is deflected by the electric field. There are no coils inside the CRT.
Is it composite video signal that comes from the processor board to the CRT block? You can replace it with a LCD screen
I know this video is very old but it's the first time I've seen it. so forgive me if all the following has been said in the comments.
It was pretty obvious from the first video with all the evidence that the scan coils weren't snug against the back of the tube, causing the vignetting. The tube base being disconnected is also evidence that the coils were loose, they probably moved back and knocked the base off, that'd also explain why the geometry magnets were loose, they'd have been knocked off when the coils went back.
I suspect it's had a new tube at some point if the coils moved that easily, there is almost always a bit of masking tape around the tube neck for the clamp to grip (or sometimes silicone to hold it in place after it's been tightened up) into.
Interesting fact, if the beam current is high enough and the coils are left like that it's possible to 'drill' through the glass of the tube neck with the electron beam.
Focus isn't magnetic, it's electrostatic BTW.
Thanks for the video :) can you make another one about the generation of high voltage inside the CRT TV ? Thanks in advance.
HP o scope CRTs seemed to be some of the most elongated ones I have ever seen. Now tighten that clamp until you hear glass breaking, I bought a scope, thought it was dead, but the thing was generating heat. I opened it up and the plug was off the back of the CRT & the deflection yoke also slid back due to a rather sharp impact in shipping. It had been encased in that expanding foam for packing.
Wow, tough audience Dave. Yes, Dave did draw an electrostatic deflection system. This raster CRT uses magnetic deflection. I think a tutorial like the one Dave's giving is easier to understand if you start with electrostatic deflection plates instead of magnetic deflection coils. The idea's the same but the forces involved are of course different. Analog scopes used electrostatic deflection. Raster CRTs use magnetic coils. Dave, I continue to be amazed at the rate you can chunk out these videos.
There you go, I didn't know the name for that, thanks. Even has it's own wiki page.
The very low shutter speed does work a treat, but no good for moving images of course.
I like the 3D CRT diagram on the white board.
Can we generate a simple driving signa for the CRT using a microcontroller?
Hi Dave, What's the slot on the front panel under the CRT ?
when are going to see a teardown of this?, i'd love to see the analog chanels!
Yeah, I was even thinking maybe a performance upgrade of the opamps or something perhaps.
Is it possible to reconstruct the image on the screen from the noise you're picking up off the CRT?
Yes, not sure how it was dropped, but yes, that neck board does not have much mass and is very tight, so would require a lot of G's to bring it off.
Hey, I have an old "valve based" scope with a round screen... it's a 1963 heathkit 1012 one of my professors handed down. He took very good care of it and built it well (his first of many heathkits) and it still works. Not super-useful, but it beats not having a scope...
How does the focus coil work ?
man that was informative. ripper video once again dave. square in the nuts.
right ive got that not exact, but it is still not an coil, they use like a pinhole shaped or cylindrical anode with i think a negative voltage. it should be behind the Wehnelt cylinder. sorry for getting it not so precise
Blow everyones mind, Dave, and give a tutorial on traditional 3-gun color tubes vs trinitron color tubes. It would be fun to see you try it in Dave-CAD
"Please excuse the crudity of the model, I don't have much time to paint it and do it in scale" like a Doc. And nice T-shirt too :)
My thoughts exactly. I didn't read anything where somebody said it was a "circular mask". I, along with others, specifically said that there is NO mask in a monochrome CRT.
On measurement equipment, I would not bet that they use interlacing. I´ve used once an old school, but in the time high tec Scope with Color CRT. After a few minutes, I had a headache. When I move my head, I even saw rainbow flashes (colour separation) so I even think that they skipped the three beams on that particular model I used. Measurement equipment isn´t a TV screen.
The common work around for this issues, is to use the VGA output and connecting a Computer/TFT monitor. Works a treat.
How does a colour one work? Are there miniaturized people inside, painting each electron individually?
We are in like flin... No problem what so ever.. and bob's your uncle. I just love these.
Love how you went all Australian at the end. :)
Nice one Dave. The Horiz. Def. is up & down. The Ver. Def. is on sides. The relation between magnetic fields and elec. fields, here is the same issue. And for the focusing, no easy way to focus electron by magnetic as the electron will drift aside. The focusing is been done by a voltage around 700V inside the tube, that is why you may find a variable resistor for focusing, its output is going inside the CRT, standing in front of the Electron Gun as a metal disk having a small hole in the center.
I think you've got x and y mixed up in the whiteboard diagram for the deflection plates -- by the right hand rule the magnetic field affects the electrons perpendicular to the field direction.
finally come back with the type of "lessons". Wish you buy more hp or tek instruments to repair, it will be very good and interesting also the terdown of them!
Why are scopes using electrostatic deflection insted of electromagnetic? Is it more accurate or something?
The main reason is expected deflection angle in CRT with magnetic deflection commonly used in tv sets is 110 degree. CRO tubes are very long comparing to screen dimensions, so electrostatic deflection is enough advantage is driving it by voltage which is much simpler.
And endlessly grateful :P even if I don't understand half of it
Does the disk drive work?
Yes they will, it's the only space in which they can really compete. Although they might end up getting stripped to the bone like Tektronix did.
it could be quite hard to explain because if we consider magnetic deflection we have helical electron beam it's no so easy to understand, but it works !
BTW I wonder why someone changed position and angle of deflection assembly
I wonder what happened to this device to make the circuit board fall off the end, as well as shaking loose and rotating the deflection coils. You said the device did not appear to be dropped but it must have suffered some trauma. If it was vibration, it must have been quite intense. Do you have any history for the device? Do you know who used it before it appeared in the auction?
"bobby dazzler" thats a new one I think I'm going to use it lol!
no problem, nice work. that Dynamic Signal Analyzer is a bargain.
Oh i found something else you have got Analyzer spelled wrong in the description
Will you continue with FPGA vids?
In the previous video you implied you didn't specialize in crt systems, never refered to the issue as vignetting, and tried to make the display smaller than the "dead spots". I don't expect someone who specializes in digital to be intimately familiar with discreet amplifiers. So I was a bit surprised when your next video was an educational video about crt's and vignetting as though you specialized in crt's, with no thanks or credit given to commenters who 'helped confirm' the issue, thats all.
Yeah that thing must have taken a pretty physical serious shock to the rear to move the yoke back and knock the back plug off, might want to look everything else over for any shock issues.
Nice !!!!
I'm sure I'm not the first to request this but... Would love to see an episode on CRT driver board repair / operation on colour teles or an arcade chassis.
I don't think Dave would do that, but it isn't that difficult from what I remember.
Pretty smart guy, great video. From Aviation Electronics 73"
I'm also affected by that.I hear the CRT monitor when it's on, sometimes even from another room :(
Good quality CRTs like the one here are painted inside from the deflection coils to the edges of the front of the tube. I've seen countless cheaper tubes that did not have this and any overscanning caused a reflection in the corners or even the sides in more extreme cases. It would look as though the corners of the image instead of being missing would be folded back toward the middle of the screen like a dogeared page in a book.
Not composite, it is HSYNC, VSYNC, and data bit. Possibly to replace with LCD some some effort.
If you want to understand how a colour CRT works Google for: Mullard it's the tube that makes the colour - a great 1960s film on the design and manufacture of shadow mask CRTs. It's on RUclips - an old VHS copy but still watchable.
Amazing what you can do when you know how stuff works. Just cracking it open and pushing a couple of components back into place, and you can prevent throwing away this piece of equipment worth hundreds of dollars! It's not always that easy of course, but when it is... feels good to be in complete control of your tools. I feel the same way when I can whip up a Greasemonkey script to fix a website I use (I'm looking at you, RUclips!!!)
wish i had just 10% of this guys electronic engineering knowledge.
Great video... I don't care if you got all the names right... this is info "close enough" to learn how to do simple repairs... thanks for sharing !... it takes quite some balls to talk to nerds about a topic where you don't have a black belt.... I could be doing videos myself if I had such balls...
With CRTs on certain refresh rates I can see the screen refreshing with my normal eyes, my eyes are very sensitive because of me having autism. So with CRTs I sometimes have sensory issues with them with the sight and sound. so basicly I can see it refresh and hear the flyback as a very high pitch sound.
clearly this was dropped which dislodged everything then, great video!
Welcome to in depth fucking friday !!!!!!!!!
On a CRT, if toy remove some of the coils (tried on an old 5 inch black and white portable CRT TV, you can quickly cause a burn in on the display) (I actually took it apart while it was running :) ) (though I did not get a chance to play with the high voltage stuff (should have ignored the warning around it and see if it would really hurt to get shocked by it :)
Yeah, I noticed that immediately too. Electrostatic and magnetic deflections are rather different beasts. Electrostatic moves electrons even if they are not moving, but magnetic only deflects moving electrons. After one more generation, will we have all forgotten how CRTs work? Guess we'll still have mass spectrometers to keep us on our toes about charged particle deflection.
What a Bobby Dazzler that one!
Electrostatic deflection is good at deflecting random small angles very quickly, but doesn't handle large angles well.
Magnetic is good at large angle deflection, The circuits are designed for a single or a small number of angles.
You are a lucky guy ;)
Really makes me wonder who dropped that unit. loose connector, loose deflection assembly... Suprised it still works
Focus coils went out with buggy whips; your monitor uses electrostatic focus. And rather than bicker over the correct pronunciation of "vignetting", call it "neck shadow" instead. If you want to perfect the centering of the raster, there's a couple of ring magnets clipped to the back of the deflection yoke; rotate them to center things up. Other adjustable magnets around the periphery of the yoke are to correct pincushion.
Its way more complicated to setup a colour CRT as well. You've not only got convergence (getting all three beams in line and geometry adjustments), but purity (getting the beams to hit their phosphors as accurately as possible). Computers were used for setup towards the end of colour CRT production. Minolta, Image Processing Systems of Canada and another Japanese company I've forgotten the name of used to make kit for this. Sad to see they are long gone already.
Yes it is. Even from quite some distance away. In the good ol' days of CRT computer monitors this was used for spying regularly.One of the reasons why some major technology companies went to LCDs so rapidly, even while they still were small and expensive.
Well, that is untrue. The Wehnelt cylinder forms part of the electron emitter assembly, and provides you with a narrow beam focused at al short distance of it (its position depends of some voltages I do not want to get into). The focusing coil exists, it is much more voluminous, and its purpose is to focus the beam on the screen. In this case it may well be fixed magnets.
CRTs have both AFAIK.
Where did the missing electrons go!?
Don't know, need a 3.5" floppy to test it!
Hey friend!
this vignetting is a french word...
that first "g" has to not being spelled...
hate to point, but i do it for serving point - not to hurt Your feelings, Man!
Keep on cool videos!
Greetings from Ukraine!
16:34 its super effective!
Funny... was screaming at the video about the yoke alignment and seating. I used to repair 1st and 2nd Gen Macintosh video systems quite a bit. It is not uncommon for a short, sharp, shock to knock the yoke and Cathode connector off the tube. Especially when the assembly is old... lots of thermal cycles makes everything loose. Doesn't take much of a knock to bugger the whole thing.
16:34 it's super effective!
crts are awsome and provide the best colours
bobby dazzler.... lol what a phrase.
I've never seen a tube with focusing coils, only focusing plates.
I had it right!
Though spelt vignetting, it's pronounced 'vinyetting' Dave. :)
Click and Clack, the Tappet Brothers would probably disagree, whilst driving a "Chevro-LET" of course. :P
sounds horrible... so, do you glow in the dark? :)-
The closest I got to the insides of a CRT was when I tried to rebuild one but putting a new gun on. Interesting experience w/ glass blowing and the whole process of drawing a vacuum bake into the tube. Tube worked too at the end which was really amazing. This was back when I was repairing TVs.
It's an endlessly tough audience!
Bloody ripper! Bobby Dazzler!
Looked to me like he got electrostatic and magnetic deflection mixed up.
Are there any Chinese makers trying to make such high quality and relatively low volume equipment as this? HP just had a big divisional shakeup. Will the newest corporate incarnation of HP still make such high quality lower volume gear?