EEVblog #524 - Vignetting on a Cathode Ray Tube - Page 1
Forum section: electronics / general

author: EEVblog
author_rank: Unknown
timestamp: September 21, 2013, 10:32:16 pm
content: Dave explains what causes CRT vignetting problems and how to fix it.A follow up to the HP 35660A Dynmaic Signal Anayser video here: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.

author: RupertGo
author_rank: Regular Contributor
timestamp: September 21, 2013, 11:25:40 pm
content: Yay! Fixed!Two things, though - I've always pronounced it ving-yett rather than vig-nyett, because it's really a French word and that's how it's said in la belle France. Could certainly be wrong here too.However, there won't be a focus coil inside the tube. It'll be electrostatic focussing, so there'll be some sort of plate system to tighten up the beam. CRTs can have any mix of electrostatic and electromagnetic beam control after it leaves the gun, and in the early days of long-neck CRTs there were some that used coils to focus the beam and plates to deflect it. This is quite a short modern tube, though, so it's got coils for deflection but plates for focussing. I don't know of any CRTs that had internal coils - but again, open to re-education.Bonus fun: when I was much younger, I used to take old portable TVs, slide the deflection yoke off the neck and put a salvaged yoke back on, which I fed from a stereo audio amp. You end up with a really cheap Lissajous display that produces lovely patterns in time to any music you play into it. Quite dangerous if you're not confident of what volts live where, but I managed to get invited to various parties on the strength of being able to put on a freaky sound-to-light show. In the end, I even had a system with a cheap video camera and a junked projector that could throw the images around an entire room, which meant I met far more interesting girls than I ever managed with my ham radio gear.Result.

author: c4757p
author_rank: Super Contributor
timestamp: September 21, 2013, 11:34:14 pm
content: Quote from: RupertGo on September 21, 2013, 11:25:40 pmI just let Dave have his Aussie-isms - they're as just bad as we Americans are!

author: N2IXK
author_rank: Frequent Contributor
timestamp: September 21, 2013, 11:42:25 pm
content: Thanks for the update video, Dave!There isn't a focus "coil" inside the tube, but a focusing anode. This an electrostatic focus tube, not an magnetic focus type. Magnetic focus CRTs pretty much went away back in the 1950s, because it was much easier to supply a simple DC focus voltage (at essentially no current) than the substantial DC current needed to energize an electromagnetic coil. Not to mention the savings in copper and steel over a wound focus coil.

author: Hydrawerk
author_rank: Super Contributor
timestamp: September 21, 2013, 11:44:10 pm
content: Those CRTs are gonna be like vacuum tubes nowadays... Our children will see them only at a museum, while we grew up in a world full of CRTs in TV sets or oscilloscopes and other signal analyzers...

author: Bored@Work
author_rank: Super Contributor
timestamp: September 22, 2013, 12:01:23 am
content: Quote from: Hydrawerk on September 21, 2013, 11:44:10 pmThese days try to explain rotary phones to kids.

author: c4757p
author_rank: Super Contributor
timestamp: September 22, 2013, 12:15:28 am
content: Quote from: wilfred on September 22, 2013, 12:00:00 amAh, OK then, "Dave-isms". I've got some pretty cringeworthy Chris-isms, so we're still even

author: N2IXK
author_rank: Frequent Contributor
timestamp: September 22, 2013, 12:16:09 am
content: Quote from: Hydrawerk on September 21, 2013, 11:44:10 pmSome kid eventually has to wonder why that big streaming video site is called You*TUBE*, right?

author: c4757p
author_rank: Super Contributor
timestamp: September 22, 2013, 12:22:01 am
content: Well, it's online, and the Internet is a series of tubes... so...

author: free_electron
author_rank: Super Contributor
timestamp: September 22, 2013, 12:31:17 am
content: glad to see it's fixed.i think what happened is that magnet assembly got jolted loose with the yoke and probably a bit of plastic broke off.when you re-set the yoke the first time that prevented it from going forward. pulling the yoke back and then re-set it solved that.

author: IanB
author_rank: Super Contributor
timestamp: September 22, 2013, 12:49:36 am
content: The traditional solution for CRT flicker was to run an external sync from the CRT to the camera. Do modern cameras have an external sync input?

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 04:59:30 am
content: Quote from: IanB on September 22, 2013, 12:49:36 amNot at the Prosumer end. You have to go up to the pro market to start to get them with a genlock input these days. Mostly done because they are so small that there is no room for a connector likely to be in most cases unused. You only find spare space on large broadcast studio units which definitely will all be locked to a master clock.

author: carbon dude oxide
author_rank: Frequent Contributor
timestamp: September 22, 2013, 07:17:20 am
content: Quote from: Hydrawerk on September 21, 2013, 11:44:10 pmIm quite young and i enjoy vacuum tubesive finished building a clock using nixie and i have a freind my age who works in a high grade audio shop so i get to see all the vaccuum ampsmy girlfriend however does not understand why i like them XD

author: Fezder
author_rank: Regular Contributor
timestamp: September 22, 2013, 07:31:33 am
content: Nice video! yet another tip, if i ever encounter this problem....none of my broken in-service scopes have this problem :S but they all have screen problem however haha...

author: G7PSK
author_rank: Super Contributor
timestamp: September 22, 2013, 08:32:49 am
content: My father was/is a professional photographer and he always leaves the g silent. The word is late middle english, and is a small ornamental design filling a space in a book or carving typically based on foliage. The word is derived from the French for vine (vigne). How it came to be used to describe a photograph that fades into the background I do not know.

author: EEVblog
author_rank: Unknown
timestamp: September 22, 2013, 08:36:22 am
content: Quote from: wilfred on September 22, 2013, 12:43:23 amI usually set manual exposure for most shots. I've set it as a dedicated button on the back of my camera. You'll often see me push the button during a video. For example, if there is a white envelope on mailbag, and then the contents are pulled out and it's under exposed, I'll then hit the exposure button to lock it back to the new contents. If I know a shot will benefit from autoexposure then I'll just leave it on that. The majority of shots are best shot with manual exposure though. For example, a PCB teardown and my (pale white) hand comes into the shot. On auto exposure you'd have the camera compensate for that and the board darkens and lighens every time I move my hand into the shot, and that can be really annoying to watch.

author: JoeMuc2013
author_rank: Contributor
timestamp: September 22, 2013, 09:01:26 am
content: Good one, Dave! I knew you wouldn't leave it half-done.Anyway, why would a manufacturer ever allow for this to happen at all? It does look like a 100% solid device otherwise. So wtf... they saved a few pennies at the wrong place.

author: SeanB
author_rank: Super Contributor
timestamp: September 22, 2013, 09:36:38 am
content: The monitor is a bought in unit, made by a good manufacturer, Matsushita ( National Panasonic) and is about one of the best you could get for reliability and life. No compromises on parts, no overrunning and no bad capacitors. Was designed for a 10 year life on 24/7/365 in a rack. i have seen many that have had a burn on them such that you could read the menu when off, but they still were working.

author: tecman
author_rank: Frequent Contributor
timestamp: September 22, 2013, 12:34:10 pm
content: One other note. On the back of the deflection coils you can see a black ring. It is most likely two black rings, which are weak permanent magnets. You can rotate them to deflect the entire raster; in other words to center the image. If you notice that the raster was not quite centered in the CTR, by adjusting the rings you could better center the image. The other magnets that had broken loose are for geometric correction. To reach the corners of the screen rectangle, it takes some correction since the distance from the gun to the corner is longer than to the center of the screen. These magnets perturb the sweep's magnetic field to correct this error and linearize the display geometry.I have a DSA as well. When I was looking the price on the used HPs was just way beyond my range. I eventually found an Advantest R9211C. The menus are not as logical as on an HP, but performance is as good. The Advantest units were sold by Tektronix in the 90's. Smaller CRT but more math functions available compared to HP and a built in thermal printer make it a descent unit. Best of all is that it is 13"x7"x17", quite a bit smaller than the HP and only 35 lbs. I can actually carry it other locations without a hoist.paul

author: N2IXK
author_rank: Frequent Contributor
timestamp: September 22, 2013, 12:59:14 pm
content: Quote from: Fezder on September 22, 2013, 07:31:33 amOscilloscopes don't use magnetic deflection coils. In order to achieve higher bandwidth, they incorporate 2 sets of electrostatic deflection plates internal to the tube.The only neck-mounted coil you might see on a scope CRT is for trace rotation.

author: Fezder
author_rank: Regular Contributor
timestamp: September 22, 2013, 01:57:02 pm
content: Quote from: N2IXK on September 22, 2013, 12:59:14 pmahh, i see. didn't know that eitherthanks for info!

author: FrankBuss
author_rank: Unknown
timestamp: September 22, 2013, 05:56:54 pm
content: Nice that you fixed it.Dave, you mentioned something about to be careful about charged capacitors. How did you discharge them or is it sufficient to turn it off and wait an hour, or a day and then poking around with a neon-lamp tester screw driver at suspicious parts to see if there is any charge left? I plan to repair my Vectrex, modifying it forbuzz reductionand I don't want to get electrocuted.

author: mrflibble
author_rank: Super Contributor
timestamp: September 22, 2013, 06:10:42 pm
content: Quote from: FrankBuss on September 22, 2013, 05:56:54 pmIf you do that, remember to use the right one.

author: steve30
author_rank: Frequent Contributor
timestamp: September 22, 2013, 06:14:10 pm
content: Nice video. I like the look of that green CRT. Looks nice and glowy.

author: FrankBuss
author_rank: Unknown
timestamp: September 22, 2013, 06:33:01 pm
content: Quote from: mrflibble on September 22, 2013, 06:10:42 pmOk, maybe not a good idea to use it, I imagine the sausage in the video would be my fingerFound the service manual:http://www.vectrex.nl/docs/vecman.pdfThe voltage at the CRT is 5.8kV. But I don't plan to power it up when the case is open, so I hope that I can still do it when I know how to discharge it safely.

author: jahonen
author_rank: Super Contributor
timestamp: September 22, 2013, 06:56:16 pm
content: Quote from: FrankBuss on September 22, 2013, 06:33:01 pmYou can discharge the tube HV by connecting a screwdriver to the grounded metal structures around the tube with alligator clip wire, and then sticking the screwdriver to HV anode connection (usually under plastic cup). I used that method in my previous life when I used to fix TV's etc., worked just fine. Also note that amount of stored energy is not large enough to kill normal healthy people, but the shock may cause some other collateral damage. It sure feels very nasty.Regards,Janne

author: Dave
author_rank: Super Contributor
timestamp: September 23, 2013, 12:27:54 am
content: I believe you got the orientation of the deflection coils wrong in your explaination. You actually need to energize top and bottom coil to deflect the beam along the x axis, and the side coils for the y axis. Basically the opposite of deflection plates.Also, kudos on the great videos lately. I really liked the last two dozens.

author: hikariuk
author_rank: Unknown
timestamp: September 23, 2013, 04:13:15 am
content: Quote from: mrflibble on September 22, 2013, 06:10:42 pmI think the "oops" might have been the best of that video

author: FrankBuss
author_rank: Unknown
timestamp: September 23, 2013, 04:22:44 am
content: Quote from: jahonen on September 22, 2013, 06:56:16 pmThanks, I think I can do this. I found a video about your technique:Good other tip in the video: one arm behind you to avoid that it discharges through your heart, if anything goes wrong. And my Vectrex was turned off (and the mains cable unplugged) for 3 weeks now, so should be safe anyway.

author: Grapsus
author_rank: Regular Contributor
timestamp: September 23, 2013, 06:25:03 pm
content: Very nice tip for repairing CRTs !I have a small remark about deflection coils/plates : deflection coils and deflection plates are very different. An electron moving with speed v experiences a force caused by magnetic and electric fields : f = -e*(E + v^B). Deflection plates use only the electric field to cause delfection, the force is then proportional to the potential difference between the plates. This is a poor method as it gives very little deflection even with very HV, that's why very old CRO tubes are so long and have such small screens, they used plates. On the other side, deflection coils use the magnetic field to deflect electrons and they're very efficient at it. Recent CRT screens, are pretty big and flat, and the electron ray reaches the corners without any problems. In order to do so, the CRTs use a particular configuration called Helmholtz coil that creates a constant magnetic field inside the tube. In this video we obviously have deflection coils and not plates.

author: jahonen
author_rank: Super Contributor
timestamp: September 23, 2013, 06:42:13 pm
content: Another small error in the Dave's video was that horizontal deflection coils are actually at "bottom" and "top" of the neck, and vertical coils at sides. This is due how charged particles behave in a (dipole) magnetic field. This is in contrast of electrostatic deflection where it is other way around.http://en.wikipedia.org/wiki/Lorentz_forceRegards,Janne

author: JackOfVA
author_rank: Unknown
timestamp: September 24, 2013, 12:57:24 am
content: Quote from: FrankBuss on September 23, 2013, 04:22:44 amFor those of us that started tinkering with electronics in the vacuum tube days, the rule was to keep one hand in your pocket if working on live gear.

author: Stonent
author_rank: Super Contributor
timestamp: September 24, 2013, 01:30:54 am
content: Quote from: N2IXK on September 22, 2013, 12:16:09 amMaybe call it YouPanel?

author: vk6zgo
author_rank: Super Contributor
timestamp: September 24, 2013, 08:26:27 am
content: Quote from: FrankBuss on September 22, 2013, 05:56:54 pmIn the big,bad,evil world where real Techs work,nobody has an hour to do anything that isn't directly involved with actually fixing the thing.Most of the really high voltages around a CRT type TV or Monitor are generated by the horizontal deflection circuit.The H & V deflection circuits are supplied by what we used to call "Boost HT" in the valve days,which is derived from an overwind on the Horizontal Output Transformer (H.O.T.).The H deflection circuits are initially supplied with a lower supply voltage to get them started.Once that happens,a voltage of around +150v is obtained by rectifying line rate "ac" from an overwind on the H.O.T. .This is a halfwave rectifier,the ripple is still around 15kHz,so it doesn't need a very large value capacitor for filtering.Low value means not a lot of charge left in the cap when it isn't being constantly replenished.The tube EHT is derived from the EHT winding of the H.O.T in a similar way,this time,the filter cap is the capacitance between the inner & outer coatings of the CRT---a very small capacitance,which again,is all that is needed!Again,not a lot left when it isn't being constantly replenished.Wait!----there's more!The CRT doesn't immediately turn off when you remove power to the Monitor---after all,it still has hot filaments,& some EHT,so it draws current for a while tending to discharge the tube capacitance.If you,in turn,discharge the tube with a screwdriver,there is very little charge left.In a similar manner,the deflection circuitry (mainly vertical) tends to continue,disharging the "Boost HT" filter cap.OK,you can get a "bite" from stored charge on a CRT,& it might cause you to say @##$$$$%%!!!!!!,but it is unlikely in the extreme to do you any real harm,unless it causes you to drop the CRT.Even this is very unlikely to cause a damaging "implosion" in any TV type CRT made since the mid 1960s.Now we come to the part whichcanzap you!A mains operated TV or Monitor will probably have a switchmode power supply.(Dave's one is not a problem because it operates from 12vDC from the equipment mainframe)Normal Mains operated SMPS rectify the 230/120 vac Mains directly,then use the resultant several hundred vdc to charge some seriously big caps.These are needed because the ripple is around 100/120 Hz.The dc voltage in turn is used to supply the switching devices.These capacitors are the ones which constitute a danger in a "switched off" monitor.PS:-Next time I do a posting this long,I'm doing it offline,then cutting & pasting it here.--Too many typos to edit out!!

author: SeanB
author_rank: Super Contributor
timestamp: September 24, 2013, 09:09:44 am
content: I measured a 24 inch CRT before it went into the bin, and the tube capacitance was just under 2nF. The only time this will be a problem is if there is a fault which gives EHT but does not warm up the heaters. There are very few faults that do this though, as generally the heaters are fed by a dedicated winding on the LOPT that only does this job, as this reduces the capacitance of the heater ground system to a lower value so allows a higher bandwidth for the driver with a particular peak current. Thus the only time the heaters do not light with EHT are if the heaters all go open circuit ( on a colour CRT there are 3 heaters, monochrome tubes generally have a thicker single heater filament) or the wires go totally open circuit, and the heaters generally are either 6.3V or 12V, and run at around 300ma per heater.

author: G7PSK
author_rank: Super Contributor
timestamp: September 24, 2013, 05:11:08 pm
content: A CRT is like a leyden jar,it will pick up a charge even when removed from the TV/monitor and can still sting you weeks or monthes later.

author: M0BSW
author_rank: Frequent Contributor
timestamp: September 24, 2013, 06:38:56 pm
content: Quote from: mrflibble on September 22, 2013, 06:10:42 pmI remember this video as quite scary, and since never used mine for mains voltage

author: Fezder
author_rank: Regular Contributor
timestamp: September 24, 2013, 06:46:57 pm
content: haha, teachers at school almost forbidded/banned these screwdrivers, just for safety reasons, and we were like ''what harm can these do?''

author: Steffen
author_rank: Regular Contributor
timestamp: September 24, 2013, 07:02:51 pm
content: In Germany these voltage indicating screw drivers are by law forbidden in professional use like in the industry or for electro-companies. Selling them is not forbidden, but using them as educated electrician is forbidden.IMHO the production and trade of this screw drivers should also be forbidden.

author: Fezder
author_rank: Regular Contributor
timestamp: September 24, 2013, 07:07:06 pm
content: Quote from: Steffen on September 24, 2013, 07:02:51 pmafter seeing this video, i agree...

author: c4757p
author_rank: Super Contributor
timestamp: September 24, 2013, 07:15:22 pm
content: Wow.... "these are dangerous, so you can only use them if you don't know what you're doing".

author: Stonent
author_rank: Super Contributor
timestamp: September 24, 2013, 07:18:00 pm
content: Quote from: Fezder on September 24, 2013, 07:07:06 pmI used one last night to verify a 900V circuitTruth be told, the circuit produces so little current that even a digital multimeter will cause a drop in voltage of several hundred volts unless you increase the impedance to about 1000M Ohm.

author: Fezder
author_rank: Regular Contributor
timestamp: September 24, 2013, 07:44:41 pm
content: hmm, interesting

author: Steffen
author_rank: Regular Contributor
timestamp: September 24, 2013, 07:52:23 pm
content: The thing with this screw drivers is that you can't be sure to see the truth. If you are on an insulated floor or with insulated shoes there will be no current trough the lamp inside even if the circuit is fully powerd. Then you think that the circuit is switched off and you touch it and maybe you make short circuit between L an N/PE and BOOM. All this happend and killed people. Therefore you have to probe bipolar with Duspol or DMM.

author: G7PSK
author_rank: Super Contributor
timestamp: September 24, 2013, 08:14:59 pm
content: Those neons will glow even if you are standing on a rubber matt, the body is a big enough earth/sink to make a good neon glow without passing current through the body directly to ground, I expect that when you are fully charged the neon will go out, I have not tried that as it is probably some time required as voltage will leak of into the atmosphere. I cannot see that as long as they are used as specified that there is any more risk than any other instrument that is used for checking power circuits. I have never heard of any one coming to harm as a direct result of using one correctly. I would not rely on one though to indicate that there was power on a circuit as they will light up with just stray currents, holding them in front of a crt will light some of them even.I did blow a neon screwdriver up when I was 13 0r 14 years old, but not in any way you would think of, it involved a sand pit a shoe and a 12 gauge shotgun cartridge and a stupid dare, the result was the shaft was driven up through the neon tube.

author: Steffen
author_rank: Regular Contributor
timestamp: September 24, 2013, 08:51:14 pm
content: My grandfather has a probe screw driver with some kind of LED inside. It still lights up by measuring from the left hand to the right one. So much about accuracy and quality of that lyingsticks "Lügenstift" in German.

author: vk6zgo
author_rank: Super Contributor
timestamp: September 25, 2013, 05:00:19 am
content: Quote from: G7PSK on September 24, 2013, 05:11:08 pmYes,but the pertinent word is "sting"---annoying,yes,dangerous,no!

author: amyk
author_rank: Super Contributor
timestamp: September 25, 2013, 10:44:38 am
content: Quote from: Stonent on September 24, 2013, 01:30:54 amThen the logo is still a bit of anachronism... like the floppy disk "save" icon.

author: rsjsouza
author_rank: Super Contributor
timestamp: September 26, 2013, 09:02:20 pm
content: Quote from: G7PSK on September 24, 2013, 08:14:59 pmMe neither. This is the one I inherited from my dad. It has an interesting neon bulb shape (looks more like a glass fuse). The resistor is 1Mohm(edit: reduced the photo size)

author: G7PSK
author_rank: Super Contributor
timestamp: September 27, 2013, 08:06:28 am
content: Quote from: rsjsouza on September 26, 2013, 09:02:20 pmJust like the one I bust all those years ago.I have just been playing with a neon screwdriver or rather making measurements, the resistance is 1meg just like the esd wrist straps, I have just found I need to get one of Dave's ucurrents as the current flow on the neon I have is somewhere around 0.2 and 0.3 uA as measured with my amprobe set to UA. When standing on a 4 inch thick rubber block the neon dims to a barely visible glow after 48 seconds, must be the charge time for me.