EEVblog #524 - Vignetting on a Cathode Ray Tube

[jahonen]

If you do that, remember to use the right one.

Ok, maybe not a good idea to use it, I imagine the sausage in the video would be my finger

Found the service manual: http://www.vectrex.nl/docs/vecman.pdf The 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.

You 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

[Dave]

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.

[hikariuk]

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?

If you do that, remember to use the right one.

I think the "oops" might have been the best of that video

[FrankBuss]

You 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.

Thanks, 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.

[Grapsus]

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.

[jahonen]

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_force

Regards,
Janne

[JackOfVA]

You 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.

Thanks, 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.

For 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.

[Stonent]

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...

Some kid eventually has to wonder why that big streaming video site is called You*TUBE*, right?

Maybe call it YouPanel?

[vk6zgo]

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 for buzz reduction and I don't want to get electrocuted.

In 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 which can zap 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!!

[SeanB]

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.

[G7PSK]

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.

[M0BSW]

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?

If you do that, remember to use the right one.

I remember this video as quite scary, and since never used mine for mains voltage

[Fezder]

haha, teachers at school almost forbidded/banned these screwdrivers, just for safety reasons, and we were like ''what harm can these do?''

[Steffen]

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.

[Fezder]

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.

after seeing this video, i agree ...

[c4757p]

Wow.... "these are dangerous, so you can only use them if you don't know what you're doing".

[Stonent]

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.

after seeing this video, i agree ...

I used one last night to verify a 900V circuit

Truth 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.

[Fezder]

hmm, interesting

[Steffen]

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.

[G7PSK]

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.

[Steffen]

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.

[vk6zgo]

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.

Yes,but the pertinent word is "sting"---annoying,yes,dangerous,no!

[amyk]

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...

Some kid eventually has to wonder why that big streaming video site is called You*TUBE*, right?

Maybe call it YouPanel?

Then the logo is still a bit of anachronism... like the floppy disk "save" icon.

[rsjsouza]

I have never heard of any one coming to harm as a direct result of using one correctly.

Me 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)

[G7PSK]

I have never heard of any one coming to harm as a direct result of using one correctly.

Me 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)

Just 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.