Working with un-powered CRTs. - Page 1
Forum section: electronics / projects

author: Vtile
author_rank: Super Contributor
timestamp: July 30, 2017, 06:57:41 pm
content: Hello!I though I ask before going to poke around as I have no connection no more to people who have 1st hand expertice on working with CRT devices. I know enough to know the most obvious risks of the CRTs, but I have no good knowledge of the structure(I can keep a short lecture of the physics of electron beam deflection and acceleration though)of the cathode ray tubes to know what are the physical parts causing the safety hazards while working with these. Can some older (or younger by age, but old as working age) gents shed some light to the subject, which I can then sceptically read.I know the following.* The CRT is a vacuum device.* Made of glass and the accelerator end is most fraqile mechanically.* The beam accelerator is driven with several to tens of kilovolts of DC.* The tube glass shell forms a highly charged capacitor by some mechanic, which potential is close or the same as the acceleration potential.* The dipoles in glass dielectric? do form a certain level of dielectric absorption so the tube will have some self-recharge (and considering the tens of kV of nominal driving potential it might climp rather high). Is it the glass shell that actually is the dielectric or is it some other layers on the device.* The flyback transformer circuitry obviously have also some capacitors that might be in high potential.* In case of Scope CRT it is not magnetically deflected, but by electric field deflected.My questions are... Which parts of the tube have ability form a discharge loop aka electric shock hazard (There might be variations between models/types).What is the right method of discharge the tube. I know that the jumper wire and skrew driver to short is not a proper method. As it should have resistor (which value/magnitude were used) to prevent arching and other unwanted effects. Is it really done from under the suction cup style insulator at the side of the tube or is it just a handyman method. I do lack of proper tens of kV test equipment so I can not safely attach the measurement devices to the CRT while it is charged.I'm going to change some reticle lights of my old scope (which aren't in the CRT, but close enough that one needs to take consider the CRT as a work hazard), but I would like get some more understanding of the CRT as a device before going to work near of its potentially exposed parts.

author: cdev
author_rank: Super Contributor
timestamp: July 30, 2017, 08:04:00 pm
content: Can you get a copy of its service manual? Thats the authoritative source for that kind of information on any given model.I dont ever work with high voltage anything any more but - IMHO its good practice to use a high value power resistor - not held in your hands - to discharge capacitors.If you use a screwdriver or needlenose pliers, sometimes the spark is large enough to fuse your tool to the capacitor. That causes pitting on your tools.You may have to tap it with a resistor a few times to fully discharge the voltage.They should use a bleeder resistor on high voltage equipment but they often don't.A non contact voltage indicator- many DMMs have them now- is useful.

author: helius
author_rank: Super Contributor
timestamp: July 30, 2017, 08:06:23 pm
content: The CRT capacitance (exposed on the EHT lead) can give you quite a jolt, which is especially hazardous because there are sharp metal parts and a glass bell full of vacuum nearby. The safe way to handle this is high voltage 1000:1 probe and DMM, to ensure that the CRT is discharged. As long as the CRT is discharged, you cannot receive a shock from the EHT lead, flyback, or the anode. There still may be charged electrolytic capacitors in the power supply section, which are more dangerous because they hold much more charge. The capacitance of the CRT itself is only a few pF, which even at 50 kV is not as hazardous.The correct way to discharge the CRT is from the anode cap to the metal frame that is clamped around the front of the screen. Discharging to other "grounds" like the earth lead is not safe and can damage the display electronics. A flat screwdriver bonded to a clip is the most common discharge tool, and probably the best that can be improvised. The problem with discharging through a resistor is the high voltage rating necessary.The other possible shock is from the plate and grid pins in the neck. A correctly functioning CRT will drain that charge away after being turned off, but while it is on you can be shocked by those wires.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2017, 08:24:02 pm
content: On the oscilloscope CRTs I have tested, the anode is on the inside and includes a resistive coating between the anode and ground which controls the electrostatic field. This results in a relatively low time constant of 10s of seconds so the anode only remains charged for a short time.

author: floobydust
author_rank: Super Contributor
timestamp: July 30, 2017, 08:32:01 pm
content: Don't forget the implosion hazard if you break the glass around the base or neck, a CRT is not really strong there.I would use safety glasses today and wear gloves. But a long time ago, CRT work you just were extra careful.Scope CRT does not have much capacitance/2nd anode voltage compared to a colour TV CRT, so I don't do anything but discharge the anode after a power-up.

author: Vtile
author_rank: Super Contributor
timestamp: July 30, 2017, 09:11:42 pm
content: Hmm.. So the EHT cable with the suction cup style insulator is the anode/mask connection? and the actual charge (lets say "in typical" device) is between a anode and cathode. I were in believe that the connection is inside the tube.. Live and learn.I must ask about the glass bell/body. Is it so that it doesn't form a capacitor itself (doesn't have any electrical structures between inner and outer surface) and that were just a typical internet BS that I picked up from somewhere.I need to digest and search further on the subject. ...And finally get the 50kV probe (build or buy 2nd hand. DIY might be safer in this case, but it is another subject in its own) to one of my multimeters. ..and propably get a "Cathode ray tube handbook" from yesteryear.Yes, I do have service manual IIRC. I'll of course look what it did say, but I'm also interested in broader level than what service manual of specific product will provide.I'll try to come up with more specific questions after I have digest the information so far.

author: helius
author_rank: Super Contributor
timestamp: July 30, 2017, 09:48:04 pm
content: The focus and deflection of the CRT requires the anode voltage to be very stable. To keep it stable, there is a capacitor as close as possible: in the glass envelope of the CRT itself. The inside and outside are coated in a conductive paint called "aquadag" and the outside is bonded to a ground point, while the inside is connected to the anode cup, and from there to the flyback transformer. It works well: it's so stable that it can remain charged for days after the display is turned off.The EHT is more between the whole unit and the anode, rather than the cathode and anode. The cathode is part of the electron gun and uses a potential of just 1-2 kV to fire out electrons, which are then focused and deflected. It's what happens after the deflection to accelerate the electrons into the screen (Post-Deflection Acceleration or PDA) that requires the EHT anode voltage.

author: brainwash
author_rank: Frequent Contributor
timestamp: July 30, 2017, 11:49:04 pm
content: As a quick note, though probably everyone already knows this: when discharging a large high-voltage capacitor make sure to do it multiple times. The charge 'regenerates' after a while. 'Newer' designs (>'70) have a bleeder resistor but a lot of the older ones don't.

author: David Hess
author_rank: Super Contributor
timestamp: July 31, 2017, 01:03:51 am
content: Quote from: floobydust on July 30, 2017, 08:32:01 pmAs I recall, I measured about 200 picofarads using my impedance bridge.Television CRTs are of course a completely different matter and are much more hazardous.

author: tautech
author_rank: Super Contributor
timestamp: July 31, 2017, 01:32:16 am
content: Quote from: Vtile on July 30, 2017, 09:11:42 pmFor better understanding of the relationship between the anode and cathode watch these vids:https://www.eevblog.com/forum/chat/another-fantastic-tektronix-crt-manufacturing-film/You'll see them lay down a resistive track in the neck of the CRT from the anode terminal towards the cathode but the two do not interconnect in the CRT.Their voltages are relative to one another, the cathode being highly negative and the anode being highly positive which provides the beam acceleration necessary for higher BW CRO's.By the way the multiplier provides the anode voltage and that there is no physical connection of the anode to the cathode they do hold a charge and other members have given the appropriate advice on how to discharge it.

author: drussell
author_rank: Super Contributor
timestamp: July 31, 2017, 03:50:35 am
content: Really, other than physically breaking it (the CRT) or having dangerous voltages being present in the supporting electronics, as for theCRT itselfreally the only thing you need to do is discharge the anode (the connection under the suction-cuppy thing) to ground (which typically includes the aquadag coating on the outside of the tube).Basically if you short that anode terminal of the tube that lives under the suction-cuppy connector to chassis ground, you're probably fine. Even if you don't, it is extremely unlikely to kill or maim or even lightly char a small speck of you unless you arehumorouslywell connected to itandground since the voltage potential may be high but the absolute maximum current available is extremely low. More like a good static electricity shock at worst, for a bare CRT tube, anyway....

author: Richard Crowley
author_rank: Super Contributor
timestamp: July 31, 2017, 06:21:12 am
content: All CRTs have a conductive ("Aquadag") coating on the INSIDE of the bell which forms the final (accelerating) anode.Smaller CRTshave ONLY that inner conductive element and residual charge may be coming from the high voltage generating circuit external to the tube.But larger CRTsalso have a conductive coating on the OUTSIDE of the bell which is typically grounded. The capacitance (through the glass bell) from the outside coating to the inside coating forms a high-voltage capacitor which is used to smooth the rectified AC (typically the horizontal sweep signal) into DC for use as the beam acceleration voltage.It is the potential residual charge on this large capacitor between the inner and outer conductive layers of the tube that make it hazardous to work around even when it is disconnected from all the rest of the circuit.I don't think I have ever seen a small CRT like for an oscilloscope use the outer coating as a smoothing capacitor for the EHT. But that doesn't mean that you can let your guard down. You must still use appropriate caution around the high voltage, the high vacuum, and the fragile glass envelope.

author: CJay
author_rank: Super Contributor
timestamp: July 31, 2017, 09:37:57 am
content: Lots of useful advice here but some of it is a tad misleading.A CRT can and will charge itself up in free air, it works in a similar way to a Leyden jar and can accumulate enough charge to give you an unpleasant surprise, not something you want to find out when you're carrying or working on one.*SOME* CRTs have a discharge resistor buit in (usually monitor CRTs).Most TV CRTs I've worked with did not, way into the late 80s.Definitely sensible to assume it doesn't have one and that it may have a charge.It is a very sensible precaution to discharge the anode and attach a jumper wire from the metal mounting lugs to the anode connector.

author: Ian.M
author_rank: Super Contributor
timestamp: July 31, 2017, 10:26:28 am
content: My boss favoured the two screwdriver method - put the tip of one long screwdriver on a CRT mounting lug and while maintaining contact between the shafts, slide the flat tip of the other under the EHT cap.Frankly I thought that was too close to suicidal, as one slight slip + a hairline crack in the screwdriver handle would give you a high energy 25KV shock straight across the chest.I used the EHT meter if available - count seconds down to 1KV then maintain contact fot twice as long again, otherwise I used a discharge stick (or improvised one from a flat bladed screwdriver with good handle insulation, a small rubber band and a lead with heavy duty alligator clips on the ends. The rubber band was to make sure the clip on the screwdriver shaft could not accidentally come off as that would again be very hazardous).CRT Discharge stick:Take a 3" flat strip of brass and file a V notch in the end. Mount it in a slot cut in the end of a perspex or similar *insulating* plastic rod 15" long. Solder about 2' of test lead wire to the strip at the rod end, and strain relieve it by leading it through a hole drilled crosswise through the end of the rod. Put a heavy duty alligator clip on the other end of the wire.Wrap the other end of the rod with grip tape for 4".Usage: Connect the ground clip to the grounding spring across the exterior aquadag, or to the mounting lug it goes to. Slide the strip under the EHT capIts critically important NOT to discharge a CRT to anywhere except its aquadag coating or the spring contacting the aquadag or the mounting lugs, as if you discharge to the chassis or any other ground point, the very high transient currents can cause enogh ground bounce to destroy MCUs, digital logic and other sensitive chips.Anyone unfamiliar with CRT technology should readhttps://www.repairfaq.org/sam/crtfaq.htm, especially the section 'CRT Safety Issues'

author: Vtile
author_rank: Super Contributor
timestamp: August 02, 2017, 02:30:26 pm
content: I think I do not have any further questions as the answers were so good and broad. Thank you.I'll give a bit back. The "self-charge" of the high-voltage capacitor in CRT is most probably an effect of dielectric absorption, which is surprisingly unknown phenomenon (while scientifically understood, as far as I know). The momentum of the atomic scale molecular particles in the insulator (dielectric) in capacitor causes that it doesn't fully discharge while you short-circuit it for a short time. The energy in those slow molecules creates the voltage again between the capacitor plates while they slowly re-orient themselves. Since glass is a such a good insulator the leakage current is low and the effect will be seen even a long time after the discharge is made. The effect is more known in electronics etc. with acronym DAR (Dielectric Absorption Ratio) and short description can be found on the wikipedia. In example the DAR value for electrolytic capacitor is 10 to 15 % which means it can build back 10 to 15 % of the original voltage from where it was discharged (there is a standard for this).

author: Zero999
author_rank: Super Contributor
timestamp: August 02, 2017, 05:44:09 pm
content: I was under the impression that CRTs don't have a huge capacitance so aren't that hazardous. I thought the capacitors in the TV's power supply charged to a lower voltage but stored more energy and were more dangerous?Another thing about capacitors: quite often TV flyback transformers have a voltage doubler and capacitors build-in, so that's another thing you should be wary of.

author: Richard Crowley
author_rank: Super Contributor
timestamp: August 02, 2017, 06:05:17 pm
content: Quote from: Hero999 on August 02, 2017, 05:44:09 pmI try to stay away from old fire-bottle gear (I have no nostalgia for the bad old days). But, IME, LARGE CRTs (like the ones you find in TV receivers) use the inner anode coating as one plate, the glass bell envelope as the dielectric, and an outer (grounded) coating as the other plate to create a capacitor that can kill you. Between the EHT charge in that internal capacitor, the high vacuum, and the sharp shards of glass (if broken) it can be lethal if you are particularly unlucky.OTOH, SMALL CRTs (like those you find in CROs, etc.) which have no outer conductive ("Aquadag") coating rely on filter/smoothing capacitance external to the tube. I don't know that I have seen any flyback transformers with built in capacitors but, of course, it is hard to tell if they might be included inside the plastic over-mold enclosure of the transformer.