EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: josh132 on January 25, 2021, 09:06:53 pm
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So while trying discharging a CRT, I got shocked, I was laying down on the floor for a while. After getting up, I relazed my left arm is numb. Its kind od hard to type, now. Is this normal? I got shocked on my left hand.
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Yes, it's normal.
It's described in every "101 for TV repair apprentices".
I suggest you ask here first before killing yourself.
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If you still have problems after some time, you probably should go to a doctor.
Depending on the energy of the discharge it is possible you have nerve damage (hopefully not permanent)
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If you still have problems after some time, you probably should go to a doctor.
Depending on the energy of the discharge it is possible you have nerve damage (hopefully not permanent)
To be honest, I'd go to the ER. The main problem is heartbeat fluttering. Here, you'll normally be admitted for 24 hours with ECG monitoring.
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to safely discharge a CRT you need to make special well made probes. It means you have an understanding of soldering, gluing plastics and choosing dielectrics.
I suggest you buy one.
https://www.amazon.com/Huhudde-Voltage-Capacitor-Discharge-Electronic/dp/B07W7JT7WC (https://www.amazon.com/Huhudde-Voltage-Capacitor-Discharge-Electronic/dp/B07W7JT7WC)
You might want to find one from a better brand. And when you use it every time you need to do a detailed inspection of the wiring for faults.
I suggest for HV work you keep a special set of probes (that you only use for HV), of brand name, and a discharge tool. And that you keep them in a bag (not a box with a hinge that can smash insulation) and take care of them with deoxit and cable cleaning wipes. Kind of expensive but its a really easy way to get your self killed in liue of spending say $60 for a HV kit.
I mean this under 1000V. Over that you need some even more specialized stuff.
https://www.zoro.com/fluke-disposable-paper-wipes-6-sheetspack-mc6/i/G1448806/feature-product?utm_source=google&utm_medium=surfaces&utm_campaign=shopping%20feed&utm_content=free%20google%20shopping%20clicks&gclid=CjwKCAiA9bmABhBbEiwASb35Vzv9WWGkiFVp7zb2d2h6TLSgBu2vw1rFYU-Rz_lxQds3RyVbnBglXRoCPEgQAvD_BwE (https://www.zoro.com/fluke-disposable-paper-wipes-6-sheetspack-mc6/i/G1448806/feature-product?utm_source=google&utm_medium=surfaces&utm_campaign=shopping%20feed&utm_content=free%20google%20shopping%20clicks&gclid=CjwKCAiA9bmABhBbEiwASb35Vzv9WWGkiFVp7zb2d2h6TLSgBu2vw1rFYU-Rz_lxQds3RyVbnBglXRoCPEgQAvD_BwE)
https://www.amazon.com/Hosa-D5S-6-DeoxIT-Contact-Cleaner/dp/B00006LVEU (https://www.amazon.com/Hosa-D5S-6-DeoxIT-Contact-Cleaner/dp/B00006LVEU)
This should ensure you get a good measurement, and your meter takes minimum damage from insertion. One thing I noticed that could get you killed with a fluke scopemeter is that the banana jack solder joint broke, so it was floating from the PCB. It could tell you there is a low voltage vs a high voltage. You want to limit insertion force to keep them in good order on your HV measuring equipment. Of course I recommend having a 9V battery or something to test the probes and meter with before measuring the capacitor connected to some kind of screw, crimp, etc (that will 'verify' that your probe tip is not too oxidized or whatever). Don't let the 'nice parts' of HV like oxide breakthrough lul you into a false sense of security by using dirty equipment with shoddy connections. Some people might even say you should keep 2 identical meters around to verify each other for intermittent faults related to interconnects.
Yay, you got a reason to collect multi meters now, you no longer have a hording problem.
Basically you need to do every stupid fucking simple thing people never do with 5V circuits powered from batteries on HV capacitors.
Can I ask what happened exactly that you got shocked? did you get fooled by a dirty connection or something insidious like that? BTW, also, when you verify the equipment is denergized, and its crusty, make sure to clean or replace the connection near the capacitor if its dirty, i.e. resolder joint, redo crimps on wire, even if you are not sure the equipment will work in the future, in order to give yourself a good probe point on the capacitor and not have to deal with a difficult connection more then once. Like if you made a connection on some really badly oxidized barrel crimp that is bearly accessible from the insulation, just cut it off and put a new one on (once you know how to make good crimp connectors), so its easy to probe the 2nd time. That drain resistor can break... at any time.. so you are never safe, just confident, minimize the number of times you need to probe nasty circuit points to 1. Only 50 cents in crimps and takes 2 minutes.
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It happens. I repaired a lot of CRT displays back in the day, got shocked a handful of times, it rarely causes injury but it sure doesn't feel good. One thing to be really careful of is even after you discharge they can build up a charge again. I always clip a ground wire from the dag strap to the anode and leave it on there until I'm ready to reconnect the anode wire.
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I got zapped COUNTLESS times while servicing crappy CRTs... :popcorn:
The good news is that all parts are very low energy
UNLIKE TODAY POWERFUL SMPS ...
The discomfort usually vanishes in a few hours..
The sarcastic hilarious part is that I have always zapped myself
... while trying to discharge that crappy CRT HV plug..
Therefore.. is never too much to advice. : USE F*** GLOVES.
Paul
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I've never worn gloves and it's unlikely to stop a discharge from a CRT anode anyway. I just ground a wire to the dag FIRST and then use the wire to ground a screwdriver that I poke under the anode cap. As long as the wire is grounded you can't get a shock.
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This kit comes with an adapter that slides under the anode cap. 1G so it discharges slowly but safely. I've been fried one time by an old color TV (unplugged and sitting in the junk, no less) when I was a kid, haven't had it happen since. I can still feel that zap today.
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I've never worn gloves and it's unlikely to stop a discharge from a CRT anode anyway. I just ground a wire to the dag FIRST and then use the wire to ground a screwdriver that I poke under the anode cap. As long as the wire is grounded you can't get a shock.
Exactly. That's the "101" part.
The problem is that people who've not dealt with CRTs before think that switching off the thing and discharging the PSU caps is enough.
But the whammo is the anode charge which does not go away when the device is off.
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I like using a wire with no resistor, the little pop tells you it discharged successfully and gives peace of mind. "Modern" color CRT monitors will self discharge after some time due to the focus divider but monochrome monitors can hold the HV charge for days. The shock is unlikely to harm you but there is significant risk of secondary injury say from dropping the tube so it implodes or yanking your hand back and slicing it open on a sharp edge.
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I got zapped by an undischarged CRT many years ago. It was on my car back seat sitting on its screen, going for a replacement tube. I opened the car door and grabbed the tube without thought and a spark jumped to my hand. I was surprised to say the least but luckily didn't drop the tube nor had any adverse effects.
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I've never worn gloves and it's unlikely to stop a discharge from a CRT anode anyway. I just ground a wire to the dag FIRST and then use the wire to ground a screwdriver that I poke under the anode cap. As long as the wire is grounded you can't get a shock.
Exactly. That's the "101" part.
The problem is that people who've not dealt with CRTs before think that switching off the thing and discharging the PSU caps is enough.
But the whammo is the anode charge which does not go away when the device is off.
Yeah, if not explicitly explained it isn't really apparent that the CRT envelope itself is the HV supply filter cap with 20+kV on it.
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For a normal, healthy person, the HV charge on a CRT shouldn't be dangerous, but it's a very unpleasant surprise. As others have said, just ground a screwdriver to one of the metal straps that touches the outside graphite coating and slip it under the rubber HV cup.
Even after discharging it, it will pick up a small charge later. It's not nearly as big as the initial charge, but it's good to be aware of it so you don't drop the CRT if you brush against the HV connector while picking it up.
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The warning labels are there for a reason. Now you know the reason. ;-)
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There are warning labels on practically everything these days, it's hard to blame people for ignoring them or not even seeing them. The SNR is pretty low, especially with anything intended for sale in the state of California.
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Yeah, they pack a serious punch. Not something to be carelessly messing around with.
I would love to know if an EE is less likely to die from a shock than a non-EE person assuming the current and other circumstances are all otherwise identical.
My reasoning is that EE's know what shocks feel like, they have had them before, they kind of know what's happening to them when the shock occurs.
A non-EE person has no idea what the hell is happening and must think the world is ending.
I'm wondering just how much the negative emotional and metal impact of getting an electric shock effects survival.
People can die from fear and being scared to death, literally.
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There are warning labels on practically everything these days, it's hard to blame people for ignoring them or not even seeing them. The SNR is pretty low, especially with anything intended for sale in the state of California.
Fair enough.
CRTs have always scared me. That... and car batteries.
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Fair enough.
CRTs have always scared me. That... and car batteries.
I've never found either one scary, I was messing with both from the time I was maybe 8 or 10 years old. They can be dangerous and should be respected but it's not necessary to fear them. Wear goggles when doing anything with a car battery that involves having the caps removed, and take care not to short circuit one. Discharge a CRT before disconnecting the anode and keep a ground clip in place to bleed accumulated charge. Also take care not to damage the bell of the tube which can cause an implosion. Other than that they are not all that dangerous.
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Would something like this do the job ?
https://www.youtube.com/watch?v=gLDCV-n1BDw (https://www.youtube.com/watch?v=gLDCV-n1BDw)
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Sure but all you need is a wire and a screwdriver. Strip a few inches off one end of the wire and wrap that around the metal shaft of the screwdriver, then if you want to get fancy install an alligator clip on the other end of the wire. In a pinch you can just strip that end and hook it on the grounding strap that goes around the bell of the tube.
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If you still have problems after some time, you probably should go to a doctor.
Depending on the energy of the discharge it is possible you have nerve damage (hopefully not permanent)
To be honest, I'd go to the ER. The main problem is heartbeat fluttering. Here, you'll normally be admitted for 24 hours with ECG monitoring.
+1
I had some zaps but never ended on the floor. ???
Edit: I would suggest using gloves as well. A bit pain in da arse till you get used to them. Now I can't work without them. Maybe it's OCD
kicking in. ;D
(https://i.imgur.com/e9T8j7v.jpg)
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Would something like this do the job ?
https://www.youtube.com/watch?v=gLDCV-n1BDw (https://www.youtube.com/watch?v=gLDCV-n1BDw)
Bad idea to have nothing but a piece of heat shrink between you and 20+kV, the tool shown is for much lower voltage work. The point of using a screwdriver (like the old school clear acrylic handle ones) is you have the thick plastic handle to carefully hold in case your ground wire pops off or something.
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I do not discharge CRTs unless I am going to be removing the anode cap or working in the vicinity of the LOPT/flyback. You should also avoid touching any of the neck pins as the screen (G1) voltage is there, but the tube does not store that voltage.
I am somewhat leery of the "screwdriver" method, for the reason that the spark may induce high currents in nearby wires or PCB traces. One approach I have used is to use a high-voltage probe with banana plugs and put it into the milliamp jacks of a DMM. The approximately 1 Gohm of the probe will slow the discharge down to a very safe level and it can be simultaneously monitored.
For all the talk about CRT discharging, a very important safety issue is usually overlooked, which is that the power supply section of the monitor chassis has much more dangerous capacitors. With 250VDC and thousands of uF, they can in fact kill you. The same caution needs to be exercised as with all mains connected power supplies. If there are no bleeders or they are open circuit, you need a way to discharge these caps before working in the PSU area.
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Hm, josh132 created this thread 5 days ago. I wonder is he ok ? :-//
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Hm, josh132 created this thread 5 days ago. I wonder is he ok ? :-//
According to his forum profile, he was last active today. I'd say he is fine.
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I am somewhat leery of the "screwdriver" method, for the reason that the spark may induce high currents in nearby wires or PCB traces. One approach I have used is to use a high-voltage probe with banana plugs and put it into the milliamp jacks of a DMM. The approximately 1 Gohm of the probe will slow the discharge down to a very safe level and it can be simultaneously monitored.
For all the talk about CRT discharging, a very important safety issue is usually overlooked, which is that the power supply section of the monitor chassis has much more dangerous capacitors. With 250VDC and thousands of uF, they can in fact kill you. The same caution needs to be exercised as with all mains connected power supplies. If there are no bleeders or they are open circuit, you need a way to discharge these caps before working in the PSU area.
Well it's anecdotal of course, but I've been using the screwdriver with a length of plain wire method for ~30 years on hundreds of different CRT displays and never had any issues with it but YMMV. The friend who taught me how to repair monitors worked in a repair shop back when those were a thing still and that's how they did it too.
You're absolutely right about the power supply. People tend to fear the EHT and overlook the fact that the "low" voltage in the power supply is a lot more dangerous. One of the worst shocks I've ever had was from the bulk filter capacitor in an Electrohome G07 monitor. Those have a dangerous failure mode where a bad flyback transformer or HOT blows a fuse that is on the *output* of the B+ supply and the result is a large electrolytic capacitor charged to 170V with no load to discharge it, only the internal leakage of the capacitor itself.
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Hm, josh132 created this thread 5 days ago. I wonder is he ok ? :-//
Probably got the answer he came here for and left the thread.
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Hey, josh132! Guy, people are worried! Let us know how you feel!
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Well, he is apparently fine judging by his other post...
https://www.eevblog.com/forum/chat/what-did-you-buy-today-post-your-latest-purchase (https://www.eevblog.com/forum/chat/what-did-you-buy-today-post-your-latest-purchase)!/msg3545073/
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so for all of you it was a Tektronix scope CRT i was trying to discharge to i can work around it. It was not from an old tune TV with the removable cap. The CRT used a plug for the anode and i had to use pliers for removing it. I got shocked and f##king dropped it on the board... Which killed the sweep generator. I have since visited a doctor and had no major adverse effects. Though my hand does feel a little weak. I am ok. No need to worry.
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The last time I got a HV bite was opening the connector on a Tek 22xx scope, I have plenty experience with other CRT's and haven't been caught in decades.
I consider that Tek design a wicked F'ing fail... No bleeder resistor and a connector that's just short enough to let the bloody arc reach you!
Now when I have to open those I use gloves and wrap a grounded guard ring of aluminium foil around the connectors before tugging on them.
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You're absolutely right about the power supply. People tend to fear the EHT and overlook the fact that the "low" voltage in the power supply is a lot more dangerous. One of the worst shocks I've ever had was from the bulk filter capacitor in an Electrohome G07 monitor. Those have a dangerous failure mode where a bad flyback transformer or HOT blows a fuse that is on the *output* of the B+ supply and the result is a large electrolytic capacitor charged to 170V with no load to discharge it, only the internal leakage of the capacitor itself.
Oh, that reminds me. I once grab some small adapter board with charged 400V (230V mains so something over 300V charge), just how the board nice fit to palm. And suddenly capacitor touches the skin and left two nice marks >:D |O
Oh and good old CRTs they can accumulate charge just on its own from the environment or hold charged for hours even days ::)
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Oh and good old CRTs they can accumulate charge just on its own from the environment or hold charged for hours even days ::)
Back in the early '80's I worked with NCR and was sent to their training center in Dayton Ohio to learn, amongst other things to repair the CRT terminals used with the mini computers - the instructor showed us how to discharge the tube before removing it (slide a screwdriver with attached ground wire under the anode cap), and then set it aside. He did this at 5:00 pm, at the end of the day, and when class resumed at 9:00 am the following morning, before he did anything else, he discharged the tube a second time, so we could all see the charge that had accumulated overnight.
Another one of his favorite tricks was to "walk" each student one-by-one through the power supply adjustment - he'd come over to your workstation, instruct you to put one hand in your pocket, hand you a plastic screwdriver and offer to hold the meter leads on the test points while you adjusted the voltage - the adjustment potentiometer was a few inches below the anode cap on the side of the CRT, right alongside your knuckles as you held that screwdriver, almost guaranteeing that as you twisted the pot, eyes on the meter display, that the knuckle of your pinky finger would hit the anode cap where there was just enough voltage to startle you.
The lesson there was to always be aware of exactly where you were relative to the high voltage components.
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The lesson there was to always be aware of exactly where you were relative to the high voltage components.
And then you see pictures with train surfers jumping under wires with 22kV
They have no idea what they are playing with (https://i.insider.com/5601c4f46da8111530b4dc1a?width=1106&format=jpeg)
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Another one of his favorite tricks was to "walk" each student one-by-one through the power supply adjustment - he'd come over to your workstation, instruct you to put one hand in your pocket, hand you a plastic screwdriver and offer to hold the meter leads on the test points while you adjusted the voltage - the adjustment potentiometer was a few inches below the anode cap on the side of the CRT, right alongside your knuckles as you held that screwdriver, almost guaranteeing that as you twisted the pot, eyes on the meter display, that the knuckle of your pinky finger would hit the anode cap where there was just enough voltage to startle you.
The lesson there was to always be aware of exactly where you were relative to the high voltage components.
Also that there are kinda degrees of HV insulation, and a lot of what you run into isn't really human rated. Similar problem in automotive with old pre coil on plug engines, aging ignition parts can and do arc through. You can tell who's been bit before by how close they're willing to come to the nominally insulated spark plugs wires and boots on a running engine.