What exactly happens when you discharge a CRT incorrectly? - Videokarma.org TV - Video

venivdvici · #1 12-09-2011, 06:56 PM

A woman is barefoot on a concrete floor. A late 50's portable tv is unplugged. She removed the back. She reaches in and pulls the (lead?) at the suction cup end from the CRT. Maybe her hand touches the CRT. I'm not sure what her other hand s/b doing (if anything). Will she get a shock enough to throw her back a few feet?

I'm not trying to kill someone.

Really. Thanks.

Electronic M · #2 12-09-2011, 07:21 PM

I did that once.... It did not throw me back, but I yanked my arm back real hard and swore like a pirate for a moment.

To be clear what you describe would be discharging the HV of a CRT through ones body. Normal safe discharging is when one takes a wire connects one end to the TV's chassis ground and the other to a plastic handle screw driver's metal shaft, then slip the driver under the suction cup untill you hear a electric snappping sound and or hit metal. Once one has done the screw driver trick one can SAFELY pull the HV lead and suction cup off.

If one does it wrong one can get zapped just the same as I was. However the HV can also "recover". I have a 1964 Silvertone roundy that likes to do that. I would pull the chassis do some work on it, and put the chassis back to test repeatedly. To pull the chassis I had to disconnect the HV lead (which meant I had to discharge it first) by the time I would put the chassis back (and likely much sooner) the CRT HV cup was back to having HV on it, and would give me a good ZAP when I reconnected the HV lead. I initially took it as a fluke, then got real mad when it kept happening. And I was beginning to think the damn thing was possessed! I ended up leaving a jumper from the cup to CRT ground in place when ever the chassis was out and that ended that.

venivdvici · #3 12-09-2011, 07:44 PM

Quote:

Originally Posted by Electronic M

I did that once.... It did not throw me back, but I yanked my arm back real hard and swore like a pirate for a moment.

To be clear what you describe would be discharging the HV of a CRT through ones body. Normal safe discharging is when one takes a wire connects one end to the TV's chassis ground and the other to a plastic handle screw driver's metal shaft, then slip the driver under the suction cup untill you hear a electric snappping sound and or hit metal. Once one has done the screw driver trick one can SAFELY pull the HV lead and suction cup off.

If one does it wrong one can get zapped just the same as I was. However the HV can also "recover". I have a 1964 Silvertone roundy that likes to do that. I would pull the chassis do some work on it, and put the chassis back to test repeatedly. To pull the chassis I had to disconnect the HV lead (which meant I had to discharge it first) by the time I would put the chassis back (and likely much sooner) the CRT HV cup was back to having HV on it, and would give me a good ZAP when I reconnected the HV lead. I initially took it as a fluke, then got real mad when it kept happening. And I was beginning to think the damn thing was possessed! I ended up leaving a jumper from the cup to CRT ground in place when ever the chassis was out and that ended that.

Thanks, Tom. On You Tube, I've viewed a lot of the correct ways to discharge the HV of a CRT. I was curious about how severe an incorrect discharge would be. I'd like a situation where the jolt might be enough to knock the woman over so she could drop the gun in her other hand. What if she wore a ring and it touched the CRT as she was pulling the lead? A different result maybe?

Electronic M · #4 12-09-2011, 08:16 PM

The ring will likely not have a signifficant effect. I could see some one pulling back on reflex and that reflex causing them to drop or fire the gun depending on how they were holding it and the combination of reflex and reaction.
Later on that same Silvertone I was doing a purity adjustment, and happened to touch a bare spot on the vertical winding on the yoke. My reflex caused me to yank my arm out hard enough to break an ancilary ground wire that was blocking my motion, and mildly mess up some other things I've since fixed and forgot. That is among the worst jolts I've gotten.

If the set was just turned off then the charge on the HV system will likely be stronger BTW.

Edit: Touching the chassis of the set will likely worsten any jolt from the HV, B+, etc.

venivdvici · #5 12-09-2011, 08:52 PM

Quote:

Originally Posted by Electronic M

The ring will likely not have a signifficant effect. I could see some one pulling back on reflex and that reflex causing them to drop or fire the gun depending on how they were holding it and the combination of reflex and reaction.
Later on that same Silvertone I was doing a purity adjustment, and happened to touch a bare spot on the vertical winding on the yoke. My reflex caused me to yank my arm out hard enough to break an ancilary ground wire that was blocking my motion, and mildly mess up some other things I've since fixed and forgot. That is among the worst jolts I've gotten.

If the set was just turned off then the charge on the HV system will likely be stronger BTW.

Edit: Touching the chassis of the set will likely worsten any jolt from the HV, B+, etc.

Ouch.

The guy that's tied to a chair in the fallout shelter wants the woman to put the gun down so when she falls from the jolt, he can kick the gun away from her and tackle her. While he's on her, he can demand she untie him. But I think I'm gonna have her put the gun, barrel up, in her waistband, which messes him up. The tv will be turned off right before she pulls off the back.

I can state that he was just going to have her pull the lead and get a shock, but now he's gonna have her first touch the chassis and then pull the lead so she'll get that big shock. He'll expect her to fall and then he'll kick the teevee on her. (Hmm, I wonder what would happen to her then.) His plan is to then use his mouth to lift the phone receiver and his tongue to dial the operator for help. (It's 1961.)

However, when she gets the shock, she falls over and the gun in her waistband goes off right into her midriff. She'll be dead. The guy will still need to use his mouth and tongue to call for help.

Thanks for helping me with this scene, Tom! I really appreciate it.

bgadow · #6 12-09-2011, 11:06 PM

She wouldn't have to be barefoot; earth ground doesn't matter much in this case. The HV from the crt doesn't want to get to earth ground, it wants to get to the outside of the crt, which is attached to the chassis. Thankfully I've never got in its way! I have touched the terminals of a yoke while a 21" bw TV was running, and I did not care for it, not one bit!

DaveWM · #7 12-10-2011, 12:24 PM

the yoke scares me more than a CRT (not that I want to get a jolt from HV) I got yoke bit once, been over a year and I still have not fully recovered (this was a color solid state set), I think I did perm damage to my arm.

venivdvici · #8 12-10-2011, 02:15 PM

Quote:

Originally Posted by bgadow

She wouldn't have to be barefoot; earth ground doesn't matter much in this case. The HV from the crt doesn't want to get to earth ground, it wants to get to the outside of the crt, which is attached to the chassis. Thankfully I've never got in its way! I have touched the terminals of a yoke while a 21" bw TV was running, and I did not care for it, not one bit!

Okay, I'll have her keep her shoes on. I'll make them heels. She'll tumble easier! Thanks.

venivdvici · #9 12-10-2011, 02:32 PM

Quote:

Originally Posted by DaveWM

the yoke scares me more than a CRT (not that I want to get a jolt from HV) I got yoke bit once, been over a year and I still have not fully recovered (this was a color solid state set), I think I did perm damage to my arm.

Wow, with two of you talking about a yoke, I had to look it up to see what it was. Couldn't find it in my books, but I found this in Wikipedia:
In detail, scanning of CRTs is done by magnetic deflection, by changing the current in the coils of the deflection yoke.

It looks like it's the copper-colored wire around the section of the picture tube right before it fans out into the screen. Is that it?

If this gives a nastier jolt, I'm all for that! If it burns her, all the better.

Hope you get better, Dave. Sorry you got stung.

bgadow · #10 12-10-2011, 10:12 PM

Generally the terminals on a yoke are insulated, but not always. The set that got me is a mid-50s Motorola. The plastic that covered the back of the yoke had deteroriated, exposing the soldered connections. The bad plastic also made the yoke want to shift side-to-side, making the picture lopsided. I guess I was trying to straighten it out when I got bit. I don't think I'd use that particular excuse in the book (the bad plastic) because I don't think that stuff rotted that quick.

DaveWM · #11 12-12-2011, 07:39 AM

the big diff is the crt can bite with power off from the stored charge. the yoke will only bite with the power on.

Electronic M · #12 12-12-2011, 11:15 AM

That means that the CRT will likely only bite once for only so long (usually just an instant) with the power off. And that the yoke will continue to bite if the power is on indefinitly.

leadlike · #13 12-12-2011, 07:47 PM

Typically, the concern from getting bitten from HV was not so much the actual injury from the electrical shock, but your reaction to it, such as your arm flying back uncontrollably into another part of the set where some high amperage current could get you, or just slicing your arm open on a wire-wrap stake or the sharp edge of a metal cabinet.

zenith2134 · #14 12-13-2011, 09:21 PM

Right, i'm more worried about working on the inside of a medium powered pentode hifi amplifier with plate voltage around 500 dc and hi-current filament supplies. The crt can indeed hold it's charge plus build it back up weeks after power down..unless it's got a bleeder.
best zapp ive gotten yet was 120 mains at a porch light fixture which was wired with no ground. i was on a stair made of concrete with sneakers on. both hands were involved which is why this happened (a novice mistake, this was a few yrs back) one holding the fixture and one unscrewing the decorative cover's aluminum thumbscrew. for those interested, there was originally a ground stake in the garden for all the front lights but it appeared to be disintigrated(gone completely, with wiring still present but left unconnected to the ground plane) the house was made in 1910. Now I grounded it all to the cold water entrance below, via really nice 10awg solid core copper w/outdoors jacket inside metal shield.

timmy · #15 12-14-2011, 02:54 PM

how about ever getting shocked with 120 volts of battery dc power from an electric pickup i converted. i leaned over the battery pack on a hot day and my arms were sweaty and touched both terminals neg& pos, and that is a real shock and if you survive it then you are a real man. much to my surprise i did survive it but stats say i should not have.being there is 1500 amps available at the terminals it was bad and scary and never felt a shock like that ever.crt or yoke ill take any day over pure dc at that voltage.