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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 24, 2012 3:46 am 
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Just unplug the set before sticking your hands into the chassis! If the insulation on the high voltage lead to the CRT is in good condition and that big suction cup is in good condition, then you should only have to worry about the high voltage if you are going to disconnect the high voltage lead at the CRT or work inside the high voltage cage. If you are not sure about the insulation, then you should discharge it to be safe.

As stated above, it is easy to discharge the high voltage. Just connect a wire between the chassis and a thin bladed screwdriver. Work the screwdriver under the big suction cup and hold it there for perhaps 30 seconds. Sometimes after disconnecting the high voltage lead I will connect a clip lead between the chassis and the button on the CRT where the lead was connected.

If the CRT does recharge because it was not completely discharged, the voltage will be quite a bit less then the full charge. So don't worry too much.

Note that the high voltage on an early black and white set is around 12000 volts. A color set on the other hand has around 25000 volts (except for some very small screens), so you have to be more careful of a color set.

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 24, 2012 4:30 am 
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Location: Niagara Falls, NY
I worked Avionic systems (communications, navigational and RADAR) while serving fulltime in the ANG, and one tool we always had on hand in the shop was a HV discharge rod. It was phenolic or sometimes fiberglass, about three feet long, had a brass probe mounted on one end where a heavy guage insulated ground wire was attached to it. The other end of the wire had a good sized alligator clip attached. This can be easiler made at home using a cut off broom handle, a large nail or screw hook, large alligator clip or small car battery clip, and lamp cord.

To properly discharge a high voltage component, the ground wire is attached to a good ground FIRST, then while holding the (non conductive) rod, the metal probe is placed on the object, terminal, component or whatever needs to be discharged. This is repeated a few times to ensure it is completely discharged. Attaching clip leads or using screwdrivers really are not the best methods IMHO. You have a better chance of being the discharge path using those methods.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 24, 2012 5:46 am 
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Thank you both for the help, it would seem easy enough, now what about the tubes in the HV box, especially the HV tube (such as a 1B3) Do I discharge the same way? Do those tubes in the HV box also contain High voltage just like the CRT once in the off position?

Jon


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 24, 2012 5:49 am 
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I think that the high voltage you were working with was much higher than that in a TV set. The high voltage in a TV set will only jump 1/4 of an inch at the most. When working the screwdriver under the big suction cup insulator you have to nearly touch the metal before you hear the snap.

Edit: posting at almost the same time as ketron.

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 24, 2012 5:52 am 
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Once you discharge the CRT you have discharged everything. The lead to the CRT is connected to the tube socket of the high voltage rectifier, often a 1B3..

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Wed 25, 2012 6:07 am 
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ketron281989 wrote:
You guys are making me nervous about digging into the HV, but respectively I appreciate all the help with my question.

Jon

As everyone has said, just be careful and you should have no problems.

philsoldradios wrote:
As long as you avoid doing something truly dumb...


That reminds me of my one bad high-voltage experience in over 30 years (so far) of working on CRT displays. One type of 23" black-and-white video monitor that I repair has a removable chassis on a metal base, and everything unplugs from it for removal (CRT socket, yoke, HV wire, ground wire, front-mounted controls and power switch). In the late 80s, I was installing a good chassis in one of these monitors after removing the bad one as I had done a couple of hundred times before. Well, while connecting the various plugs I had somehow plugged in the AC power as well, and the power switch (part of the brightness control, so they are seldom if ever turned off). As I was reaching to connect some other plug, I heard a bit of sizzle or crackle and then BAM! My hand touched the still-loose HV terminal on the PLUGGED-IN chassis. It felt like I got hit by a truck. Luckily, my other hand and body were outside the cabinet so I did not get a shock all the way through my body, and I did not go flying (I was a few steps up a ladder, too). You can be sure I never made that mistake again!

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Fri 27, 2012 3:51 pm 
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Location: Powell River BC Canada
With TV high voltage most of the minor injuries I had were cuts and scrapes trying to reach
a tuner tube close to the front of the set. Some chassis needed contortions to get your hand
there on a service call. The jolt made my arm pull back over some sharp metal. Also some
very few had metal cone tubes, and those woke you up if you didn't notice.

As to hazard, my simple answer is all high voltage is dangerous in a TV powered up or not
because I have no idea how it affects you.

What you can do, is buy a neon test stick and use it on your project. If you are asking about
shock in TV sets, it is assumed that are not going to be working on live chassis using a cheater cord
to power it.

Back in the day, when TV repair paid the rent, you needed one of these tickets to work
in the trade in many places.

Attachment:
BCTQTV.jpg
BCTQTV.jpg [ 166.41 KiB | Viewed 687 times ]

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Fri 27, 2012 11:30 pm 
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Neat certificate thanks for the advise, could you tell me a bit more about the neon stick idea you were talking about?

Thanks

Jon


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 12:27 am 
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Location: Pewaukee, WI
I regard all voltages in a powered down set as more of a annoyance than a danger.....I've discharged the HV of a just-powered-down 80's color set with my hand once while I did jump back (more by decision than reflex) and it did hurt it was not all that bad as far as some of the shocks I've gotten go (the surprise of it was possibly the worst part).

I've had to deal with anode recovery in a roundy once.....I'd discharge it set the screw driver down pop the cap off and get zapped or would get zapped on reassembly. The CRT is mounted to the cabinet on that set and I had to repeatedly pull and reinstall the chassis to track down a bug so I kept getting zapped until I was convinced it was not me forgetting to discharge or botching the discharge. I swore like a pirate for the better part of that trouble shooting session until I got really mad and decided to leave the ground connection in place on the CRT when the anode wire was not connected.

I don't like to get shocked by HV so I take precautions, but compared to a LARGE jolt I got taking down live Christmas lights one wet evening (busted bulb got me on the arm) I'm not all that bothered by HV.

I did once have a reflex reaction on that same metal cabinet roundy that I mentioned earlier. I was adjusting the yoke for a level picture with the set on, and touched a corroded spot on the vertical winding.....I yanked my arm back so hard that I knocked a convergence magnet off AND ripped loose a ground wire that went from the convergence board to the blue lateral magnet! :shock: I could see the blue glow of the electricity meeting my thumb when I grabbed on, and afterwards there was some black soot on my skin where I got zapped.
Always be extra careful on live chassis as when you think you are being careful and start to relax that is when you are apt to end up making a mistake that can be dangerous.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 1:17 am 
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Neon high voltage stick 300 Volt to 33 kVolt AC or DC.

http://www.seaward.co.uk/products/high- ... ators/kd1e

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 3:13 am 
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There is another way to use a neon bulb for testing. If a bare neon bulb is brought near the high frequency high voltage on the plate cap of the high voltage rectifier (1B3 or similar), it will light up. It does not need an electrical connection to anything. So if you want to find out if the horizontal / high voltage section is sort of working, you can use a neon bulb as an indicator. Just attach a neon blub to the end of a long plastic or other insulating rod. With the set running and the high voltage cage open, bring the neon bulb near the 1B3 and see if the bulb lights up.

The bare neon bulb only reacts to the AC high voltage at the plate of the high voltage rectifier and to a lessor extent that at the plate of the horizontal output tube. It will not react to the DC voltage going to the CRT. This kind of tester is useful if you can't get any raster on the CRT and want to know if high voltage is present. Of course if the 1B3 is dead you can have the AC high voltage but not the DC high voltage.

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 6:51 pm 
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Tom Schulz wrote:
I think that the high voltage you were working with was much higher than that in a TV set. The high voltage in a TV set will only jump 1/4 of an inch at the most. When working the screwdriver under the big suction cup insulator you have to nearly touch the metal before you hear the snap.

Edit: posting at almost the same time as ketron.


That's what I thought until I worked on an old Dumont metal-CRT tube set. My hand was 2" away from the CRT, but a thin spark jumped from the bell to my hand, it felt like I had walked across a carpet and touched a wall plate. It was a warm summer day, and I think humidity from my hand or the air created a path for the 2" spark. I soon got rid of that set, it took four men to lift it and carry it to the curb.
I have worked on a lot of CRT sets and computer monitors, and never got any shocks from the CRTs since I knew to avoid or discharge the high-voltage leads.
Don

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 7:02 pm 
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Gee, Don. You had hv. Why did you quit? Especially on a Dumont??
Oh, well.
Bill Cahill

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 7:09 pm 
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That sounds almost more like leakage through humid air than a real arc. The full jolt you get with contact is quite a lot stronger. Certainly a metal bell CRT exposes you to much more risk than a glass CRT.

In any case you certainly do not need a 3 foot insulated rod and a heavy gauge ground strap. And you need something thin to get under the big rubber suction cup that most sets have at the CRT end of the high voltage lead.

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sat 28, 2012 7:40 pm 
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Yeah, it was just leakage, but I thought the bell was grounded. Actually this was in 1970. I had the huge 21"? Dumont set working with a good picture but the sound was not too loud, good enough though. Inputuner picked up FM as well, with the fancy eye tube. It had a constant-voltage power transformer that I gave to a friend for his ham transmitter. It was the only set I saw with two tubes each of 5U4, sweep, and HV rectifier. I was so pissed-off about the potential for a severe shock, that I got rid of it. It was not considered valuable in 1970, a friend gave it to me along with a two-chassis 16" Admiral floor-model that I also fixed and kept for a few years.
Don

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 29, 2012 5:03 pm 
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Most of us who have worked with TV repair and for that matter High Power transmitters tend to get careless.

60 cycle (Hz) power across the heart muscle will often cause it to go into Fibrillation which can result in sudden death. (60 Hz is not a magic number, any Pulsing works well).

Many a Police Man has Tazere'd a Perp and killed him/her.

A DC voltage across the Heart Muscle tends to reset the heart rhythm although it can also trigger Fibrillation. The Doctor uses DC when He yells "Clear".

So Here is the risk:

TV high voltage from older B/W sets using a Flyback transformer derived HV tends to be lower (15 KV or less, low current) and it will wake you up, and cause you to flinch and rip a gash in your hand or hit your head or knock the chassis on the floor and break the CRT which will impale you with glass shards :) .

The B/W projection sets have up over 25 kV and they bite harder not to mention X-Ray eye damage.

Some earlier sets obtain the high voltage from a winding on the power transformer (like the early 40's Philco 7") and route this around under the chassis so don't get cocky with one of these either.

Flyback derived HV won't usually kill you but it will knock you on your butt and may cause an RF burn and then the AC side of the rectifier is 60 Hz and a good 500 volts +.

Then the Hot Chassis sets (no power transformer) need to be treated with care, I use an isolation transformer and ground the isolated chassis.

Us old guys usually test Flyback HV by arcing the plate of the 1B3 or H-out tubes to an insulated screwdriver. dont touch or arc it to the chassis. Don't hook up a scope or meter here (HV meter is exempted here)

When you are working on 7" electrostatic sets be aware the high voltage is all over the place under the chassis and it will vause you to flinch and fry the input to your meter or scope.

I believe RCA started the trend to Flyback HV and their center grounded B+ Voltage to the chassis was done specifically to improve the safety.

After the power is off I will take a clip lead and discharge the HV to the chassis, never bother much wit B+ as it is usually depleated by the time the tubes cool.

Jim


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 31, 2012 12:54 am 
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could one paint the outer metal cone on one of those old CRTs to prevent getting blasted?


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 31, 2012 2:52 am 
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I wonder if the high voltage would rise if you painted the shell. The corona load of the surface
would vanish. My book tells that the shells were spun from 17 to 27 % Chrome Steel. Wonder
what kind of paint would stick to that?

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 31, 2012 3:28 am 
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Yah, there may be a reason why no manufacturer painted those metal cones.

My Scott combo TV uses an insulating diaper on its metal 16AP4:

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Tue 31, 2012 3:40 am 
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radiotechnician wrote:
I wonder if the high voltage would rise if you painted the shell. The corona load of the surface
would vanish. My book tells that the shells were spun from 17 to 27 % Chrome Steel. Wonder
what kind of paint would stick to that?

Since the metal is quite smooth with no sharp points, there should be no corona. To get adequate insulation with paint, the paint would have to be applied quite thick. Using sheet insulation formed into a cone, as in the preceding post, would be much easier. My DuMont had similar insulation. I decided not to trust it.

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