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 Post subject: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 9:54 pm 
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Location: Lexington, KY
Hi guys a question for you. I am going to take my working philco chassis out of the cabinet sometime this week, I think there is something wrong with the audio stage. I bought this set restored last month but am now having my doubs about this individual who is suppose to help me with my GE restoration. Well he talks a lot of BS and I think he performed a lazy restoration as well as not knowing much on tv restoration at all. I want to take a look underneath.

I have never handeled a live chassis or a chassis that is still "charged". My question, what do you not touch on the set, what can shock you, and how can I ground everything if I need to handle the HV box and CRT.

This is a very serious question, and answers would be great. Maybe I should wear some rubber gloves :oops:


I can also tell you all that I will be undertaking my GE by myself, that TV guy doesn't know how to properly restore a set from what I can now gather and I refuse to let him touch any of my sets. You all are nice to help me with input on my questions, I greatly appreciate that.

Thanks for the help everyone

Jon


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 10:07 pm 
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Assuming the set was not powered up briefly which charged the filter caps, the only thing that usually maintains a charge is the CRT itself... The filter caps will be discharged by the current draw of the tubes as long as they were hot... The CRT can be discharged by connecting a grounded clip lead to a screwdriver and slipping it under the anode connector on the CRT... If the HV wiring is in good condition, as long as you don't stick your finger under the anode lead or play around inside the HV cage, you aren't likely to get shocked even if you do nothing...


Tom


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 10:25 pm 
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My experience with tube sets (1958-1988) was that all the primary DC discharges itself, as the cathodes are still emitting when the source is cut and the capacitors are relatively small (~50uf). By 'primary' we mean 125-350VDC plate supplies originating on the rectifier cathode(s) and their resistored and decoupled branches.

If you're poking around while the set is operating, MUCH caution is required. Better would be to connect test eqpt while it is NOT operating, then turn it on and observe the results without touching anything.

While operating, don't get within a foot of any tube with a cap on top. When off, temporarily ground the cap of the horizontal output tube in case there is a boost capacitor still holding charge (I never observed any but I also never worked on a Philco).

The big bugaboo you probably already know, is the CRT anode (flying lead). Not only will it kick a frightening SNAP when you ground it, but space charge will rebuild on it AFTER you've temporarily grounded it so KEEP it grounded OR reground it before you go anywhere near it. Once discharged it won't seriously injure you electrically any more but it can still be VERY uncomfortable, cause you to jerk away uncontrollably, result in physical injury like scraping skin across sharp edges.

That's all I knew when I was 14 and it obviously worked, I'm still here.

(co-post)


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 10:31 pm 
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While HV can really bite you, make sure you don't grab a hold the 120v AC line while working on or repositioning the chassis. It's easy to forget that the AC line has a greater ability to take you out.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 10:50 pm 
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First, it isnt' the horizontal output plate cap you need to worryabout. That hv goes away when set is disconnected.
The second anode, and, believe it, or, not, the anode connection on the picture tube, are really the only two major worries.
Never have the set plugged into the line while taking apart.
There are numerous precautions, but, if the set was in plying order, when you shut off the power, the filters will lose most of their charge in short order.
I've seen them back up a few volts, like about 6-12 volts dc, but, that's not dangerous, and, usually loses that static charge fairly quickly.
If testing voltages, always connect the equipment with the set turned off.
Keep one hand behind your back, or, in your pocket.
Never handle chassis while it is on, or, live.
Hope this helps some.
Bill Cahill

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 10:57 pm 
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Yes it helps greatly, so I do not touch the cable going from the HV box to the bell of the CRT, as well as all HV tubes.

Now do I have to discharge all of those I just stated above.

Thanks

Jon


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Sun 22, 2012 11:16 pm 
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Only the second anode, and, perhaps the hv connector on the crt.
That you may short to chassis with an insulated screwdriver. As 35Z5 stated, connect a clip liead from the blade of the serewdriver to chassis.
If you recently had it playing, and, turn it off, no need to discharge the electrolytics. And, the plate caps on horizontal output tubes do not hold the hv.
The hv section is the only thing which normally does.
Bill Cahill

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 12:07 am 
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Location: New York
I posted about this in the Test Equipment forums a while back:

"True story-

In the late 1960's, I was gifted with an old 'roundie' Zenith color TV that needed a picture tube. I was in high school, working offhours and weekends in an electronic distributor. The owner of the shop got the picture tube, and gave me instructions on what to do, First order, unplug set. second step, discharge picture tube anode, and so on. Ground wire to chassis, wire wrapped around my long Craftsman screwdriver. Big SNAP when I got it under the rubber cup. Took a lunch break. Came back, and reached in to pull back the anode cup to unplug the HV lead.

I don't know how long I was unconscious. I apparently did the high school biology class experiment to myself, where you feed electricity into a dead frog's leg to see it move. I was wearing long sleeves, and when I reached in with my right hand, I KNOW I didn't lean on the metal chassis, I remember looking at the wood cabinet to find a place for my left hand, since the chassis was very long and wide, so I had to reach in pretty far. Perhaps a knuckle brushed it, or it arced to my finger or perhaps the back of my hand as I leaned in.

When I did the "frog", I had been sitting on my heels, to get inside the console. When I woke up, I was lying on the floor on my back, and yes, it tasted like I had eaten a whole roll of aluminum foil. I was dizzy, probably from a concussion, as the top of my head had broken through the plaster wall, which was 5 or 6 feet behind the console TV. I had also cracked the wooden lath behind the plaster itself.

I then tried to discharge it again, after insuring that the TV WAS unplugged from the outlet. Heard another healthy SNAP. Then I got a HV meter, and it popped right up to 37KV. Remember, this is after several discharges. I don't know how high the anode voltage had actually gotten to. The set had a LOT of issues, the B+ regulators had failed, etc. I spent weeks on that set before it worked well. And I often wonder if that thing was producing soft X-rays, due to runaway voltages.

Lesson? Safety is totally YOUR responsibility. That discharge, I am sure, was across my chest. It wasn't just my arm or hand that jumped. If I had an undiagnosed cardiac issue, that could have been a big deal. And I hit a wall. I may have jumped back against something else, triggering a bigger episode. Like a hot soldering iron close to me while I was out cold. I NEVER completely trusted that boss, or school teachers, anybody, ever again.

Develop REAL good habits about what you do and how you do it, and all this stays a lot of fun."

I did edit out a bit, pertaining to the original post I was responding to. I was 16 at the time, and I had gotten a job working at an electronic parts distributor. The owner told me I would get a "complete education" from him. Damn right I did. When I told him what happened, he replied that I "should have known" that some of the old CRT's could hold a significant charge on the anode.

Gee, thanks!

So beware a "live" chassis, where the metal chassis is connected to one side of the 110 volt line, and high voltage. I have also heard about where a tech wanted to tap a tube to see if it was causing an intermittent issue, and he used a pencil. The high voltage really liked the carbon in the center. All that tech did was bump his head on the wooden cabinet. And be the butt of jokes at the shop for a while.

Oh, and yes, I DID fix that color Zenith, as we used it for a long time. I did also learn how to handle high voltage properly, and have made many repairs over the years. Don't be afraid of it, just treat it with respect.

John S.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 12:17 am 
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Thanks John that is just what need to hear, I know from what you all have told me, HV on tv's are quite dangerous. I appreciate the help. I think about this when I plan on taking out that Philco tv chassis next weekend. I need to do all the research on precautions and proper ways on how to discharge before next weekend, I wish I had a experienced individual by my side when I do it, I am quite the visual learner.

Thanks for your help and a good story I will remember when getting around the CRT and HV of these old tv's.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 12:24 am 
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Another true story....
While a teen ager, just having gotten my job at the tv repair shop, I decided to try to resurect the old family set.
I got a smimilar chassis, spkr., and, usable pix tube.
I put them into cabinet, and, actually got a raster. Nothing else, but, a raster. That means the front of pix tube lit up.
I was asking my new boss which tubes to try.
He told me to try the two tuner tubes, which on the particular chassis I am talking about, KCS 47, are dangerously close the the mtal bell of that round picture tube. The metal bell gets the hv.
I had just unplugged the set. I was in my bare feet on a tile floor, on cement basement floor.
I was holding a metal bench lamp in my left hand to try to see what I was doing.
Well, this little finger on my right hand managed to touch the bell of that picture tube....
When it did, I felt a strong jolt up my arm, accross my back, and, down to me left hand.
I remember letting out a yell as I went flying. I landed against an oposite wall, and, knocked myself out. When I came to, I was a number of feet away from the set,, and, lamp was on other side of room.
I called the shop. Gene Petuna, the other tech there, laughed, and, said, you dummy. You need to discharge the hv first!
I asked him if I should do that. He laughed out loud, and, said, Heck, No! You already did!.
Bill Cahill

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 12:31 am 
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He He :lol: good story Bill


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 2:57 am 
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Bill, how long after turn-off and unplug from AC do wires/components in HV cage and first anode/pic tube pose a shock hazard? To put it another way, without doing any manual discharge, how long do you leave the set unplugged before assurance that all hazards are absent? I did some tube-tapping with the wooden end of a long screwdriver while the chassis was hot to detect tube ringing, being careful not to contact any part of chassis with the blade...


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 3:46 am 
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Location: keansburg n.j. usa
Depends on the set. I remember getting zapped 6 mos later on a rca xl100. Just take an allagator clip attached connected to the chassis and to a screwdriver and slide it under the anode cup. Make sure you have a decent plastic handle on the screwdriver.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 5:21 am 
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Location: Ft Worth TX
Tube set CRTs can hold a nasty charge almost indefinitely. SS sets have bleeders, IF they are still operational. Never ASSUME safety. Or U and ME can get knocked on our ASS.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 6:45 am 
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As you may have gathered from the above posts, when you discharge the high voltage keep it grounded for a little while. The coating on the inside of the CRT and the coating on the outside of the CRT form a capacitor that holds the charge. The coatings are just sprayed on and there can be sections that are not well connected to other sections. It can take a minute to for everything to discharge.

Also, it you ever work on a set with a metal CRT, all of the metal is attached to the high voltage. On those sets there is a separate capacitor that will hold the charge.

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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 3:08 pm 
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I spent 20 years designing high voltage power supplies for CRT displays (hence my user name), and can vouch for all of the safety tips above. A typical 21" or 23" picture tube has an anode capacitance of a few thousand picofarads. The stored energy is 1/2CV^2. If you do the math, at 25,000 volts and say 2000pF, the stored energy works out to something like 1.25 Joules. With the TV turned off and just the stored charge on the tube. That's a lot of energy potentially running through your body. :shock:

Safety first.


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 8:34 pm 
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If the HV was really dangerous I'd be dead, been hit enough times the proof is in the pudding... The B+ or AC line is far more dangerous... A good 2nd anode is about like a modern automotive iginition system(been bit by plenty of those as well)...

I got used to the Warwick sets with the voltage divider/bleeder focus circuit that killed the CRT charge on power down, so wasn't discharging the anodes when I pulled a chassis(CRT remains in cabinet on consoles)... Well we got new Sayno chassis sets at Sears in '77 so the first time I slipped my thumb under the anode cap to disconnect one, SNAP!!! That was a good one and hate to admit it, but I did it not once but twice...

There were several others but one of the most memorable was a 17" Toshiba color set the customer said shocked them if they touched the top... This was a metal cabinet set and I asked, just on the top??? Yep only the top, don't touch it when it's on... Well being the doubting Tom, I touched the top near the edge, nothing... Customer says it shocks near the center, well I laid my hand on the PLASTIC handle and promptly saw stars, POW... WOW that bit!!! Turned out the anode connector had split open would arc to the metal handle bracket, that was insulated from the cabinet... On a positive note, it did keep the cat off the set... :lol:

Tom


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 9:31 pm 
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:wink: The same as Tom's post before mine , I too have been bit a couple of times by the various tv voltages , and so have a few others ive known who also work on old electronics . Yet ive never heard of anyone being killed by one ... yea , im sure in rare instances it does happen , but it seems the majority of deadly zappings I read about always involve good ol house current of the 100 or 220 variety .

Has anyone here ever personally known of anyone takin the big dirt nap after being whacked by a tv's picture tube ?


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 Post subject: Re: What not to stick your hands into when it comes to HV
PostPosted: Jan Mon 23, 2012 10:01 pm 
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You guys are making me nervous about digging into the HV, but respectively I appreciate all the help with my question.

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 3:30 am 
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Those stories are amusing, but the everyday reality is that many, many, many people work safely on these old TVs without getting shocked. All it takes is common sense and a few simple precautions.

As long as you avoid doing something truly dumb, you're at much greater risk driving to the store for groceries.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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