Opening up a CRT TV - precautions

Discussion in 'Technical Repair' started by hang-on, Jan 19, 2015.

  1. hang-on

    Thread Starter New Member

    Oct 18, 2014
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    Hi,

    I'm opening up an old CRT TV. A VHS cassette is stuck in the built-in VHS recorder/player, and I'm going to get it out. Also I would like to repurpose the CRT part of the TV for an arcade machine in the future. My intentions are to mess as little as possible with the actual circuit and components, so I'll just build a new frame around it.

    I know that one should approach a CRT circuit with caution. Capacitors inside can hold some serious charge (thousands of volts). I have unplugged the device from the mains plug. The attached picture shows a part of the CRT (in the foreground, to the right) marked "LIVE PART" on the PCB, and the shield in front of the blue capacitor is marked with a lightning. As far as I can see, this is where the AC is converted to DC.

    One question:
    - I suspect that the high voltage stuff to avoid is encapsulated in the grey plastic container to the left? With thick white and red wires coming up from this container, and connecting to the CRT part and the back of the screen (ground?).

    I'm taking no chances here, so I'm approaching this thing slowly and carefully :) I'm not going to split everything apart to salvage components, etc. I just need some info with regards to where/what to pay special attention/avoid.
     
  2. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the large red wire going to the side of the crt that ends in what looks like a suction cup is where the high voltae is. get a thin screw driver and ground it with a wire to the chasis, push the screwdriver blade under the suction cup, if there is any residual high voltage, it will make a pop. leave it under the suction cup for a few minuits or hours, whatever you think is safe. the high voltage should be discharged by then,.
     
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  3. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    As long as you dont remove the suction cup on the end of the EHT lead, there is no danger, as long as its switched off first, the grey box is the line output transformer .
     
    hang-on likes this.
  4. wayneh

    Expert

    Sep 9, 2010
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    Not really. If you look at the ratings on the larger capacitors - and you need a large physical size to store a "dangerous" amount of energy - you'll see that none of them are rated to more than maybe 600VDC at the most. Still quite a jolt, but not thousands. The good news is, there is nowhere to store energy except in those big caps.

    But the bad news is, a big capacitor can store its energy for days even. I learned that the hard way, not with a shock but with a big pop when I inadvertently shorted the poles with a tool. The danger came from how fast I yanked my arm out when it popped!

    In theory that stored energy can discharge through a coil and generate a whopping voltage. That said, I don't think there is much risk after the device is unplugged for a few hours. Avoid touching or shorting the big caps, or better yet use a resistor to discharge them. Then there is no energy stored anywhere to surprise you.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    the crt is a big capacitor. the inner and outer coatings store a charge for days. the outer coating is grounded and the inner coating is connected to the high voltage (thousands of volts) rectified in the horizontal output flyback transformer. being made of fairly thick glass, it has very low leakage and will hold a charge a long time.
     
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  6. #12

    Expert

    Nov 30, 2010
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    and that's why you never wear any jewelry when working on these.
    No rings, no watch, no cufflinks, no necklaces, no dangly bits, etc.

    You don't need to discharge the CRT because you aren't taking this all apart, but, if you ever discharge a CRT and come back the next day or 3 days later, discharge it again! They probably won't seriously hurt you, but the impact from your reaction might.

    This is what I did for my first full time job. 45 years later, I still, literally, break out in a sweat if I don't have 3 layers of safety between me and the power.
     
    killivolt likes this.
  7. killivolt

    Active Member

    Jan 10, 2010
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    Thats why I didn't wear my wedding band for years. All my wives thought it strange; saying I was making excuses.



    kv

    Edit: Just to be clear not all at the same time. lol
     
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  8. #12

    Expert

    Nov 30, 2010
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    Me too. My first wife drove me to work at 8 am and picked me up at 5:15, then wondered how I was finding time to cheat on her. :(
     
  9. hang-on

    Thread Starter New Member

    Oct 18, 2014
    25
    1
    Thanks for your support, all of you! A grounded screwdriver under the suction cup sounds like a handy approach to discharging the CRT. I'm thinking that I can use an alligator-clipped wire: One end chews on the device's chassis, the other end chews on the metal shaft of the screwdriver. Then I put safety goggles on, and slowly push the screwdriver blade under the suction cup.... bumbum bumbum (heartbeat) :)

    But if I can get away with handling the CRT without discharging it first, that is even better. Theoretically, if I accidentally touch the (device unplugged, of course) 'back' of the suction cup, with my fingers or a tool, will I get shocked? Or is it only dangerous 'under' the suction cup? Dodgydave - you say that I got nothing to fear if I leave the suction cup in place...?
     
  10. bertus

    Administrator

    Apr 5, 2008
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  11. hang-on

    Thread Starter New Member

    Oct 18, 2014
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    Wow - that is a really comprehensive source, thanks! But now I'm a little confused, because

    1) In the text I read about dangerous capacitors
    "It is essential - for your safety and to prevent damage to the device under test as well as your test equipment - that large or high voltage capacitors be fully discharged before measurements are made, soldering is attempted, or the circuitry is touched in any way. Some of the large filter capacitors commonly found in line operated equipment store a potentially lethal charge. This doesn't mean that every one of the 250 capacitors in your TV need to be discharged every time you power off and want to make a measurement. However, the large main filter capacitors and other capacitors in the power supplies should be checked and discharged if any significant voltage is found after powering off
    " - and,

    2) I also read that it is not recommended to use a screwdriver to discharge?!

    Hmmm..
     
  12. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Only if you don't want a hole blown out on the operating tip!:p
    Max.

    The CRT HV does not usually have the same stored energy as a large Electrolytic.
     
  13. takao21203

    Distinguished Member

    Apr 28, 2012
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    solder a neon lamp + 100k resistor to it
     
  14. #12

    Expert

    Nov 30, 2010
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    Major limitations to that include the idea that a 1/4 watt resistor is only rated for 250 volts before arc-over and a neon bulb can be destroyed in microseconds by a few too many milliamps. So, resistor? Fine, but don't bet your life on it.
     
  15. takao21203

    Distinguished Member

    Apr 28, 2012
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    But you get the idea, right?

    DSC03079.jpg
     
  16. #12

    Expert

    Nov 30, 2010
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    What? Put enough 1/4 watt resistors in series to survive 30KV?
    or even 5KV?
    About 20 of them?
    So awkward!
     
  17. takao21203

    Distinguished Member

    Apr 28, 2012
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    I'm talking about the electrolytic capacitors not some old CRTs that supposedly hold their charge for a long time.

    Most I have seen discharge almost instantly when you turn them off.

    For discharging, both the neon lamp and 1 MOhm resistor will survive the small charge from the CRT- i doubt you'll see much of an effect. Fiddling under the suction cup with tools? I wouldnt recommend that.
     
  18. takao21203

    Distinguished Member

    Apr 28, 2012
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    they arent 1/4W by the way
     
  19. takao21203

    Distinguished Member

    Apr 28, 2012
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    from what I know recent CRT devices have a discharge coating so they make specific noises when turning them off. The TV set will attract hairs for 20 seconds or so then its all over. Rebuilding a charge? You'd need a fairly old CRT device to see that.
     
  20. wayneh

    Expert

    Sep 9, 2010
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    "Most" is a useless term when it comes to potentially lethal conditions. "Most" lightning bolts won't hit me. It's that ONE I have to worry about.
     
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