Safely disassembling CRT Monitor

Discussion in 'General Electronics Chat' started by magnet18, Jan 28, 2011.

  1. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
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    Hi everyone, I have an old crt monitor sitting in my garage and was planning on disassembling it this weekend, mainly for mosfets and that shiny flyback transformer I know is in there. (everything I need for a simple plasma speaker :) )

    I know there is dangerous stuff in there and was wondering the best way to do it safely is, its been sitting in my garage unplugged for about a month (cold garage if it matters) but I don't know if theres still large charges in there (big coils, big capacitors, scary voltage:confused:)

    Any advices, tips, safety warnings wold be greatly appreciated, thanks :)
    [EDIT]
    I know it is forum policy not to discuss potentially dangerous topics, I don't know if this falls under that category but if it does I completely understand.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,346
    Hello,

    I do not think there will be much danger, as it has been off for a month.

    But if you want to read up on safety, take a look at this page:
    http://www.repairfaq.org/sam/safety.htm

    Bertus
     
  3. marshallf3

    Well-Known Member

    Jul 26, 2010
    2,358
    201
    I seriously doubt you're in any danger but remember that a CRT can hold a charge for years since on most there's no real drain from the anode to the ground due to the integral HV diode in the flyback. For your own safety take a KNOWN GOOD cliplead, find a decent ground to the aquadag (coating around the outside of the CRT) which will usually be a wire or two around it. Connect one end of the cliplead to that and the other to a small flat blade screwdriver that has a large handle. Gently probe the screwdriver tip under the anode connector rubber insulator to the CRT until you know you'd made a connection with the actual anode connector. Do not be surprised if you end up with either nothing or a loud pop.

    I know from experience that a Sony CRT can hold a charge for a year and still enough to knock you across the room.

    Your father may have concerns that are valid but unless you're charging up a large cap the output of a flyback transformer isn't of any real current. Also remember that the output will be DC and I think you may want AC depending on your design. This means hitting up an old TV repair shop and asking for an old flyback from a set that used a separate tube (such as a 1B3 or 3A3) for the rectifier of the HV.

    An older TV flyback transformer was designed to operate at 15.734 KHz as were the old CGA monitors. The newer higher resolution monitors can often accept up to 96 KHz or better. To drive any of them requires a pretty well tuned circuit but in all cases there is an amount of safety involved as the "one hand in the pocket" rule doesn't apply, it just means that touching the HV output will flow right through your body all the way to the floor you're working on.

    Regardless I see this solution as far safer to the way some do this by taking an old microwave, neon sign or oil burner ignition transformer to make a plasma generator. Far cheaper to make but you'll learn a lot more and a common flyback isn't going to kill you - if they would I would have been dead 50 years ago.

    HV takes a lot more respect than mains voltages and if you're going that route I really see no potential for injury so long as you treat it with the respect it deserves. There are dangers even working with 24 VAC circuits.

    Would you care to share a circuit schematic or link of what you're wanting to build?

    Shoot, I won the state science fair competition three years in a row back in high school but I'll swear the guy that built his own CO2 laser from scratch was going to beat me out as that thing could burn through sheet metal but at least he had the common sense to enclose it such that the 10.6 μm output wouldn't get out.

    In time I'm building a copper vapor laser as well as an argon-ion one just for the fun of it. There's a lot of info about those available here: http://www.repairfaq.org/sam/laserclp.htm and I've got most of the parts and materials, I just have to get set up to do some glassblowing and put the darn thing together if I ever get time to semi-retire. I've made a nice ruby and ND-Yag laser in the past. Perhaps the easiest is a simple Nitrogen laser but they're generally worthless as the output is in the UV region but good for pumping dye lasers which opens you up to new worlds.

    A subject well worth studying but again you'll be dealing with high voltage again to drive one. I picked up a bunch of old oil burner excited transformers from eBay years ago, about 6 KV out from 120V in and totally isolated so can add the rectified outputs in series.

    Best of luck convincing your father that there are always dangers involved when experimenting but you can always tell him that another thing you want to build is a helicopter based on a couple of motorcycle engines. My father never inhibited me but my mom sure did, she knew that at the age of 12 I was making nitroglycerin and TNT in my garage chemistry lab and after she heard the explosion I made (and huge hole in the ground) she pretty much left me to my own devices. Besides that I had been working on the TV they gave me for my bedroom when I was 4 years old. Every week I'd take all the tubes out of it and ride my bicycle down to the local drug store to test them all but had to beg to get them to buy a new one for one I found that was bad.
     
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  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    One thing I have not seen yet is a way to safely let air into the CRT and do away with the implosion danger. If a CRT breaks, the pieces get sucked into the inside, and then rebound back out at high speeds.

    To get rid of that danger, you take a screwdriver with a long shaft and tap it into the center of the plastic cap at the rear of the CRT (removing the connector first). If you go in at a slight angle at the base of the locating key, the screwdriver will knock off the tit where the vacuum pump was attached. There will be a little crunch and a couple of seconds of hiss. After that, the CRT is no longer dangerous to handle.
     
  5. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
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    @beenthere, thanks, if i do disassemble it ill do that

    (guessing smashing it with a hammer isn't too safe? :D )
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
    4,670
    804
    IIRC all modern CRTs are made to be safe and not implode when shattered, so even smashing it with a hammer COULD be safe, but there is no way I would reccomend you to do it ;)
     
  7. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
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    Mind...
    Assaulted...
    With ideas...
    :D

    (none of witch involve smashing CRTs of course ;))

    Actually, I have it on good authority (the same man who thinks flybacks are to dangerous to mess with) that you can drop a T.V. CRT off a 3 story parking garage into an alley and it will bounce almost all the way back up.
     
    Last edited: Jan 30, 2011
  8. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    Don't do this as its only a general way how its done, things differ from one TV to another one

    We dont discharge flyback (EHT) transformer,we discharge the High voltage filter capacitor and that's the picture tube itself,we do it by shorting the anode to ground (wall coating) sometimes some people use a resistor to slowly discharge the tube rather direct way which may cause arc,but we dont do it every time very rare i.e.. in some special case(like if the EHT may be damaged),normally EHT have an internal resistor to slowly discharge the HV filter cap's current when the set is turned off.

    Your television system is different than mine.
    In my country we use 625-B monochrome and PAL-B color system (by the way this is what 625 in debjit625 means).
    And in you country I think its 525 monochrome and NTSC color system.

    Now EHT frequency also depends on the system,the EHT frequency is taken from the horizontal output section which drives the horizontal deflection coil at a rate of 15625 Hz ,this frequency is a result of the Hor scaning lines 625 and the frames per second 25 in this case,so you can see my EHT is within audio range not out of that,In your case the lines will be 525 and the frame rate may be 30 half of supply line frequency 60Hz which cause your EHT to operate at 15750 Hz.Their are more things but I will not go into details but what I am trying to say is flyback (EHT) is not for plasma speakers.

    I think it may help
    http://forums.parallax.com/attachment.php?attachmentid=69682&d=1272020720

    Good Luck for your plasma speakers:)
     
    Last edited: Feb 6, 2011
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  9. debjit625

    Well-Known Member

    Apr 17, 2010
    790
    186
    I think you have already noticed but then also I will like to say that touching the screwdriver under the suction cup will not make the Picture tube discharged,you have to ground it properly,on the first picture a wire is connected between the screwdrived and the wall coating of the picture tube i.e.. the ground.

    Normally our forum do not let us to discuss this kind of topic,but in this case debe picture was really helpfull so that you dont make any mistake.

    Another usefull stuff is that when ever we open a television set at first we discharge a power capacitor in the smps based power supply section,these contains a high voltage which could shock you ,in older TV they used transformer based power supply which I think had a low voltage power filter capacitor so they may not cause it(any way I have never worked those old sets).

    So when openning the set check if its using HV filter capacitor in power supply section and if yes discharge it.Many time I got shock from it

    Good Luck and be carefull.
     
  10. r d

    New Member

    Feb 9, 2011
    2
    0
    The real danger in that CRT set is the CRT has at least 11,000 lbs of vacuum pressure. They can implode if not handled properly. We were taught to rap them in a thick packing quilt and use a screw driver or nut driver to break the neck of as gently as possible. You will be able to hear the air going into the tube.
     
  11. beenthere

    Retired Moderator

    Apr 20, 2004
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  12. magnet18

    Thread Starter Senior Member

    Dec 22, 2010
    1,232
    124
    sorry, I hijacked my own thread... :rolleyes:
     
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