rfi emi etc

Discussion in 'General Electronics Chat' started by paulmars, May 26, 2011.

  1. paulmars

    Thread Starter Member

    May 13, 2010
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    Inside the device (flat screen monitor, in this case), the PC boards are covered with metal enclosures with small holes. Holes for cooling I presume. Small holes to prevent rfi interference I presume, just like the holes in the metal door of a microwave oven. The holes smaller then a microwave, to stop them from passing thru and cooking the people in the kitchen. My question regards the metal PCB cover inside the monitor. Are the holes to prevent interference from getting in or out? Is it a health hazard to use monitor w/o the pcb covers?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    It's to keep it within the monitor. The old CRT monitors generate a good bit of electrical noise. The shielding helps to minimize the radiated noise.
     
  3. paulmars

    Thread Starter Member

    May 13, 2010
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    this is a flat screen monitor. I've seen these covers w/holes in other equipment too. Is it a health hazard to use monitor w/o the pcb covers?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If they weren't needed, they would not have been included; no point in running up the cost of the item. Flat panels (LCD's) frequently have CCFL's inside to provide the backlighting. They generally require several hundred volts to start/operate.

    I don't suppose absence of the shields would cause a health hazard, unless someone depended on an electronic implant device (like a pacemaker) and the RFI emissions were quite strong. However, if it's radiating electrical noise, it may cause interference with other devices (radios, TVs, cell phones, cordless phones, other electronic devices).

    It's always a good idea to put things back together the way they came apart.
     
  5. paulmars

    Thread Starter Member

    May 13, 2010
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    It works w/o the shields and back over. With them in place, screen goes all horiz lines after 5 minutes. Cant find any bad components. considering using it with the back naked.
     
  6. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    You are having an 'overheating' problem with a component. Used to have a monitor that did this. I had to keep a small fan blowing on the back of it or it would power off after 20 or so minutes. It sounds like it is the vertical deflection circuit(if they still use such things in flat panels). My knowledge is all cathode ray tube, so the circuit names may be wrong, but the overheating part isn't. If you can locate the HOT part. Examine it(after power off for some time) to see if it has adequate thermal connection with its heatsink, or perhaps it needs a larger sink, or maybe even doesn't have one at all. That should make it so you can put the cover back on the unit. And you might want to increase the size and number of holes in the cover. bottom and top.,
     
  7. paulmars

    Thread Starter Member

    May 13, 2010
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    Ive already visually inspected everything and cant find what is over heating. Everything is very hot to the touch. There are no heatsinks that I see.
     
  8. paulmars

    Thread Starter Member

    May 13, 2010
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    after 5 minutes it goes bad w/case. W/o case and covers, i've had it powered on several times for 12+ hours w/no problem.
     
  9. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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  10. Jotto

    Member

    Apr 1, 2011
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    the only reason the covers are there is for protection of the board since the outside is usually plastic. I have monitors with them and without them. The only part of the unit that needs to be shielded is the tuner. Everything else it doesn't matter.

    If its generating a lot of heat, is the heat coming from the power supply? I am surprised there are not any heatsinks in the unit. I have heat sinks on the micro processor, depending on the monitor some on the power supply are very large and some are small.

    I am not sure on your unit, but most follow this. 110vac input, 12 or 24vdc output, or both. 12vdc, 5vdc, 3.3vdc and sometimes 1.5vdc are used on the A/D board. Inverter will usually be 12 or 24vdc with 5vdc switched. You can run the unit without the panel hooked to it, run the inverter alone with the adding of a resistor to simulate the switched 5vdc. Can run the inverter with no lamps using a dummy load. These are all general without knowing what your using for voltage but it all basically works the same on all models. Have also seen 33vdc and 18vdc.

    I would first try and leave the inverter out of the circuit, it just provides light, I watch how much current the unit draws to know if its on or not. I use the PR570 by sencore for this, or you can use your meter to see the current draw. I check all the voltages produced by the power supply, and then I go to the A/D board and check the ones that are produced on that board. If I know the panel is ok, I disconnect that and hook the inverter up, again I watch for the current draw, it should be considerably more with the inverter hooked up. If I see that current goes up, and then goes back down, I know that my inverter is not balanced and see if its the inverter or the lamp by removing lamps and adding a dummy load. I don't think yours is the inverter by your description of the problem.

    I would say look at you smps, there will be a 47mf@50 volts usually, there will be a PWM and FET in that circuit, and there might be more then one. Newer units are using these in more the one spot now. Now this has changed a bit from when I first started working on this stuff, so you make have one that has the PWM and FET in one component now, hard to check. But it will still have a cap at the value stated or maybe a little different but not much. It seems a standard to use the 47mf cap, but I have seen others used just not often.

    And then there is you adding heat to different parts to watch it fail, then use something that will produce very cold air, like canned air turned upside down to cool it down an see if it comes back, but you also have to be careful doing this method, you can cause condensation that could cause damage. Its just a matter of being very careful.
     
  11. paulmars

    Thread Starter Member

    May 13, 2010
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    Tks. Plastic cover was cut to allow air and the metal shields were left off. It works like this, who knows how long. Its been put back in use. If it fails again or is replaced, I'll try some of what you suggest. I've already checked the power supply voltage.

    thanks,
    >p
     
  12. nsaspook

    AAC Fanatic!

    Aug 27, 2009
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