diagnosing a bad CRT

Discussion in 'General Electronics Chat' started by sixstringartist, May 15, 2009.

  1. sixstringartist

    Thread Starter Member

    Apr 8, 2008
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    Im looking for some tips on diagnosing a bad CRT. It is an NEC MultiSync FE1250 that is losing picture (as if I turned it off) repeatedly. As this progressed it takes longer and longer for the image to come back on and its symptoms appear more frequently. If I physically turn off the monitor and turn it back on the image with come back momentarily before again going blank. My searching seems to point to a bad flyback transformer but Im hoping to get some advice from someone more experienced.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    A bad CRT stays that way. You monitor blinking sounds more like a high voltage problem.
     
  3. DonQ

    Active Member

    May 6, 2009
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    Here's a couple pieces of advice.


    1) If it actually is the flyback, it will probably cost you more to fix it than it is worth.
    2) regular circuit components supporting the flyback would be more repairable, but difficult to troubleshoot, because:
    3) electronically troubleshooting it requires special equipment and techniques, because:
    4) troubleshooting the high voltages present in this area can damage regular meters, scopes, etc, and can also damage the operator.

    The flyback is responsible for the B+ for the CRT, and voltages in this circuit can easily be in the multiple thousands of volts. You'll probably see limits on regular test equipment for max voltages of 600V or so. The high voltage present here has a big capacitance in that the CRT itself works as a capacitor, and this turns this very dangerous voltage into an extremely dangerous voltage.


    That being said... I have had a number of CRTs, along with a number of other devices (motherboards, cell-phones, etc) fail because of a poor lead-free soldering process in manufacture. Even now, I have an NEC multisync that has a bad joint that I have been unable to locate, but I do know where it is near. I took a long piece of wooden dowel (to keep my fingers far from the lightning producing circuits) and used it to press on the main circuit board enough to make it flex. I found several places that made it "fixed" when I pressed hard, but I finally found a place where only a gentle press "fixed" it.

    I extracted the circuit board and did a visual inspection in the area with the help of a magnifyer, but couldn't find the cause. Other times this exact procedure led me to a place where a cracked solder joint could be seen and resoldered.

    It might be worth a try. Actuall troubleshooting is only for when you can't find the problem by just looking at it.

    Note: only extract the circuit board after allowing sufficient time for the B+ capacitance to discharge. This can sometimes take an extremely long time.
     
    Last edited: May 15, 2009
  4. sixstringartist

    Thread Starter Member

    Apr 8, 2008
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    Thanks for the replies. I know the monitor is likely not worth repairing but I am interested in the learning experience and the monitor itself is rather nice. Im aware of the precautions that must be taken when dealing with CRT monitors but how long does it take to discharge the B+ capacitance? Other places Ive read instruct on how to quickly discharge the B+ with a screwdriver. Would you not recommend that approach?
     
  5. Ratch

    New Member

    Mar 20, 2007
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  6. sixstringartist

    Thread Starter Member

    Apr 8, 2008
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    If it was a bad joint wouldnt it be sporadic behavior? What Im experiencing is a screen blackout after a short, semi constant length of time. When the monitor is cool it takes longer for it to occur so Im leaning towards a voltage problem.
     
  7. DonQ

    Active Member

    May 6, 2009
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    A couple of more notes.

    The timing thing does make it sound like a component thing. My first suspicion would be an electrolytic capacitor somewhere. They are notorious for being among the first to go, and when they do, they can be very temperature dependent.

    If it is leaking, it will dissipate power, so it might actually get warm, which would explain the restart behavior. Hard to feel it while it is turned on (without attracting sparks), but you may be able to just spray it with a refrigerant spray and see if you can find one that makes the problem change.

    You may be able to tell something about the circuit area where the problem is. If the B+ is the problem, there might be other signs. Lots of CRTs will "crackle" when the B+ drops, like when you turn it off. If you don't hear this when it blinks to black, but you do hear it when you turn it off, then it probably is not the B+.

    Losing the B+ somewhere after the horiz osc would proably do a fade to black, rather than a blink off, especially if there is not much of the screen illuminated.

    As long as the Multi-sync is set for one of the slower screen refreshes, and your ears are young (or you can borrow a young pair of ears for cheap), you may be able to hear the horiz frequency in the flyback. Set it at as low a resolution and refresh rate as you can and see if you can hear it. It will be way up there, 15k-20kHz. Then, does it go away when it fails?

    It may be that the video loss is in some other circuit. If the sweep goes away, you might be able to see curved traces just before it blinks out. Brightness/contrast circuits might cause color/brightness warping just before it fails. Things like this.

    Whn you don't have the right equipment, you just have to be creative.


    As far as B+...

    I'm not going to recommend discharging the B+ with a screwdriver for the same reason that I can't see any wood on my new ladder (too many warning labels, and it's just a ladder, not 20,000V!) I also wouldn't tell you to put a physically large resistor in series when you don't do this. And when you don't finish this, I wouldn't tell you to leave a jumper hooked between B+ and ground.

    And nobody would trust just a fraction of an inch of plastic on the handle of a screwdriver when it's so far off the end of any normal voltmeter reading.
     
  8. sixstringartist

    Thread Starter Member

    Apr 8, 2008
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    Is there anything I should be aware of before spraying the refrigerant around?

    It is doing a fade to black. When the problem first appeared it was a fade to black and then back to normal after a few seconds. Those few seconds eventually turned into dozens of seconds and then it would never come back until the monitor was powered off for a period of time.

    On the lowest res/refresh I can hear a faint buzzing but not a whine like something high frequency. I cant tell if the buzzing is coming from near the yoke or the middle of the mainboard (which I believe is where the flyback is).

    At low res, there is a slight but noticeable jitter of the image in the horizontal. I notice this mainly with my desktop icons. It doesnt appear to be vertical at all. At higher resolutions this becomes unnoticable. Also degaussing fixes the low res jitter but Im suspicious that degaussing may have something to do with it.

    Actually Im a bit perplexed right now. When I first got this monitor out of storage to attempt a repair, the thing went fade to black before I even got into windows. I hooked it back up to check some of the things you mentioned and now its been going strong for 10+ minutes at high resolution.
     
  9. bertus

    Administrator

    Apr 5, 2008
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  10. DonQ

    Active Member

    May 6, 2009
    320
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    I think you might be experiencing what I call "The laying on of hands". It is the miracle cure I would experience when people would call me to service their equipment, usually hundreds of miles or several hours away. When I would get there, apparently just the act of me being there to touch it would cause the problem to disappear. "I can't fix it if it aint broke."

    So what you have to do is provoke it. Is it cool now? Turn on the heat, or use a hair dryer. You can also use the hair dryer like the opposite of spraying a coolant, except the coolant can be more precisely placed.

    If it crackles while it's running, you might have a high-voltage leak on the board. It is generally a good idea to blow the dust out anytime you take off the cover. A backwards vacuum cleaner is probably enough. If you have and use compressed air, do it from a distance. A close blast of compressed air can damage things. A small paint brush can dislodge accumulations. Sometimes just getting a clear view can let you see bad components. Heat damage, swollen components, leaking fluids, discolorations...

    Spraying a coolant is fairly safe, but just cool one component at a time so that you know what's what. Leaking some on to the circuit board is generally not a problem, just not in excess or on the flyback.

    If you don't recognize the flyback, it is either in a shielded box, probably about the size of a coffee cup, or is exposed and about the size and looks of a donut (is there a theme here?). It will also have a thick wire leading from a rubber plug on one of the side faces of the CRT. Mostly, just stay away from the wire, the flyback, and the components immediately adjacent.
     
  11. sixstringartist

    Thread Starter Member

    Apr 8, 2008
    18
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    yes bertus I appreciate those links. I found those about a week ago and have been rummaging through them since.


    As for the CRT, could I be lucky enough that blowing out the dust fixed the problem?

    Its been sitting on my desk for two days and haven't had a bit of trouble with it :/
     
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