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Intermittent fault in Eizo 19" T766 CRT

Discussion in 'Electronic Repair' started by Wiebe Cazemier, Apr 25, 2008.

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  1. Hi,

    My 4 or 5 year old Eizo T766 19" CRT began showing an intermittant fault
    yesterday. Sometimes the screen contracts a bit and comes back with wrong
    colors. An example is that the screen turns very blue, and you can see the
    diagonal vertical retrace lines in blue running over the screen (at least, I
    assume they are the retrace lines). When the screen snaps back, it's often too
    greenish, but turns normal over the course of a few minutes.

    I had it open yesterday, and a couple of times, I could reproduce the fault by
    touching the cathode assembly, so I thought it was a physical error, like a
    loose contact. And just now, when it did it again, the monitor responded to me
    hitting it (gently...). But, I couldn't reproduce it consistently, yesterday
    or now.

    I have a couple of questions.

    1) A couple of times, the fault caused the monitor to go into off mode. At
    least, the power led turned yellow, which it normally does in offmode. Is it
    likely that when the fault is somewhere in a late stage as the cathode
    assembly, that the control electronics is aware of it?

    2) Should I want to refit the plugs connected to the cathode assembly (three
    big wires; are those the RGB line voltage wires?), I would need to discharge
    the CRT. I've seen a lot of videos of people just sticking a screwdriver
    connected to the metal chassis under the anode plug, but is that safe? I've
    never discharged a CRT before, so I'm kind of apprehensive.

    3) How vital is it to discharge the power supply caps? If so, how does one do
    that?

    Any other insight is welcome, of course.

    Thanks in advance,

    Wiebe Cazemier
     
  2. James Sweet

    James Sweet Guest

    I would suspect bad solder joints in the area where you can induce the fault
    by tapping. It *may* be a short inside the CRT, but don't condemn that yet.
    Don't worry about discharging the capacitors, just unplug it for 10 minutes
    or so and it should be safe. Even when you're sure everything is dead, treat
    it as live just in case.
     
  3. And what about the CRT? Is discharging as I described safe?

    When the fault is in the CRT, I assume nothing can be done? Well, let's just
    hope it's not that; those Sony Trinitron tubes are pretty good, so I guess the
    chance is small. But on the other hand, because after a fault, the screen is
    greenish, which turns OK over the course of about 30-60 minutes, heating
    cathodes may very well be it. The cathode assembly itself doesn't heat up that
    fast.
     
  4. James Sweet

    James Sweet Guest



    You shouldn't have to discharge the CRT, that's only necessary if you have
    to remove the anode lead, even then it usually discharges itself through the
    focus divider on color monitors.

    The 22" flat trinitron tubes commonly have intermittant focus issues, mine
    acts up occasionally but has behaved pretty well lately. Haven't run into
    that with the 19" tubes.
     
  5. bz

    bz Guest

    Caution: even if you discharge the CRT, if the anode lead is disconnected,
    as the dielectric (glass in this case) 'relaxes from the stresses of being
    charged', a charge can build back up.

    I can remember drawing a nice arc many minutes after discharging the tube
    the first time.

    This can be a bit of a surprise, especially if you are removing the CRT to
    replace it. Shocking, one might say.

    Also, the comment about letting it sit for 10 minutes so all the capacitors
    will be discharged is good, assuming that there are no open bleeder
    resistors, etc. But, it might NOT be good to 'bet your life' on the
    bleeders doing their job. Occasionally, they do open, so your caution to
    treat them as if they might be 'live' is a good one for those that want a
    long life.

    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  6. James Sweet

    James Sweet Guest


    Yes if you disconnect the anode, you should definitely discharge it again
    before you touch it, I usually leave a clip lead connecting it to the ground
    strap so that doesn't happen. The zap won't hurt you, but it's enough of a
    surprise to drop the thing or cut your hand open. In this case he shouldn't
    have to disconnect the anode though.
     
  7. OK, but since I will need to solder and/or refit plugs, I can't consider it to
    be 'live' at all time. Will it be sufficient to measure the voltage between
    the RGB line wires, for example, and ground, to give me an idea of whether
    it's safe?
     
  8. So discharge through the cathode assembly won't happen?
    Focus issues are not my problem. When there is a fault, the screen flashes a
    couple of times, contracts, expands (like an old TV) and sometimes it shuts
    itself off.
     
  9. bz

    bz Guest

    Yes.




    --
    bz

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  10. bz

    bz Guest

    Sounds like you might have an intermittent short to ground on a power
    supply line, perhaps inside a bad capacitor.
    Or an intermittent connection in the voltage sensing circuits, or the AC
    line into the power supply.

    Intermittents are difficult to localize and fix.

    Perhaps you can narrow down the problem by monitoring voltages at various
    points while it is acting up.



    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  11. The problem here is, that I'm not too familiar with CRT screen electronics. I'm
    pretty well versed in audio electronics and other more common, low voltage,
    stuff, but I've never really had to deal with CRTs, so I don't know too much
    about them, or how to localize the circuits you mentioned (except perhaps the
    PSU).

    Would you happen to know of a good book/guide about CRT repair, or at least an
    explanation of the insides?
    Tell me about it... This fault only happens when it's just turned on, thus far,
    and very hard to reproduce.
    I also have a transient recorder, although only a single channel. Still, it
    should be useful. However, come to think of it, the voltages inside the
    monitor are probably too high for it...
     
  12. bz

    bz Guest

    So, what is different about 'just turned on'?
    Components are cool.
    High transient currents occur.

    Look for places that those can make a difference.
    From the symptoms, I would start by looking at the PSU outputs.
    Troubleshoot by 'divide and conquer'.

    Divide the device logically into two halves, and localize the problem to
    one of the two.

    Continue until you reach the bad part.






    --
    bz 73 de N5BZ k

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  13. I've been reading the repair faq at http://www.repairfaq.org/sam/crtfaq.htm. At
    some point it says this:

    Occasionally, small conductive flakes or whiskers present since the day of
    manufacture manage to make their way into a location where they short out
    adjacent elements in the CRT electron guns. Symptoms may be intermittent or
    only show up when the TV or monitor is cold or warm or in-between. Some
    possible locations are listed below:

    One of them is: Heater to cathode (H-K). The cathode for the affected gun will
    be pulled to the heater (filament) bias voltage - most often 0 V (signal
    ground). In this case, one color will be full on with retrace lines. Where the
    heater is biased at some other voltage, other symptoms are possible like
    reduced brightness and/or contrast for that color. This is probably the most
    common location for a short to occur.

    That appears to be exactly what's happening. The blue gun is turned on
    completely, also showing the retrace lines. Also, remember that I have been
    able to reproduce the fault a couple of times by tapping the cathode assembly.

    I will try the "put the monitor on it's face and try tapping it to dislodge the
    short" method first. If it works, it would be a very low tech, but effective
    solution :)

    The "blow out the short with a capacitor" method also seems fun, but also a
    little risky :)
     
  14. bz

    bz Guest

    If you are SURE that no vibrations from your tapping traveled elsewhere,
    that is a strong clue.
    Otherwise it can be a misleading clue.

    I have played the 'taps' game and won. I have played it and lost.
    It is best when you can make smaller and smaller taps while getting closer
    and closer to the problem.

    If the tap will only occasionally trigger/fix the problem, then it is easy
    to be misled.
    When I had a TV repair shop, in the early 70's, we had a tester for picture
    tubes.
    It had a short indicator and a 'remove short' button that discharged a
    capacitor through the short.
    Sometimes it would fix things. Sometimes it would make things worse.

    Good luck with your experiment. Hopefully, it won't drop something into the
    center of the shadow mask.

    Be aware that with 15 lb of air pressing on each square inch of the CRT,
    you are playing with a live bomb.

    I once took a picture tube out, laid it on its faceplate, got about 50 feet
    away and tossed rocks at it.

    Woomp. Dust and dirt and glass flew everywhere.

    Chunks of the face plate (glass about 1 1/2 inches thick) landed about 50
    feet BEHIND me.

    Now, I would want a thick sheet of lexan between me and any CRT that I was
    tapping upon, and safety goggles, gloves.




    --
    bz

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  15. James Sweet

    James Sweet Guest

    Don't be so quick to condemn the CRT, sure it could be at fault, but
    eliminate the external stuff first! A bad connection on the neck board can
    just as easily cause the same symptom.
     
  16. But won't a bad connection give me a black screen, because when the cathode is
    disconnected from it's line, no current can flow. How can the blue cathode be
    shorted to ground on the neck board? I can't see anything (obvious) on the
    board that could cause a short.

    BTW, I thought the RGB guns had separate line wires, but besides the focus and
    grid voltage wires, only one extra thick wire runs to the neck board. That has
    to be the cathode line voltage, right? And if so, doesn't that mean that when
    it's shorted to ground, all the guns are fully turned on, and not just the
    blue one (as is the case here)?
     
  17. I know what you mean. Just yesterday, I had another CRT open, which often has
    one or more of it's colors failing. You can fix it by smashing it, but when I
    had it open, I could fix it by very gently touching the neck board. In that
    case, it was clear where the fault was. In the case with this Eizo, I can tap
    the neck board a lot harder without it doing anything, so you're right in that
    it could very well be something else.
    I assume that would connect between chassis ground and cathode? As I mentioned
    in my other post, I thought there were three line wires, but I can only
    discern one; is that the one?

    And the cathode line capacitor BTW, where is that likely to be located? Near
    the flyback on the mainboard?
    Aperture grill mask, actually :)
    I was very careful, because I really didn't want to break anything. But anyway,
    I wasn't able to fix the problem. I'm looking at the screen right now, and as
    I'm typing this, it just flashed again. I don't know if I can or should tap
    harder, but I wouldn't feel too comfortable trying it.

    It's really annoying that I can't reproduce the fault reliably... Even if it is
    a short between the blue gun and ground, I couldn't even short it out with a
    cap, because the problem only every occurs for a fraction of second, very
    sporadically...
     
  18. Wait, I've been thinking. An intermittent short in a capacitor between the blue
    cathode line voltage and ground could of course also cause this (and there are
    some of them, ceramics, on the neck board). But one question about that, which
    I guess also applies to when the short is inside the CRT:

    Provided there is one cathode driving voltage for all guns, why aren't all guns
    fully turned on when one of the cathodes is shorted to ground? Because if one
    is shorted to ground, there is no more line voltage for the other guns either,
    right?
     
  19. bz

    bz Guest

    If I remember correctly, the short was located via the earlier testing,
    rotating a switch to localize the short.
    You would then charge up a capacitor that was built into the tester and
    then discharge the capacitor through the short via a pushbutton switch.
    [http://en.wikipedia.org/wiki/Trinitron]
    Another feature was a three cathode single electron gun arrangement, in
    contrast to the then-dominant (and still common) three gun arrangement.
    Single gun systems tend to be easier to manufacture reliably, simplify beam
    focus and control, and are less prone to inter-electrode short circuits.
    [unquote]

    Sorry, no idea. But I think you are mistaking which capacitor is going to
    be used to clear the short.


    Intermittents are very problematic.



    --
    bz

    please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
    infinite set.

    remove ch100-5 to avoid spam trap
     
  20. James Sweet

    James Sweet Guest




    It appears you need to look at the schematic for a CRT monitor or TV to get
    a better understanding of how this works. A disconnected cathode will float,
    you have to pull it down towards ground to turn it on. The transistors
    driving the cathodes isolate them from one another.


    It depends on where the bad connection is, and the design of the cathode
    driver circuits. It's also possible for one of the driver transistors to
    have an intermittant short. I would desolder the blue cathode pin on the CRT
    socket and tap the neck with the monitor on. If you can still repro the
    symptoms, then the problem is probably in the tube.
     
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