Intermittent fault in Eizo 19" T766 CRT

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

Reply to
Wiebe Cazemier
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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.

Reply to
James Sweet

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.

Reply to
Wiebe Cazemier

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.

Reply to
James Sweet

E"James Sweet" wrote in news:90vQj.50$5X.10@trndny08:

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.

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bz    	73 de N5BZ k

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bz

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.

Reply to
James Sweet

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?

Reply to
Wiebe Cazemier

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.

Reply to
Wiebe Cazemier

Wiebe Cazemier wrote in news:a1268$48130a2f$d4cc82be$ snipped-for-privacy@cache1.tilbu.nb.home.nl:

Yes.

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Wiebe Cazemier wrote in news:b2c19$48130a86$d4cc82be$ snipped-for-privacy@cache1.tilbu.nb.home.nl:

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.

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bz    	73 de N5BZ k

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bz

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...

Reply to
Wiebe Cazemier

Wiebe Cazemier wrote in news:fcc4$48135a13$d4cc82be$ snipped-for-privacy@cache3.tilbu.nb.home.nl:

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.

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bz

I've been reading the repair faq at

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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 :)

Reply to
Wiebe Cazemier

Wiebe Cazemier wrote in news:dba16$48146d43$d4cc82be$ snipped-for-privacy@cache6.tilbu.nb.home.nl:

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.

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bz

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bz

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.

Reply to
James Sweet

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)?

Reply to
Wiebe Cazemier

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...

Reply to
Wiebe Cazemier

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?

Reply to
Wiebe Cazemier

Wiebe Cazemier wrote in news:911f0$4814d036$d4cc82be$ snipped-for-privacy@cache4.tilbu.nb.home.nl:

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.

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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.

Correct.

Intermittents are very problematic.

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bz

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bz

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.

Reply to
James Sweet

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