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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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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 |
#2
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message b.home.nl... 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 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
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Intermittent fault in Eizo 19" T766 CRT
On Friday 25 April 2008 18:38, James Sweet wrote:
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. 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
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message .home.nl... On Friday 25 April 2008 18:38, James Sweet wrote: 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. 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. 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
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Intermittent fault in Eizo 19" T766 CRT
E"James Sweet" wrote in
news:90vQj.50$5X.10@trndny08: 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. 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
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Intermittent fault in Eizo 19" T766 CRT
"bz" wrote in message 98.139... E"James Sweet" wrote in news:90vQj.50$5X.10@trndny08: 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. 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. 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
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Intermittent fault in Eizo 19" T766 CRT
On Saturday 26 April 2008 04:25, bz wrote:
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. 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
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Intermittent fault in Eizo 19" T766 CRT
On Saturday 26 April 2008 02:51, James Sweet wrote:
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. So discharge through the cathode assembly won't happen? 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. 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
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
b.home.nl: On Saturday 26 April 2008 04:25, bz wrote: 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. 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? 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
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
b.home.nl: On Saturday 26 April 2008 02:51, James Sweet wrote: 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. So discharge through the cathode assembly won't happen? 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. 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. 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
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Intermittent fault in Eizo 19" T766 CRT
On Saturday 26 April 2008 13:21, bz wrote:
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. 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? Intermittents are difficult to localize and fix. Tell me about it... This fault only happens when it's just turned on, thus far, and very hard to reproduce. Perhaps you can narrow down the problem by monitoring voltages at various points while it is acting up. 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
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
.home.nl: On Saturday 26 April 2008 13:21, bz wrote: 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. 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? Intermittents are difficult to localize and fix. Tell me about it... This fault only happens when it's just turned on, thus far, and very hard to reproduce. So, what is different about 'just turned on'? Components are cool. High transient currents occur. Look for places that those can make a difference. Perhaps you can narrow down the problem by monitoring voltages at various points while it is acting up. 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... 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
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Intermittent fault in Eizo 19" T766 CRT
On Saturday 26 April 2008 19:40, bz wrote:
So, what is different about 'just turned on'? Components are cool. High transient currents occur. Look for places that those can make a difference. (snip) 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. 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
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
b.home.nl: On Saturday 26 April 2008 19:40, bz wrote: So, what is different about 'just turned on'? Components are cool. High transient currents occur. Look for places that those can make a difference. (snip) 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. 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. 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. 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 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
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message b.home.nl... On Saturday 26 April 2008 19:40, bz wrote: So, what is different about 'just turned on'? Components are cool. High transient currents occur. Look for places that those can make a difference. (snip) 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. 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 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
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 20:52, James Sweet wrote:
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. 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
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 15:14, bz wrote:
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. 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. 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. 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? Good luck with your experiment. Hopefully, it won't drop something into the center of the shadow mask. Aperture grill mask, actually 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. 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
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 21:04, Wiebe Cazemier wrote:
On Sunday 27 April 2008 20:52, James Sweet wrote: 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. 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)? 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
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
b.home.nl: On Sunday 27 April 2008 15:14, bz wrote: 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. 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. 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. I assume that would connect between chassis ground and cathode? 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. As I mentioned in my other post, I thought there were three line wires, but I can only discern one; is that the one? [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] And the cathode line capacitor BTW, where is that likely to be located? Near the flyback on the mainboard? Sorry, no idea. But I think you are mistaking which capacitor is going to be used to clear the short. Good luck with your experiment. Hopefully, it won't drop something into the center of the shadow mask. Aperture grill mask, actually Correct. 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. 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... 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
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message .home.nl... On Sunday 27 April 2008 21:04, Wiebe Cazemier wrote: On Sunday 27 April 2008 20:52, James Sweet wrote: 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. 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)? 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? 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. |
#21
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 23:47, James Sweet wrote:
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. I agree that I need to see schematics, or other sorts of documentation. As I stated earlier, I'm reasonably well versed in audio electronics (built a HiFi system from scratch), but CRTs are uncharted waters for me. Anyway, about reproducing. I haven't been able to consistenty reproduce it. Only about two maybe three times did the image seem to respond to me tapping the neckboard or neck. Not enough to be sure. And about disconnecting a gun; is that safe for the CRT? The repair FAQ warns against running the CRT without the neckboard connected. Is having the other guns connected enough protection against unintentional discharge, through the glass or whatever? |
#22
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 23:23, bz wrote:
[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] Ah, that explains it. And the cathode line capacitor BTW, where is that likely to be located? Near the flyback on the mainboard? Sorry, no idea. But I think you are mistaking which capacitor is going to be used to clear the short. The question about the line capacitor was unrelated. It had to do with another minor issue of this monitor (it slightly getting brighter over time, ever since I got it), of which was suggested it could be this cap which was degrading. While we were on the subject, I just thought I'd ask. |
#23
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Intermittent fault in Eizo 19" T766 CRT
On Sunday 27 April 2008 23:47, James Sweet wrote:
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. Taking what bz said into account, that the Trinitron tube has a single electron gun, doesn't this mean that if the screen turns fully blue (and not red and green as well, meaning white), including retrace lines, that the short can exist nowhere except inside the tube? |
#24
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote in
b.home.nl: On Sunday 27 April 2008 23:23, bz wrote: [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] Ah, that explains it. Three guns means three filaments, three cathodes, and three sets of focusing electrodes. One gun means that the three cathodes all use the same focusing electrodes. And the cathode line capacitor BTW, where is that likely to be located? Near the flyback on the mainboard? Sorry, no idea. But I think you are mistaking which capacitor is going to be used to clear the short. The question about the line capacitor was unrelated. It had to do with another minor issue of this monitor (it slightly getting brighter over time, ever since I got it), of which was suggested it could be this cap which was degrading. While we were on the subject, I just thought I'd ask. Ah. I see. I would expect a cathode bypass cap to be near the cathode. On the other hand a cap involved in driving a cathode with a signal might be near the video output. Sorry I can't help more. I remember quite a long discussion somewhere about 'monitor too bright', google may find it for you. If I remember correctly, it was a panasonic monitor being discussed and two fixes were mentioned. One involved going through the setup menu and the other putting a resistor in parallel with another resisitor. This may or may not be of any help to you. -- 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 |
#25
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Intermittent fault in Eizo 19" T766 CRT
On Monday 28 April 2008 01:07, bz wrote:
The question about the line capacitor was unrelated. It had to do with another minor issue of this monitor (it slightly getting brighter over time, ever since I got it), of which was suggested it could be this cap which was degrading. While we were on the subject, I just thought I'd ask. Ah. I see. I would expect a cathode bypass cap to be near the cathode. On the other hand a cap involved in driving a cathode with a signal might be near the video output. Sorry I can't help more. I remember quite a long discussion somewhere about 'monitor too bright', google may find it for you. If I remember correctly, it was a panasonic monitor being discussed and two fixes were mentioned. One involved going through the setup menu and the other putting a resistor in parallel with another resisitor. This may or may not be of any help to you. You were also involved in the discussion about my monitor getting brighter. But, it's no issue at all. A while ago, I turned down the G2 voltage and ran the automatic color calibration and everything was perfect again. Except for this intermittant fault issue, that is. I suppose it's got nothing to do with it, but still I think it's best to ask whether turning down the G2 voltage could have induced the failure I'm dealing with now? I turned it down only a very tiny bit. So little that it was hard to make such a subtle adjustment. |
#26
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message b.home.nl... On Sunday 27 April 2008 23:47, James Sweet wrote: 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. Taking what bz said into account, that the Trinitron tube has a single electron gun, doesn't this mean that if the screen turns fully blue (and not red and green as well, meaning white), including retrace lines, that the short can exist nowhere except inside the tube? The Trinitron tube has a "unified" electron gun, it's more marketing than anything else, but in a nutshell it uses the same focus and accelerating electrodes for all three cathodes. There are still three separate guns in effect, just bundled into one tidy assembly. Electrically it is no different than the three separate gun assemblies used in other CRTs. You will not damage anything by disconnecting a cathode, the accelerating anodes, grids, focus, etc will still be active. |
#27
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Intermittent fault in Eizo 19" T766 CRT
On Monday 28 April 2008 03:06, James Sweet wrote:
The Trinitron tube has a "unified" electron gun, it's more marketing than anything else, but in a nutshell it uses the same focus and accelerating electrodes for all three cathodes. There are still three separate guns in effect, just bundled into one tidy assembly. Electrically it is no different than the three separate gun assemblies used in other CRTs. You will not damage anything by disconnecting a cathode, the accelerating anodes, grids, focus, etc will still be active. OK, thanks. I will have a look soon if I can desolder the blue gun and see if the screen still flashes blue. But first, I will look up all the parts after the (blue) cathode driver transistor and use a strong magnifier glass to look for broken solder joins and such. |
#28
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Intermittent fault in Eizo 19" T766 CRT
On Monday 28 April 2008 03:23, Wiebe Cazemier wrote:
OK, thanks. I will have a look soon if I can desolder the blue gun and see if the screen still flashes blue. But first, I will look up all the parts after the (blue) cathode driver transistor and use a strong magnifier glass to look for broken solder joins and such. I have the monitor running now without the blue gun connected. The fault persists. When I turned it on, the screen flashed blue on and off like a stroboscope. I was able to reproduce it by tapping the neckboard. Now that it's warm, the fault is gone again. There is one difference. Before, the screen would contract and expand and eventually turn off when it did this, probably because the CRT driver (an LM2413 BTW) was shorted to ground through the blue cathode. Anyway, the fault seems to be in the tube, in the blue gun. I will try the shaking method a bit more, and hopefully I don't have to take any drastic measures. BTW, I read somewhere in one of the sci.repair faqs that you have to tap the neck quite hard: "not enough to break it, but close". I don't really dare to do that... |
#29
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Intermittent fault in Eizo 19" T766 CRT
On Monday 28 April 2008 14:11, Wiebe Cazemier wrote:
I have the monitor running now without the blue gun connected. The fault persists. When I turned it on, the screen flashed blue on and off like a stroboscope. I was able to reproduce it by tapping the neckboard. Now that it's warm, the fault is gone again. Whoops, I meant, with the blue gun disconnected... There is one difference. Before, the screen would contract and expand and eventually turn off when it did this, probably because the CRT driver (an LM2413 BTW) was shorted to ground through the blue cathode. Anyway, the fault seems to be in the tube, in the blue gun. I will try the shaking method a bit more, and hopefully I don't have to take any drastic measures. BTW, I read somewhere in one of the sci.repair faqs that you have to tap the neck quite hard: "not enough to break it, but close". I don't really dare to do that... |
#30
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message b.home.nl... On Monday 28 April 2008 14:11, Wiebe Cazemier wrote: I have the monitor running now without the blue gun connected. The fault persists. When I turned it on, the screen flashed blue on and off like a stroboscope. I was able to reproduce it by tapping the neckboard. Now that it's warm, the fault is gone again. Whoops, I meant, with the blue gun disconnected... There is one difference. Before, the screen would contract and expand and eventually turn off when it did this, probably because the CRT driver (an LM2413 BTW) was shorted to ground through the blue cathode. Anyway, the fault seems to be in the tube, in the blue gun. I will try the shaking method a bit more, and hopefully I don't have to take any drastic measures. BTW, I read somewhere in one of the sci.repair faqs that you have to tap the neck quite hard: "not enough to break it, but close". I don't really dare to do that... Yep, sounds like it really is the tube. If it's a heater-cathode short, you could isolate the heater, powering it with 2 or 3 turns of wire around the flyback core. Careful not to overdo it, it really only takes a couple of turns on these high frequency transformers. |
#31
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Intermittent fault in Eizo 19" T766 CRT
On Monday 28 April 2008 19:00, James Sweet wrote:
Yep, sounds like it really is the tube. If it's a heater-cathode short, you could isolate the heater, powering it with 2 or 3 turns of wire around the flyback core. Careful not to overdo it, it really only takes a couple of turns on these high frequency transformers. Indeed, I read about that. The downside is, that image quality will suffer, because the cathode will/can be in contact with those wires, which have stray capacitance, and noise pickup, etc. Additionally, I don't know if the Trinitrons feature of automatic color calibration touches the filament current, but if so, that will render that feature useless, and it's a feature I love about this monitor. Do you know if it does so? But I will try the shake/tap method a bit more first. The short is every so slight, that even after 20 seconds of warming up, the fault is almost completely gone and hardly reproduceable. I wonder if it also does it when it's upside-down... |
#32
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Intermittent fault in Eizo 19" T766 CRT
On Tuesday 29 April 2008 00:53, Jamie wrote:
why not use a isolation xformer for the heaters? I also read about those, but didn't investigate. It's basically the same idea as winding an extra turn on the flyback, right? Do you simply place it between the original filament power supply+gnd and filament? How does it affect image quality, also taking into account the eventual possible situation that the blue cathode will be in contact with the heater permanently? |
#33
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote:
On Monday 28 April 2008 19:00, James Sweet wrote: Yep, sounds like it really is the tube. If it's a heater-cathode short, you could isolate the heater, powering it with 2 or 3 turns of wire around the flyback core. Careful not to overdo it, it really only takes a couple of turns on these high frequency transformers. Indeed, I read about that. The downside is, that image quality will suffer, because the cathode will/can be in contact with those wires, which have stray capacitance, and noise pickup, etc. Additionally, I don't know if the Trinitrons feature of automatic color calibration touches the filament current, but if so, that will render that feature useless, and it's a feature I love about this monitor. Do you know if it does so? But I will try the shake/tap method a bit more first. The short is every so slight, that even after 20 seconds of warming up, the fault is almost completely gone and hardly reproduceable. I wonder if it also does it when it's upside-down... why not use a isolation xformer for the heaters? -- "I'd rather have a bottle in front of me than a frontal lobotomy" "Daily Thought: SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS. http://webpages.charter.net/jamie_5" |
#34
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Intermittent fault in Eizo 19" T766 CRT
On Tuesday 29 April 2008 00:51, Wiebe Cazemier wrote:
On Tuesday 29 April 2008 00:53, Jamie wrote: why not use a isolation xformer for the heaters? I also read about those, but didn't investigate. It's basically the same idea as winding an extra turn on the flyback, right? Do you simply place it between the original filament power supply+gnd and filament? How does it affect image quality, also taking into account the eventual possible situation that the blue cathode will be in contact with the heater permanently? One more question about them: the cathode signal is not DC, so won't current from the cathode driver still find it's way to ground through the transformer? |
#35
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Intermittent fault in Eizo 19" T766 CRT
"Wiebe Cazemier" wrote in message .home.nl... On Tuesday 29 April 2008 00:51, Wiebe Cazemier wrote: On Tuesday 29 April 2008 00:53, Jamie wrote: why not use a isolation xformer for the heaters? I also read about those, but didn't investigate. It's basically the same idea as winding an extra turn on the flyback, right? Do you simply place it between the original filament power supply+gnd and filament? How does it affect image quality, also taking into account the eventual possible situation that the blue cathode will be in contact with the heater permanently? One more question about them: the cathode signal is not DC, so won't current from the cathode driver still find it's way to ground through the transformer? No, it doesn't work like that, the transformer isolates it. If the primary was dead shorted, it might have some effect on it, but the video signal is in the MHz range, the transformer will be around 30KHz. |
#36
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Intermittent fault in Eizo 19" T766 CRT
Wiebe Cazemier wrote:
On Monday 28 April 2008 03:23, Wiebe Cazemier wrote: OK, thanks. I will have a look soon if I can desolder the blue gun and see if the screen still flashes blue. But first, I will look up all the parts after the (blue) cathode driver transistor and use a strong magnifier glass to look for broken solder joins and such. I have the monitor running now without the blue gun connected. The fault persists. When I turned it on, the screen flashed blue on and off like a stroboscope. I was able to reproduce it by tapping the neckboard. Now that it's warm, the fault is gone again. There is one difference. Before, the screen would contract and expand and eventually turn off when it did this, probably because the CRT driver (an LM2413 BTW) was shorted to ground through the blue cathode. Anyway, the fault seems to be in the tube, in the blue gun. I will try the shaking method a bit more, and hopefully I don't have to take any drastic measures. BTW, I read somewhere in one of the sci.repair faqs that you have to tap the neck quite hard: "not enough to break it, but close". I don't really dare to do that... As a last(distasteful)resort you could discharge a cap (~1000mfd 10-50volt)between pins with an intermittent short in the blue gun. |
#37
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Intermittent fault in Eizo 19" T766 CRT
Jamie wrote: why not use a isolation xformer for the heaters? Because some are DC. -- http://improve-usenet.org/index.html Use any search engine other than Google till they stop polluting USENET with porn and junk commercial SPAM If you have broadband, your ISP may have a NNTP news server included in your account: http://www.usenettools.net/ISP.htm |
#38
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Intermittent fault in Eizo 19" T766 CRT
On Tuesday 29 April 2008 04:06, Sjouke Burry wrote:
As a last(distasteful)resort you could discharge a cap (~1000mfd 10-50volt)between pins with an intermittent short in the blue gun. I've decided not to try this. It's dangerous as it is, but especially for heater-cathode shorts, because you often also blow out the filament. |
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Intermittent fault in Eizo 19" T766 CRT
On Tuesday 29 April 2008 05:29, Michael A. Terrell wrote:
Jamie wrote: why not use a isolation xformer for the heaters? Because some are DC. Indeed. I just measured, and it has 5 V DC on it. No AC component exists. |
#40
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Intermittent fault in Eizo 19" T766 CRT
On Tuesday 29 April 2008 01:15, James Sweet wrote:
No, it doesn't work like that, the transformer isolates it. If the primary was dead shorted, it might have some effect on it, but the video signal is in the MHz range, the transformer will be around 30KHz. How sensitive are the filaments to voltage difference? I just measured that they are powered by 5V DC. Will those two windings also give me 5V RMS? Additionally, how do I wind around the flyback; there's a plastic assembly around it and all. I was thinking. Can't I tap some power from some AC source with a small isolation transformer, and construct a small circuit to give me 5V DC? Or is that over engineering? |
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