Intermittent fault in Eizo 19" T766 CRT

[Wiebe Cazemier]

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?

 

[Wiebe Cazemier]

[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.

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.

 

[Wiebe Cazemier]

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?

 

[bz]

[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.

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.




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

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[Wiebe Cazemier]

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.

 

[James Sweet]

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.

 

[Wiebe Cazemier]

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.

 

[Wiebe Cazemier]

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...

 

[Wiebe Cazemier]

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...

 

[James Sweet]

Whoops, I meant, with the blue gun disconnected...

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.

 

[Wiebe Cazemier]

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...

 

[Wiebe Cazemier]

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?

 

[Jamie]

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?


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"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"

 

[Wiebe Cazemier]

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?

 

[James Sweet]

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.

 

[Sjouke Burry]

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.

 

[Wiebe Cazemier]

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.

 

[Wiebe Cazemier]

Because some are DC.

Indeed. I just measured, and it has 5 V DC on it. No AC component exists.

 

[Wiebe Cazemier]

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?

 

[Wiebe Cazemier]

I tried the tapping a bit more, and now the red gun also has this problem... I
didn't tap that hard I think (from the description "not enough to break it,
but close" I would think I need to tap a lot harder).

Now my question is: are those cathodes/filaments so fragile that I could have
damaged them, or is it possible I dislodged dirt which now also affects the
red gun?

The filaments were cold BTW, when I tried this. I suppose warm filaments are
more easily damaged.