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

"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.

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.

"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.

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.

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"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.

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?

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.

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.

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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

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...

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

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

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

"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.

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)?

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...

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?

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

"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.

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?

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.

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?

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

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.

"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.

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.

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...

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...

"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.

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...

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?

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"

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?

"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.

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.

Jamie wrote:

why not use a isolation xformer for the heaters?


Because some are DC.



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Use any search engine other than Google till they stop polluting USENET
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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.

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.

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?