Hi,

Over the past few days I've been analysing a problem in my Eizo T766 19" CRT
(Sony Trinitron tube), with this group's help. Unfortunately, I can conclude
nothing else besides that the red and blue gun occasionally short to something
at low potential, most likely the heater. I tried tapping it loose, but with
no success. So, now I'd like to collect as much information as possible about
zapping the short out.

I've seen suggested that you can use a neon transformer (or other kind of tesla
coil) for this: connect both pins of the heater to eachother, and connect the
transformer between it and the affected cathode (one at a time). This seems
rather dangerous to me; such arcing usually leaves everything blackened. If
this is a good approach, what voltage neon transformer should I look for?

Another method is a capacitor charged up to several hundred volts; start with a
few uF, then increase as desired.

I would like to know, based on people's experience, how much chance I have of
blowing out the cathode or filament. Bear in mind that it's an intermittant
short, that does not show up on the DMM when the tube is unpowered (not even
on the 200 MOhm range), so in the most positive situation, we're talking about
loose debris which needs to get out of the way, and not a dead short.

One last question: is the heater filament an exposed (or covered?) fragile
filament like that in light bulb, or is more robust like heating wire of an
electric stove?

Any help is appreciated, and thanks in advance.

Wiebe Cazemier

"Wiebe Cazemier" wrote in message
b.home.nl...
Hi,

Over the past few days I've been analysing a problem in my Eizo T766 19"
CRT
(Sony Trinitron tube), with this group's help. Unfortunately, I can
conclude
nothing else besides that the red and blue gun occasionally short to
something
at low potential, most likely the heater. I tried tapping it loose, but
with
no success. So, now I'd like to collect as much information as possible
about
zapping the short out.

I've seen suggested that you can use a neon transformer (or other kind of
tesla
coil) for this: connect both pins of the heater to eachother, and connect
the
transformer between it and the affected cathode (one at a time). This
seems
rather dangerous to me; such arcing usually leaves everything blackened.
If
this is a good approach, what voltage neon transformer should I look for?

Another method is a capacitor charged up to several hundred volts; start
with a
few uF, then increase as desired.

I would like to know, based on people's experience, how much chance I have
of
blowing out the cathode or filament. Bear in mind that it's an
intermittant
short, that does not show up on the DMM when the tube is unpowered (not
even
on the 200 MOhm range), so in the most positive situation, we're talking
about
loose debris which needs to get out of the way, and not a dead short.

One last question: is the heater filament an exposed (or covered?) fragile
filament like that in light bulb, or is more robust like heating wire of
an
electric stove?

Any help is appreciated, and thanks in advance.

Wiebe Cazemier

My B&K CRT analyser / restorer removed shorts by discharging a cap across
the affected electrodes. I seem to recall that g1-k shorts were actually
more common, due to 'burnt out' emissive material falling off the cathode
and lodging in the g1-k gap. The location of the short was indicated by neon
lamps, the intensity of the glow giving an indication of the resistance of
the path. When the zapping took place, I think that the CRT gun was being
run by the analyser as a basic triode. Some quite spectacular fireworks used
to occur in the neck, as the contaminating material was vapourised by the
discharge across it from the cap. It was very effective at removing
inter-electrode leakage paths. The B&K was a fine instrument in its day. I
lent mine to someone some years back, and it was never returned. I can't
even remember who it was to ...

CRT heaters are not particularly fragile, and are insulated from the cathode
cylinder, by a heat conducting coating. If you are discharging between h and
k, then the heater pins should be shorted together to avoid one of those
'unfortunate accidents' ... And of course, make sure that the base connector
is removed from the tube !!

Arfa

You can't clear a heater to cathode short, no matter what you do. No
commercial CRT restorer even attempts it. If you try to discharge a
current through the short, you will kill the filament. The only
solution is to provice a "floating" heater source that won't drag the
cathode low when the contact is made. The heater voltage will
actually rise to cathode voltage (above ground), but will remain 6.3
between the filament pins. One method is to buy an isolation
transformer from supply houses, which is basically a toroid with two
windings on it. The other is to manually wrap some wire around the
flyback core, and feed to the filament. In either case, the CRT board
must be modified to float the filament pins completely away from
ground. Very common procedure done at repair shops.

On Friday 02 May 2008 03:44, John-Del wrote:

You can't clear a heater to cathode short, no matter what you do. No
commercial CRT restorer even attempts it. If you try to discharge a
current through the short, you will kill the filament. The only
solution is to provice a "floating" heater source that won't drag the
cathode low when the contact is made. The heater voltage will
actually rise to cathode voltage (above ground), but will remain 6.3
between the filament pins. One method is to buy an isolation
transformer from supply houses, which is basically a toroid with two
windings on it. The other is to manually wrap some wire around the
flyback core, and feed to the filament. In either case, the CRT board
must be modified to float the filament pins completely away from
ground. Very common procedure done at repair shops.

Hmm, there seems to be some disagreement over whether a H-K short can be fixed.
Will the filament also be killed if you tie its pins together? That way, a
current path other than through the entire filament always exists.

Floating might work, but not in this case, because the problem exists in two
guns. Additionally, it would decrease image quality (at least when the short
exists; it might clear up when the short disappears when it's warm), and the
reason I want to fix this monitor is because of it's high quality; a quality
that not even other T766 models have/had.

Another thing; if I measure the filament voltage, it's 5V. Is that normal
deviation from the 6.3 which is common? And, when isolating, is it useful to
use some kind of regulation, like with a couple of diodes and an LM317, to
make sure the voltage is exactly what it should be?

On Friday 02 May 2008 02:01, Arfa Daily wrote:

CRT heaters are not particularly fragile, and are insulated from the cathode
cylinder, by a heat conducting coating.

The short that exists, could that be because of degraded insulation material,
or is external debris more likely?

On Fri, 02 May 2008 14:11:40 +0200, Wiebe Cazemier
wrote:

On Friday 02 May 2008 03:44, John-Del wrote:


Hmm, there seems to be some disagreement over whether a H-K short can be fixed.
Will the filament also be killed if you tie its pins together? That way, a
current path other than through the entire filament always exists.

Floating might work, but not in this case, because the problem exists in two
guns. Additionally, it would decrease image quality (at least when the short
exists; it might clear up when the short disappears when it's warm), and the
reason I want to fix this monitor is because of it's high quality; a quality
that not even other T766 models have/had.

Another thing; if I measure the filament voltage, it's 5V. Is that normal
deviation from the 6.3 which is common? And, when isolating, is it useful to
use some kind of regulation, like with a couple of diodes and an LM317, to
make sure the voltage is exactly what it should be?

Floating the heater in a monitor is not usually possible because they
run the heaters on a regulated DC supply instead of using AC from the
flyback. DC obviously won't work with a CRT heater isolation
transformer. I have seen other Sony monitor CRTs with 5v heaters, so
it may be fine. If your monitor does use AC heaters, then you need a
true RMS meter to measure this voltage.

I've seen more H-K shorts than anything else. K-G1 shorts can usually
be blown out, but H-K shorts are harder to remove reliably. The
problem with fixing a H-K short is that the heater is coated with
insulation. If this insulation cracks, then the heater will short to
the cathode. Attempting to blow out the short can blow the heater.
Also, even if the short goes away, the heater still has damaged
insulation and could short again at any time.

A CRT with two shorted guns is probably past hope. If you really want
to fix the monitor, start looking for a compatible replacement CRT.
Almost any 19" Trinitron monitor CRT should work.
Andy Cuffe

Floating the heater in a monitor is not usually possible because they
run the heaters on a regulated DC supply instead of using AC from the
flyback. DC obviously won't work with a CRT heater isolation
transformer. I have seen other Sony monitor CRTs with 5v heaters, so
it may be fine. If your monitor does use AC heaters, then you need a
true RMS meter to measure this voltage.




I did isolate the heater on a Sony monitor once years ago, it was a DC
heater but I disconnected the plug and then wound a couple turns of wire
around the flyback core and used that instead, it worked great for a day,
but the cathodes were shot so I added another turn and it was bright again
for an hour or so, then it got unusably dim again, added another turn and
the heater burned out. The picture did look fine while it worked though.

On Friday 02 May 2008 18:46, Andy Cuffe wrote:

A CRT with two shorted guns is probably past hope. If you really want
to fix the monitor, start looking for a compatible replacement CRT.
Almost any 19" Trinitron monitor CRT should work.

Also when the guns only short intermittently? And, when it's warmed up, the
problem is gone.

The problem with replacing the CRT, is that I'm almost completely sure that the
image quality will be lower. When I bought this monitor, it required three
trips to the service desk to have it calibrated properly. Out of the box, it
sucked. I've also seen other T766 models, which all sucked. If I am going to
replace the CRT, I'm going to let an Eizo qualified repair company do it,
because they can do a better job of it. Problem is, Sony doesn't manufacture
the tubes anymore, so it's possible it can't be repaired. And even if it can,
as I said, quality will probably be lower than what I'm used to.

"Wiebe Cazemier" wrote in message
b.home.nl...
On Friday 02 May 2008 18:46, Andy Cuffe wrote:

A CRT with two shorted guns is probably past hope. If you really want
to fix the monitor, start looking for a compatible replacement CRT.
Almost any 19" Trinitron monitor CRT should work.

Also when the guns only short intermittently? And, when it's warmed up,
the
problem is gone.

The problem with replacing the CRT, is that I'm almost completely sure
that the
image quality will be lower. When I bought this monitor, it required three
trips to the service desk to have it calibrated properly. Out of the box,
it
sucked. I've also seen other T766 models, which all sucked. If I am going
to
replace the CRT, I'm going to let an Eizo qualified repair company do it,
because they can do a better job of it. Problem is, Sony doesn't
manufacture
the tubes anymore, so it's possible it can't be repaired. And even if it
can,
as I said, quality will probably be lower than what I'm used to.

I would look for another monitor and get that calibrated if need be. CRT
monitors are cheap these days, plenty of really nice ones are getting
replaced by LCDs.

On Friday 02 May 2008 23:50, James Sweet wrote:

I would look for another monitor and get that calibrated if need be. CRT
monitors are cheap these days, plenty of really nice ones are getting
replaced by LCDs.

My experience with second-hand monitors is not too good. Also, the Eizo's on
the second hand market are all older models. On large Dutch advertising sites,
I can't find a single T766. Most of em are older shadowmask models, and it's
the trinitron I like.

Eizo states that they keep spare parts for, I believe, 5 years, but I don't
know if that includes the CRT. Some might call me nuts for investing money in
a CRT monitor, but the fact that to this day, I have yet to see a better
monitor, justifies me in it . The only problem is, how can I get that lazy
repair guy calibrate the monitor properly for me, especially when his shift
ends in 5 minutes, and not stuff in some old CRT they have lying around... On
their website they advertise with how well they repair monitors (warm up
period, calibrate of focus and colors, etc), but I've experienced it first
hand, it at one time, I got it returned with the colors calibrated to make
just about everything green... I wonder how they do such a sloppy calibration.

On Friday 02 May 2008 01:09, Wiebe Cazemier wrote:

Hi,

Over the past few days I've been analysing a problem in my Eizo T766 19" CRT
(Sony Trinitron tube), with this group's help. Unfortunately, I can conclude
nothing else besides that the red and blue gun occasionally short to
something at low potential, most likely the heater. I tried tapping it loose,
but with no success. So, now I'd like to collect as much information as
possible about zapping the short out.

I've seen suggested that you can use a neon transformer (or other kind of
tesla coil) for this: connect both pins of the heater to eachother, and
connect the transformer between it and the affected cathode (one at a time).
This seems rather dangerous to me; such arcing usually leaves everything
blackened. If this is a good approach, what voltage neon transformer should I
look for?

Another method is a capacitor charged up to several hundred volts; start with
a few uF, then increase as desired.

I would like to know, based on people's experience, how much chance I have of
blowing out the cathode or filament. Bear in mind that it's an intermittant
short, that does not show up on the DMM when the tube is unpowered (not even
on the 200 MOhm range), so in the most positive situation, we're talking
about loose debris which needs to get out of the way, and not a dead short.

One last question: is the heater filament an exposed (or covered?) fragile
filament like that in light bulb, or is more robust like heating wire of an
electric stove?

Any help is appreciated, and thanks in advance.

Wiebe Cazemier

Just to confirm something: the CRT repair FAQ says this about K-G1 shorts:

Cathode to control grid (K-G1). Since the G1 electrodes for all the guns are
connected together, this will affect not only the color of the guilty cathode
but the others as well. The result may be a very bright overloaded *negative*
picture with little, none, or messed up colors.

But [1] says that cathode to G1 can produce similar symptoms as heater-cathode
shorts. I ask, because I noticed that G1 is connected to ground on the CRT
socket board. I can imagine that one cathode can very well short to G1 without
the others being affected. Is that a right assement?

Can I disconnect G1 to test if it's a cathode-G1 short? There are two pins on
the CRT to which G1 connects (which I don't understand BTW).

And, what is the use of G1 being at ground potential...?

BTW, [1] also states a 25% success rate of blowing out cathode-heater shorts
with the flyback...

[1] http://www.thegleam.com/ke5fx/crt/sencrt.pdf.

On Saturday 03 May 2008 01:33, Wiebe Cazemier wrote:
Just to confirm something: the CRT repair FAQ says this about K-G1 shorts:

Cathode to control grid (K-G1). Since the G1 electrodes for all the guns are
connected together, this will affect not only the color of the guilty cathode
but the others as well. The result may be a very bright overloaded *negative*
picture with little, none, or messed up colors.

But [1] says that cathode to G1 can produce similar symptoms as
heater-cathode shorts. I ask, because I noticed that G1 is connected to
ground on the CRT socket board. I can imagine that one cathode can very well
short to G1 without the others being affected. Is that a right assement?

Can I disconnect G1 to test if it's a cathode-G1 short? There are two pins on
the CRT to which G1 connects (which I don't understand BTW).

And, what is the use of G1 being at ground potential...?

BTW, [1] also states a 25% success rate of blowing out cathode-heater shorts
with the flyback...

[1] http://www.thegleam.com/ke5fx/crt/sencrt.pdf.

I just did a test. In off state, I can't measure any short. But when I heat up
the heater with an external power supply, I can measure 1K resistance between
G1 and any of the cathodes (R, G and B). As the heater heats up, the
resistance decreases. The strange thing is, even tough they all read 1K, the
continuity meter only read complete short on red (that is, "0". The others
read "0.2"). I still couldn't measure anything between heater and cathodes.

So, as far as I can see, it's a cathode-G1 short after all (that is, that 1K
resistance should be there, right?). The only thing that stands in the way of
this conclusion, is the comment from the repair FAQ above.

Any comments on this are appreciated.

Have you tried running a degaussing coil over the shorted area (set
powered down). I recall someone telling me ages ago that he had
dislodged a wedged particle causing a short by doing so. Degauss and
tap at the same time maybe.
Jango.

On Saturday 03 May 2008 13:00, jango2 wrote:

Have you tried running a degaussing coil over the shorted area (set
powered down). I recall someone telling me ages ago that he had
dislodged a wedged particle causing a short by doing so. Degauss and
tap at the same time maybe.
Jango.

No, I haven't. But I also don't have one... And, I can remember reading
something about that being dangerous for trinitron tubes; or perhaps that was
only about using it near the aperature grill mask (since that's extra
fragile).

"Wiebe Cazemier" wrote in message
b.home.nl...
On Saturday 03 May 2008 01:33, Wiebe Cazemier wrote:
Just to confirm something: the CRT repair FAQ says this about K-G1
shorts:

Cathode to control grid (K-G1). Since the G1 electrodes for all the guns
are
connected together, this will affect not only the color of the guilty
cathode
but the others as well. The result may be a very bright overloaded
*negative*
picture with little, none, or messed up colors.

But [1] says that cathode to G1 can produce similar symptoms as
heater-cathode shorts. I ask, because I noticed that G1 is connected to
ground on the CRT socket board. I can imagine that one cathode can very
well
short to G1 without the others being affected. Is that a right assement?

Can I disconnect G1 to test if it's a cathode-G1 short? There are two
pins on
the CRT to which G1 connects (which I don't understand BTW).

And, what is the use of G1 being at ground potential...?

BTW, [1] also states a 25% success rate of blowing out cathode-heater
shorts
with the flyback...

[1] http://www.thegleam.com/ke5fx/crt/sencrt.pdf.

I just did a test. In off state, I can't measure any short. But when I
heat up
the heater with an external power supply, I can measure 1K resistance
between
G1 and any of the cathodes (R, G and B). As the heater heats up, the
resistance decreases. The strange thing is, even tough they all read 1K,
the
continuity meter only read complete short on red (that is, "0". The others
read "0.2"). I still couldn't measure anything between heater and
cathodes.

So, as far as I can see, it's a cathode-G1 short after all (that is, that
1K
resistance should be there, right?). The only thing that stands in the way
of
this conclusion, is the comment from the repair FAQ above.

Any comments on this are appreciated.

k to g1 is by far the most common leakage path as far as I recall - and I
have to say it's been a while since I have been into CRT based repairs at
this level, so what John said above about h-k shorts may well be true with
'modern' tubes, although I'm sure that I remember being able to remove h-k's
in some circumstances with my B&K 465. As far as what you are measuring,
there should be no reading between g1 and any cathode. If the grid
connection is indeed grounded, then the statement about all guns being
affected equally, is not valid in this case, and individual gun conditions
could be affected by individual shorts. What you have to remember, is that
it is irrelevant what *actual* potentials the individual electrodes are
connected to, as long as the correct *differentials* are maintained. Hence,
if the gun requires the grid to be say -40v to achieve cut-off, it doesn't
matter whether the cathode is at +40v and the grid at ground, or the grid at
+20v and the cathode at +60v, the differential of -40v is maintained, and
the bias conditions remain the same.

You questioned earlier what constituted a "firm rap". I really can't tell
you in terms of anything that you could take as 'definitive instructions'.
It's just one of those things that's a 'feel' that you gain over years of
doing it. When I was directly involved with CRT equipment repairs, it was
something you 'just did' on a daily basis, and you never actually broke a
neck as a result ... Let's say that if you rap your fingertips on the bench
just hard enough to get that sort of 'buzz' in them immediately after,
that's probably about the most force that you want to be hitting the neck
with. I would normally be using an old box spanner that I have, which has a
wooden handle, and is a bit 'kinder' to the glass. As far as putting the
monitor on its side or upside down when you do it, it's just a case of
propping or holding it in position whilst you do it. It sometimes allows
particles to dislodge under the effects of gravity coming at them from a
different direction.

Other than this, use the capacitor discharge method to try and vapourise the
particles. You might want to try this with the heater on, as you say that
the resistance of the short goes down as the heater warms up. This is
probably because the particle(s) causing the short get 'squeezed' tighter in
the k-g1 gap, as the cathode cylinder expands. A better contact between the
two electrodes, via the contamination, is likely to result in a higher
discharge current from the cap, and a better chance of vapourising the
material, rather than just dislodging it.

Arfa

On May 2, 8:11 am, Wiebe Cazemier wrote:


Hmm, there seems to be some disagreement over whether a H-K short can be fixed.
Will the filament also be killed if you tie its pins together? That way, a
current path other than through the entire filament always exists.

The current required to clear a short needs to pass through both
elements, the cathode and the heater. Even with the heater pins
shorted, current will pass from the cathode to at least a portion of
the heater, which is subject to fail at the point of contact.


Floating might work, but not in this case, because the problem exists in two
guns.

That's true. If you have contact between the filament and more than
one cathode, the tube is garbage.


Additionally, it would decrease image quality (at least when the short
exists; it might clear up when the short disappears when it's warm)

Not true. In the vast majority of tubes I've isolated over the
years, there had been no picture issues when the short occurs. The
only way that you could tell the short was still there was by
measuring the DC component of the filament above ground, and actually
watch it vary from virtually nothing to a couple hundred volts.


Another thing; if I measure the filament voltage, it's 5V. Is that normal
deviation from the 6.3 which is common? And, when isolating, is it useful to
use some kind of regulation, like with a couple of diodes and an LM317, to
make sure the voltage is exactly what it should be?


If the filament is AC supplied from the flyback, it's at the scan
rate, not 60hz. A true RMS meter is needed to accurately measure the
voltage. I use a Fluke 87 for measuring high frequency AC. Non RMS
DMMs usually read scan derived filament voltage below 5 volts. If the
filaments are DC and regulated, they will read exactly correct with
any DMM.

John

On Saturday 03 May 2008 14:04, Arfa Daily wrote:

k to g1 is by far the most common leakage path as far as I recall - and I
have to say it's been a while since I have been into CRT based repairs at
this level, so what John said above about h-k shorts may well be true with
'modern' tubes, although I'm sure that I remember being able to remove h-k's
in some circumstances with my B&K 465. As far as what you are measuring,
there should be no reading between g1 and any cathode. If the grid
connection is indeed grounded, then the statement about all guns being
affected equally, is not valid in this case, and individual gun conditions
could be affected by individual shorts. What you have to remember, is that
it is irrelevant what *actual* potentials the individual electrodes are
connected to, as long as the correct *differentials* are maintained. Hence,
if the gun requires the grid to be say -40v to achieve cut-off, it doesn't
matter whether the cathode is at +40v and the grid at ground, or the grid at
+20v and the cathode at +60v, the differential of -40v is maintained, and
the bias conditions remain the same.

You questioned earlier what constituted a "firm rap". I really can't tell
you in terms of anything that you could take as 'definitive instructions'.
It's just one of those things that's a 'feel' that you gain over years of
doing it. When I was directly involved with CRT equipment repairs, it was
something you 'just did' on a daily basis, and you never actually broke a
neck as a result ... Let's say that if you rap your fingertips on the bench
just hard enough to get that sort of 'buzz' in them immediately after,
that's probably about the most force that you want to be hitting the neck
with. I would normally be using an old box spanner that I have, which has a
wooden handle, and is a bit 'kinder' to the glass. As far as putting the
monitor on its side or upside down when you do it, it's just a case of
propping or holding it in position whilst you do it. It sometimes allows
particles to dislodge under the effects of gravity coming at them from a
different direction.

Other than this, use the capacitor discharge method to try and vapourise the
particles. You might want to try this with the heater on, as you say that
the resistance of the short goes down as the heater warms up. This is
probably because the particle(s) causing the short get 'squeezed' tighter in
the k-g1 gap, as the cathode cylinder expands. A better contact between the
two electrodes, via the contamination, is likely to result in a higher
discharge current from the cap, and a better chance of vapourising the
material, rather than just dislodging it.

Arfa

Thanks for your elaborate response.

I just had a revelation today, which also sheds light not only on this issue,
but also with earlier issues I had with this monitor; that being that it
turned brighter and brighter over time, resulting in a very washed out picture
with visible retrace lines, when at default settings. The OSD menu allowed me
to decrease cut-off and gain, but at some point that wasn't enough anymore,
and I needed to decrease G2 value on the flyback. And now I think I know
what's going on.

I think a leakage path between the cathodes and G1 has slowly been forming,
which slowly increased beam current over the years, and therefore brightness.
Now, I'm at a point where there is not just leakage, but a dead short now and
again. Tapping therefore, will not solve the issue.

Another problem I had, was that the automatic color calibrator didn't correct
for the washed out picture. And I suspect that is because the cathodes pulled
the extra current directly from ground, and therefore the circuit which
measures the beam current was oblivious to it. However, this is just
speculation.

I think I will need a proper CRT rejuvenator to clear the leakage path between
cathode and G1. I could do it myself, by using the G2 voltage for example, but
googling reveals that modern CRTs are too fragile for this, and that
professional rejuvenators are matched precisely for modern tubes.

It also appears that cathode-G1 shorts are one of the easier things to fix, so
I think I can still avoid buying a TFT . The only thing is, that it would be
better to let a professional servicer do it, but I dread the likely outcome of
it coming back with a calibration that is way off.

On Sunday 04 May 2008 02:12, John-Del wrote:
If the filament is AC supplied from the flyback, it's at the scan
rate, not 60hz. A true RMS meter is needed to accurately measure the
voltage. I use a Fluke 87 for measuring high frequency AC. Non RMS
DMMs usually read scan derived filament voltage below 5 volts. If the
filaments are DC and regulated, they will read exactly correct with
any DMM.

It's not really relevant anymore, because I think my problem is not H-K after
all, but K-G1 (see my other post, about my "revelation" ), but I have a
scope, so I could test it. But a DC test on the supply yields 4.9V, and an AC
test 0, so I think it's safe to say it's 5V DC.

On Sunday 04 May 2008 02:54, Wiebe Cazemier wrote:
Thanks for your elaborate response.

I just had a revelation today, which also sheds light not only on this issue,
but also with earlier issues I had with this monitor; that being that it
turned brighter and brighter over time, resulting in a very washed out
picture with visible retrace lines, when at default settings. The OSD menu
allowed me to decrease cut-off and gain, but at some point that wasn't enough
anymore, and I needed to decrease G2 value on the flyback. And now I think I
know what's going on.

I think a leakage path between the cathodes and G1 has slowly been forming,
which slowly increased beam current over the years, and therefore brightness.
Now, I'm at a point where there is not just leakage, but a dead short now and
again. Tapping therefore, will not solve the issue.

Another problem I had, was that the automatic color calibrator didn't correct
for the washed out picture. And I suspect that is because the cathodes pulled
the extra current directly from ground, and therefore the circuit which
measures the beam current was oblivious to it. However, this is just
speculation.

I think I will need a proper CRT rejuvenator to clear the leakage path
between cathode and G1. I could do it myself, by using the G2 voltage for
example, but googling reveals that modern CRTs are too fragile for this, and
that professional rejuvenators are matched precisely for modern tubes.

It also appears that cathode-G1 shorts are one of the easier things to fix,
so I think I can still avoid buying a TFT . The only thing is, that it
would be better to let a professional servicer do it, but I dread the likely
outcome of it coming back with a calibration that is way off.

One more thing. I may just decide to try to fix it myself (I was planning to
use the capacitor discharge method, starting with a few uF at about 230V or
so). But I have a question:

Most pin-outs of CRTs I've seen, contain only one G1 pin. But the G1 connection
on this neck board connects to two pins on the CRT. And, with the socket
removed, I can also measure 0 Ohms resistance between those pins. Is it normal
that there are two G1 pins? When zapping, should I connect them together?

"Wiebe Cazemier" wrote in message
b.home.nl...
On Sunday 04 May 2008 02:54, Wiebe Cazemier wrote:
Thanks for your elaborate response.

I just had a revelation today, which also sheds light not only on this
issue,
but also with earlier issues I had with this monitor; that being that it
turned brighter and brighter over time, resulting in a very washed out
picture with visible retrace lines, when at default settings. The OSD
menu
allowed me to decrease cut-off and gain, but at some point that wasn't
enough
anymore, and I needed to decrease G2 value on the flyback. And now I
think I
know what's going on.

I think a leakage path between the cathodes and G1 has slowly been
forming,
which slowly increased beam current over the years, and therefore
brightness.
Now, I'm at a point where there is not just leakage, but a dead short now
and
again. Tapping therefore, will not solve the issue.

Another problem I had, was that the automatic color calibrator didn't
correct
for the washed out picture. And I suspect that is because the cathodes
pulled
the extra current directly from ground, and therefore the circuit which
measures the beam current was oblivious to it. However, this is just
speculation.

I think I will need a proper CRT rejuvenator to clear the leakage path
between cathode and G1. I could do it myself, by using the G2 voltage for
example, but googling reveals that modern CRTs are too fragile for this,
and
that professional rejuvenators are matched precisely for modern tubes.

It also appears that cathode-G1 shorts are one of the easier things to
fix,
so I think I can still avoid buying a TFT . The only thing is, that it
would be better to let a professional servicer do it, but I dread the
likely
outcome of it coming back with a calibration that is way off.

One more thing. I may just decide to try to fix it myself (I was planning
to
use the capacitor discharge method, starting with a few uF at about 230V
or
so). But I have a question:

Most pin-outs of CRTs I've seen, contain only one G1 pin. But the G1
connection
on this neck board connects to two pins on the CRT. And, with the socket
removed, I can also measure 0 Ohms resistance between those pins. Is it
normal
that there are two G1 pins? When zapping, should I connect them together?

Probably for no other reason than there being a 'spare' pin. If they read
zero to one another, then it's pretty safe to assume that they are both
connected to the same place. Can you not find a data sheet for the tube on
the 'net somewhere, or a schematic for the whole monitor ?
www.eserviceinfo.com might be a good place to look for one. As far as your
calibration fears go, I would suggest that when these shorts are removed, it
is going to want a good set-up anyway ...

Arfa

On Sunday 04 May 2008 19:19, Arfa Daily wrote:

Probably for no other reason than there being a 'spare' pin. If they read
zero to one another, then it's pretty safe to assume that they are both
connected to the same place. Can you not find a data sheet for the tube on
the 'net somewhere, or a schematic for the whole monitor ?
www.eserviceinfo.com might be a good place to look for one.

I can't seem to find one. But I think it is indeed just a spare pin, especially
because the neck board also connects them together.

As far as your
calibration fears go, I would suggest that when these shorts are removed, it
is going to want a good set-up anyway ...

What I meant was, that most repair guys aren't really interested in delivering
a perfect monitor. They just want their paycheck and be done with it. I have
experience with the repair service in question (the only company in The
Netherlands that is authorized by Eizo), and I know what shabby calibration
they can do. On the other hand, I can't do any at all (the correct way), so
for that matter...

BTW, I found something on impregnated cathodes, which is what Sony Trinitron
tubes use:

Impregnated cathodes are much more robust. They can be applied at a higher
cutoff voltage and thus deliver a smaller spot without premature wear. They
are
more sensitive to a too high heater temperature, however, because they are
operated at a higher temperature to begin with. They do evaporate more metal
during their lifetime. At one time there was fear that they would deposit too
much metal on the glass around the electrodes, leading to leakage currents.
These can cause drift of focus and screen voltages and can disturb the cutoff
current measurement. Those can influence the picture too.

I guess this depositing is what's been happenning. Can someone tell me if the
cap-discharge method, or professional rejuvenation, is perhaps particularly
dangerous? (just trying to rule out damaging this monitor...)

On Sunday 04 May 2008 19:51, Wiebe Cazemier wrote:

I guess this depositing is what's been happenning. Can someone tell me if the
cap-discharge method, or professional rejuvenation, is perhaps particularly
dangerous? (just trying to rule out damaging this monitor...)

I meant, for this type of metal build-up?

Later on in the repair FAQ, it says this:

Second, an impregnated cathode, being highly conductive compared to an
oxide cathode which is a semi-conductor, can handle a much higher peak
current since the cathode material is not locally heated up by this peak
current. An oxide cathode can be destroyed by a too high peak beam current!

So, I guess with impregnated cathodes I run less risk of burning them out.

I'm going to build my own K-G1 zapper. I thought I'd put my idea up for
comments here.

- Get 325Vdc by rectifying 230Vac mains (provided by 1:1 isolation tranny).
- Charge 2.2 uF up to that DC potential.
- Discharge through two 400 Ohm resistors in series (to limit current to 400
mA. The repair FAQ has a comment in it, saying that if you need more than 400
mA, the cathode is usually toast. Two resistors because one resistor has a max
voltage of 250V. I found that Sencore's CRT regenerators use a current
limiting resistor [1], but I don't know which value (or which value cap).
- Increase value of cap step by step should the short not go away.

[1] Also says that it turns the heater off before attempting the discharge, to
protect the cathode. Is this really a problem, seeing as how the short does
not register without the heater on in my case?

Comments about resistor and cap value are most particularly requested.

Thanks in advance, as always


[1] http://www.sencore.com/newsletter/Oc...RejuveCRTs.htm

Wiebe Cazemier wrote:
Hi,

Over the past few days I've been analysing a problem in my Eizo T766 19" CRT
(Sony Trinitron tube), with this group's help. Unfortunately, I can conclude
nothing else besides that the red and blue gun occasionally short to something
at low potential, most likely the heater. I tried tapping it loose, but with
no success. So, now I'd like to collect as much information as possible about
zapping the short out.

I've seen suggested that you can use a neon transformer (or other kind of tesla
coil) for this: connect both pins of the heater to eachother, and connect the
transformer between it and the affected cathode (one at a time). This seems
rather dangerous to me; such arcing usually leaves everything blackened. If
this is a good approach, what voltage neon transformer should I look for?

Another method is a capacitor charged up to several hundred volts; start with a
few uF, then increase as desired.

I would like to know, based on people's experience, how much chance I have of
blowing out the cathode or filament. Bear in mind that it's an intermittant
short, that does not show up on the DMM when the tube is unpowered (not even
on the 200 MOhm range), so in the most positive situation, we're talking about
loose debris which needs to get out of the way, and not a dead short.

One last question: is the heater filament an exposed (or covered?) fragile
filament like that in light bulb, or is more robust like heating wire of an
electric stove?

Any help is appreciated, and thanks in advance.

Wiebe Cazemier

I've had mixed results with a stun-gun. Lotsa volts, but not enough current
to break much. I just arc'd it between all combinations of pins until
I got bored with the light show. No idea where the short was, but it
worked afterwards. YMMV.

--
Return address is VALID!
Bunch-O-Stuff Forsale He
http://mike.liveline.de/sale.html

On Monday 05 May 2008 16:42, mike wrote:

I've had mixed results with a stun-gun. Lotsa volts, but not enough current
to break much. I just arc'd it between all combinations of pins until
I got bored with the light show. No idea where the short was, but it
worked afterwards. YMMV.


Haha, I like that . Using any kind of tesla coil would do about the same, I
guess. I'm working on constructing a device which gives me a current limited
jolt at 325V (described in another post in this thread). Let's see how that
works out. 350V is the amount a Sencore rejuvenator uses, so that seems pretty
safe to me.

But about your stungun method: if it worked afterwards, what are the mixed
results you speak of?

BTW, I've always wondered, what does an arc look like in a vacuum. In air, the
air ionizes, giving the thunderbolt like arc. But in a vacuum, this can't
happen. So, will the arcing be invisible when the short has been completely
blown out?