"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 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 02:54, 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