Strange CRT Short

[Chris F.]

Set is a '95 Zenith, forget the model but the CRT is an A68ACT00X. The
original CRT was completely shot, but I had another one in an old 1987
Zenith console. Remarkably, it was a perfect match. I thought I was in luck
to have found a replacement so easily, but I'd soon find out otherwise. Once
installed, it seemed to work fine - for a few minutes. Then the picture
turned into a super-bright greenish color with retrace lines. It only seems
to happen after the set warms up. Much troubleshooting later, it didn't seem
to look like any of the usual shorts (H-K, K-G1, etc). Then I got the idea
to pull the CRT board off while the set was running, after the problem
appeared. I was quite surprised to see a steady arc (not a spark) of about
1" jump from pin 11 (blue cathode) of the tube to the corresponding pin on
the socket. This only happened after the set had warmed up and problem
appeared, otherwise there was no arc. I powered the set again, waited until
it went bright greenish-white, and this time pulled the CRT board and
immediately stuck a HV meter probe to the area around pin 11. The meter
jumped to nearly 20kV, and stayed there for several seconds before slowly
fading away.
What kind of short is this anyway, and is there any chance it can be
removed?
In a perfect world, there would be no Zenith CRTs.......
Thanks for any advice.

 

[JURB6006]

When you remove the CRT socket in that matter you're simply getting the
conduction of the tube. Then it dies out because you have removed the filament
supply. You could have a simple G1-K or H-K short and this would happen.

In fact even on a good tube, if it's strong and has hard video drive (bright)
this will still happen. This is because once you remove the socket you are
doing nothing to turn off the tube by properly biasing the grid off, or more
appropriately, raising the cathode voltage to bias it off. (with respect to the
grid)

I don't recommend removing the CRT socket while the set is running for those
reasons. Find some other way to isolate it, like unsoldering something. What's
more when you unplug the socket running you cause quite a bit of static
electricity, which is not our friend if you know what I mean. Might cost you a
chip doing that one day.

There are many test methods for those without expensive CRT testers. In fact
I've had to devise some of these for those wierd shorts in Zeniths. Firstest
and bestest is to interrupt the cathode in question. Also isolating the G1, but
having it clipleaded together so you can determine if it's an H-K or a G1-K
short. H-K is easy to isolate, a G1 short needs to be blown out. This
frequently destroys the cathode or clogs the grid unless you use just the right
current, voltage and the right R-C discharge time constant.

The only time you need to float the G1 for these tests is so you know which
element the K is shorted to, and even then you need to use a 100K resistor, if
it burns up you <u>know</u> it's a G1 short.

Find out and we'll go from there.

JURB

 

[Andre]

When you remove the CRT socket in that matter you're simply getting the
conduction of the tube. Then it dies out because you have removed the filament
supply. You could have a simple G1-K or H-K short and this would happen.

In fact even on a good tube, if it's strong and has hard video drive (bright)
this will still happen. This is because once you remove the socket you are
doing nothing to turn off the tube by properly biasing the grid off, or more
appropriately, raising the cathode voltage to bias it off. (with respect to the
grid)

Once saw that problem on a games monitor. I attached a DIY mini HV
generator to all the pins (similar to a Tesla Coil) and Gnd to G.
That fixed it. You could tell it had worked because you saw a nice
load of little purple arcs jumping around inside the tube then they
stopped.

(found this tip on usenet)

-A

 

[Chris F.]

I tried disconnecting the green cathode from the circuit (by cutting a
trace on the CRT board) but the symptom still appeared. Also tried isolating
the filament supply (by making my own winding on the flyback) but this
didn't help either. Even tried the "remove G1 shorts" function on my Sencore
CR-70. What's more, when the screen goes super bright, it seems to be all
three colors (though the green is somewhat more prevalent).
I seem to remember some video problems in the old console, from which I
got this CRT. The console, a digital system 3, had the special circuits
which would adjust the CRT voltages automatically. I remember it going
bright sometimes but soon going back to normal, probably just the circuit
adjusting to compensate for the intermittent short. I junked the console due
to tuner problems, and the fact that console sets are just not profitable
anymore.....

 

[H. R. Bob Hofmann]

I tried disconnecting the green cathode from the circuit (by cutting a
trace on the CRT board) but the symptom still appeared. Also tried isolating
the filament supply (by making my own winding on the flyback) but this
didn't help either. Even tried the "remove G1 shorts" function on my Sencore
CR-70. What's more, when the screen goes super bright, it seems to be all
three colors (though the green is somewhat more prevalent).
I seem to remember some video problems in the old console, from which I
got this CRT. The console, a digital system 3, had the special circuits
which would adjust the CRT voltages automatically. I remember it going
bright sometimes but soon going back to normal, probably just the circuit
adjusting to compensate for the intermittent short. I junked the console due
to tuner problems, and the fact that console sets are just not profitable
anymore.....

I think you may be barking up the wrong tree. If there is an internal
open to G1, that gun will pretty much run wide open and give the
screen the color of whichever G1 has gone open. A good rap on the
neck of the tube, after it is on, but before the trouble shows up
might do something to change things, either for better or worse. Just
not so hard that the glass breaks!!!

H. R. (Bob) Hofmann

 

[Jason D.]

Snip.

Snip


I think you may be barking up the wrong tree. If there is an internal
open to G1, that gun will pretty much run wide open and give the
screen the color of whichever G1 has gone open. A good rap on the
neck of the tube, after it is on, but before the trouble shows up
might do something to change things, either for better or worse. Just
not so hard that the glass breaks!!!

H. R. (Bob) Hofmann

What that?

G1 screen is basically a metal disk pierced with three small holes for
electrons to pass through emitted by the cathodes/heaters. G1's disk
spacings between cathodes and the G2 is only less than 1mm apart. If
G1 open then all guns is wide open?

G1 is ususally grounded or pulled below by negative supply (rare).

Cheers,

Wizard

 

[JURB6006]

When it shorts you should turn the G2 down quickly, as you bring it back up
slowly you will see one color first. This will almost unequivocably be the gun
in which the short exists.

Once this is determined, you have several options. One is to ground the grid,
wait until it shorts then apply some current source to the cathode, this could
be the 220V video output supply, but frequently it doesn't have enough current
to do it.

With the cathode floating and the set running check the resistance (voltage
should be gone if it's shorted) if it's in the hundreds, you might want to use
a 100/200 cap charged to about 160V. If it's 3 ohms for example a 4700/35 might
be better, or something in between. The trick is to apply the current when the
short is active, this is where even the $2000 Sencore falls down. The CR7000
actually lets it cool down, then usually it's not shorted anymore.

Another course of action would be to go ahead and simply rejuvinate the
affected gun with a Sencore or a B & K. Like a barber "just burn a little off
the " whatever.

It usually works unless the tube has a gas problem, and those problems are not
usually confined to one gun. They might be in the beginning though, the weakest
gun will have the different voltage on it and the construction of the grid
itself also come into play. A gas problem could as easilty affect the strongest
gun first,

 

[H. R. Bob Hofmann]

When it shorts you should turn the G2 down quickly, as you bring it back up
slowly you will see one color first. This will almost unequivocably be the gun
in which the short exists.

Once this is determined, you have several options. One is to ground the grid,
wait until it shorts then apply some current source to the cathode, this could
be the 220V video output supply, but frequently it doesn't have enough current
to do it.

With the cathode floating and the set running check the resistance (voltage
should be gone if it's shorted) if it's in the hundreds, you might want to use
a 100/200 cap charged to about 160V. If it's 3 ohms for example a 4700/35 might
be better, or something in between. The trick is to apply the current when the
short is active, this is where even the $2000 Sencore falls down. The CR7000
actually lets it cool down, then usually it's not shorted anymore.

Another course of action would be to go ahead and simply rejuvinate the
affected gun with a Sencore or a B & K. Like a barber "just burn a little off
the " whatever.

It usually works unless the tube has a gas problem, and those problems are not
usually confined to one gun. They might be in the beginning though, the weakest
gun will have the different voltage on it and the construction of the grid
itself also come into play. A gas problem could as easilty affect the strongest
gun first,

But a gassy tube would not come on suddenly, it would be present all
the time. The capacitor discharge/rejuvenation technique is worth a
try. The poster is right, it must be done with the tube filaments
hot, even a little higher than normal voltage is frequently used in
commercial rejuvenators. My experience has been that rejuvenation
only lasts a few months, but it is always worth a try if it is your
own set and not something that a customer will bring back in a few
months with a complaint.

H. R. (Bob) Hofmann