Burning out an intermittant heater-cathode short in a CRT

[Wiebe Cazemier]

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...)

 

[Wiebe Cazemier]

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.

 

[Wiebe Cazemier]

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/Oct03/RejuveCRTs_files/RejuveCRTs.htm

 

[mike]

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.

 

[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.

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?