Re: Burning out an intermittant heater-cathode short in a CRT

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.
.

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