Burning out an intermittant heater-cathode short in a CRT

Wiebe Cazemier · 2008-05-01T23:09:54+00:00

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 concludenothing else besides that the red and blue gun occasionally short to somethingat low potential, most likely the heater. I tried tapping it loose, but withno success. So, now I'd like to collect as much information as possible aboutzapping the short out.I've seen suggested that you can use a neon transformer (or other kind of teslacoil) for this: connect both pins of the heater to eachother, and connect thetransformer between it and the affected cathode (one at a time). This seemsrather dangerous to me; such arcing usually leaves everything blackened. Ifthis 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 afew uF, then increase as desired.I would like to know, based on people's experience, how much chance I have ofblowing out the cathode or filament. Bear in mind that it's an intermittantshort, that does not show up on the DMM when the tube is unpowered (not evenon the 200 MOhm range), so in the most positive situation, we're talking aboutloose 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?) fragilefilament like that in light bulb, or is more robust like heating wire of anelectric stove?Any help is appreciated, and thanks in advance.Wiebe Cazemier

May 01, 2008 24 Replies

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Wiebe Cazemier 18 years ago

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.

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:

are

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...)

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Wiebe Cazemier 18 years ago

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

Later on in the repair FAQ, it says this:

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

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Wiebe Cazemier 18 years ago

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]

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mike 18 years ago

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.

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Wiebe Cazemier 18 years ago

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?

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