Blowing out a cathode-G1 short, the extreme way

I may be going in over my head here but...

You've run 3A of current through this short, and it's still there?

I wouldn't think the flyback circuit would be able to deliver even 3mA, let alone 3A, and certainly not for 10 seconds.

I'd definitely want other opinions first but my thought is that you're going to have to throw current at this problem - if 3A isn't enough, use more - and just hope the short burns out before anything that's actually necessary for the CRT to work. I would suggest precautions against damage to the envelope of the tube, possibly up to implosion. (probably not likely but with dangerous consequences if it does happen)

Still, 25000V of the flyback can ionize air and form an arc, 3A at 4V can't do that... I hope 25000V can ionize the short as well.

I think high voltage low current is less destructive to the elements inside the tube than low voltage high current. I could run more amps through the short, but I kind of have the feeling it won't last... The cathodes may be impregnated, and therefore able to handle more current, but still... Perhaps I've already damaged them.

BTW, I was thinking. When using the Tesla coil method, can one even think in terms of connecting wires to the CRT pins? The 25000V will arc between the connectors anyway. The FAQ speaks of "running the coil around the pins". Does that simply mean: connecting Tesla coil ground to G1 and move the anode near the pins, so an arc will form between any pin that has a connection with G1?

You know, like this:

Wiebe Cazemier wrote in news:1b384$48341d0e$d4cc82be$ snipped-for-privacy@cache5.tilbu.nb.home.nl:

I think you will need a LARGEr value capacitor, charged up with plenty of coulombs of electrons.

Keep your wires thick and short.

With the dead short, you don't need high voltage, you need high current but only for a short time. You want to try to blow out the weakest point in the circuit and to hope it is the recently created short.

It is too late for the Tesla coil. That might have been useful when there was not a dead short.

--
bz    	73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an 
infinite set.

bz+ser@ch100-5.chem.lsu.edu   remove ch100-5 to avoid spam trap

What about using a stun-gun or taser?

I guess the effect is the same, but I have a spare monitor with flyback, I don't have stunguns lying around...

That tube is a goner. If you managed to heat the internal wires up red hot, you've welded whatever bits were shorting together. You could try charging up a large electrolytic to a couple hundred volts via rectified mains and dumping that into there, but something other than the short will probably act as a fuse.

I just tried that, of my own accord, and I think the weakest point in the circuit was the internal connection to the cathodes... The shorts are gone, but it doesn't display an image anymore. That is, it displays a blueish grey image, with visible retrace lines (somewhat blurry). And when I turn it off, there still is this characteristic contraction of the image, which ends in a blue, red and green spot in the middle.

Agreed! Time to either search for a new CRT for the item or off the original to Video Display Corp to have it re-gunned. Either way it probably will not be cost effective to do so.

Regunned? Is that possible?

I already called the repair service. It's going to cost me ? 500 to replace the CRT... The irony is, if I can be sure I don't get it back with all kinds of convergence errors and such (which a lot of T766's suffered from), I would be willing to spend it, to avoid getting TFT for a while longer.

Don't worry, everything I did was with a disconnected CRT socket. Furthermore, testing with the CRT on was with the driver disconnected, so it didn't short out.

Wiebe Cazemier wrote in news:d595a$48355326$d4cc82be $ snipped-for-privacy@cache5.tilbu.nb.home.nl:

Google for CRT repairs

-- bz

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an infinite set.

bz+ snipped-for-privacy@ch100-5.chem.lsu.edu remove ch100-5 to avoid spam trap

You'd pay $500 for a CRT monitor?

(And to think I gave away two 19" and one 21" a few months ago!)

You have no control over the voltage/current ratio. E/I=R, where R is the resistance of the short. (I'm sure much less than one ohm.)

If, somehow, you were able to find a supply big enough to actually deliver

25,000 volts across this short, it *would* draw 25,000 amps. Or much more.

(and I'd be quite confident it would clear the short. Probably very spectacularly.)

A flyback power supply won't do this. Its internal resistance is way too high. If it worked at all when connected to this short, its output would be limited to WAY less than 25,000 volts. Probably to way less than *one* volt.

There are lot of 19" monitors available for almost nothing. Mostly shadowmask types, which I dislike. A popular site among "geeks" in the Netherlands here, is tweakers.net. Their second hand offering doesn't include a single T766 or

109P model, both of which use trinitron(-like) tubes. Other second-hand sites do include 109P's and older Eizo models, but not newer Eizo models like the T766's, or T966 (21" model). And the 109P didn't meet with my demands, because of severe convergence problems (I tried about 8 of them...)

The point is, you can get a lot of average CRT's for almost nothing, but the quality that my T766 offered is hard to get. One reason is that Eizo has very strict rules when it comes to the quality of the CRTs they use; they select the best ones from manufacturing batches.

And to top it off, 500 EU is 783.85 USD... Think of it this way. I will absolutely refuse any TN TFT panel (the "standard" kind), and preferably I want an IPS panel if I am forced to buy a TFT. But then I need to come up with at least ? 1000, and still have a black level I can never be satisfied with. And IPS has slower response than TN.

If you need a few amps of current to blow out a direct short, get a good

5 Amp or heavier 12 volt supply. Get enough capacitors that you can put in parallel to build up at least 60,000 uF or more. Put a 3 or 4 ohm 10 Watt resistor in series with the caps to the power supply. This way, when switching on to charge the caps, you don't blow the "@ss" off the power supply.

After about 20 to 30 seconds or so, connect your clip leads directly from the caps to whatever you want to blow the short out of.

If the above does not work, and you want to feel like a really weird mad scientist (I am serious), get a few fully charged 120 Amp automobile batteries. Connect them in parallel. The peak full load output should be more than 1200 Amps. If this does not blow out the short, then you know that probably nothing will.

With a few automobile batteries in parallel the peak output can exceed

14 kW. Just take care that you don't injure yourself or start an electrical fire with this!

--

JANA _____

Because all else failed, I want to go to the extreme in blowing out the cathode-G1 short, from which my Eizo T766 19" CRT suffers. After the capacitor discharge, the short has gone from 1K ohm, to 0.0 Ohm, and I can make the internal wiring of the tube red hot by running 3A of current through it...

The repair FAQ mentions using a Tesla coil. The nearest thing to a Tesla coil I used, was the 750V G2 of another (scrap) monitor, but that can only be sustained for a fraction of a second before the monitor shuts off; nowhere near the 10 seconds the repair FAQ states.

My question is this: can I use the anode and ground wire of another monitor to act as my Tesla coil? How do I prevent breakdown of air and/or vacuum around the CRT pins and such (I don't want arcing in the wrong place). And, very importantly, can the flyback sustain such an arc? I've seen videos on the internet in which they use the anode of a monitor to create a sustainable arc between anode and probably ground, but I don't know if it needs to be modified for that (because an arc is basically a short).

I am aware of the dangers BTW; when I do this, I will take proper safety measures.

Regards,

Wiebe Cazemier

I had already tried this, but in an elegant way. I charged up a 4700 uF cap with my current limited supply to 30V and used that to blow out the short. The short is gone, but so is the connection to the cathodes. Bye bye tube...

That's because you've now effectively welded the elements of the gun assy together.

It's not going to help. There's no way a home Tesla coil is going to develop anywhere near 3A (you wouldn't want to be near it anyway). With the short now reading zero ohms, the only way to eliminate the short is more current. The problem is is that once you run enough current into the tube to eliminate the short, one of the elements (or the internal connecting wires) will fail before the short will, and the tube will be left with zero output.

Sorry.