Discharging a CRT - easily!
 

Hi All,

I saw a computer tech ages ago pull out a "biro" from his toolbox. Nothing special there, but it had been modified to have a wire with an aligator clip coming out the back of the pen, and a straightened out paper clip used as a probe coming out of the writing end. The aligator clip attaches to the aquadag strap or ground and you can safely slip the paper clip probe under the anode cap to discharge the tube.

The best thing about this though is that he used a high value resistor inside the pen between wire and probe that he'd probably taken from the HT section of a colour TV. (4.7Meg or so) This means that the CRT discharges "gently", rather than a big spark and for some reason, it will NOT recover some charge after a few hours or so! How many here have forgotten about recovery and been zapped slightly?

Best of all, it costs nothing to make! :D

Please move this post if it's not relevant here.

Good idea. And I think I'd leave the short tool connected while working around the tube! No chance whatsoever to recover charge.

I always just used a 10" screwdriver with a 1/8" flat blade, and a clip lead to GND. Some charge may have been left. I never gave it much thought and always re-discharged a CRT before moving it after being exposed to that little spark a few times.

Maybe a "sciency" person can tell us why the tube won't regain any charge when discharging it with a high resistance rather than a short? I have to agree with opcom, always better to play it safe, that spark hurts! (I'd hate to drop a CRT when pulling away from the anode cap quickly by reflex action!) :sigh:

Never heard about a discharge through a resistor preventing recovery, and not sure why that should be true. Interesting, though, as I have been bitten by recovered charge before.

I never discharge through a resistor. I like being able to hear that pop, as it's verification to me that it DID discharge.

That's not to say it's possible to discharge and have no pop, but if I ran the set not too long ago, I like that sound just to be sure.

The charge will partially recover after the first 'pop'. :eek: A few more 'pops' of diminishing intensity over the next hour or so is usually sufficient to drain the charge. Or else just leave the bleeder or shorting clip in place for that length of time.

Yup, What he said!

I use my HV probe. Slow discharge, an the meter lets me know it's discharging. After that, a shorting wire to the dag for a while to suck up the few electrons that still want to move around.

Interesting when discharging through a resistor. You certainly don't get the pop, but there is definitely a reaction. You know that static feeling and accompanying crackle sound if you get too close to the face of an operating CRT? It's like that, but your hair doesn't try to stand on end!

I definitely recommend you try this, as I've never once been zapped by recovery charge, I'd really love to know why this is!

But all methods work ultimately, and that's the main thing. :thmbsp:

Funny thing about high voltage: one Saturday morning years ago it was raining. I had the double garage door up and was standing inside the garage looking at the rain. Then, slowly, in spite of the humidity, I felt my hair start to stand up, hair on my arms tingle and stand up, and my ears go pop..pop and then WHAM, lightning hit the CB antenna of the guy across the street.

Yipes! When that happens, I think it's time to crouch down with your feet close together.

As the bleeder draws down the positive charge, it is in effect going negative, causing the CRT face to exhibit a negative charge. That's the crackly, staticky effect you hear. It's the inverse of the positive-going charge on the CRT face when the HV comes up.

So I'm not discharging the CRT safely. I'll need a bleeder resistor screwdriver.

Nuthin' wrong with the way you're doing it. I was just 'splainin' why the crackly sound during the bleed-down.

I agree with old_coot88, no probs with the way you're doing it, but this way seems to prevent recovery and the paper clip is nice n' easy to slip under the anode cap. :thmbsp:

Does anyone know what kind of current goes through the screwdriver when a fully charged CRT discharges? Using my HV meter only draws around 40uA, because the meter impedence is about 800 megohm. Can the tube be damaged by the spike of current from a direct short? I doubt it can, but I don't know so I have to ask.

My college professor once warned me in the middle of a lecture to "never; under any circumstances" use a screwdriver to discharge stored current. I later told him about all of the television repair literature that I had read in my life which stated to use a screwdriver, but he stuck with his guns and said to always use a resistor and that a screwdriver was a safety hazard. I still use a screwdriver; I just never use an old one with a wooden handle like I did once when I was a kid! ZZZAPP!!

TVs are designed to take the occasional CRT arc without killing anything, so the energy in one discharge is not going to destroy anything unless it's a bad design. There is usually a series resistor in the cathode circuit to limit arc current. The professor's rule not to use a screwdriver is a good general one and definitley applies to items with an unknown and possible large amount of stored energy; so, if you didn't *know* discharging a CRT with one is safe, it would be better to use a resistor.

The Prof's sage advice is probably more applicable to charged electrolytic caps than pic tubes.

I'm a screwdriver and alligator jumper guy....And after about a half-dozen bad recovery zaps from my first roundy(I was beginning to think the thing was possessed), and after getting real mad I formulated my current SOP of leaving the alligator clip lead on indefinitly(I defy any CRT to recover while that lead is on).

I guess this would be a good time to ask what I should be doing with an electrostatic CRT. Having no Aquadag on the outside, and no accessible HV line, does it need to be discharged?
- Winky

I've never tried on my Moto VT-71....I just unplug the tube and avoid touching the pins(doubt they hold a charge but I don't want to learn the hard way LOL).