Discharging a CRT - easily!

[Down Under]

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!

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

[Reece]

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

[Opcom]

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.

[Down Under]

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!)

[old_tv_nut]

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.

[lnx64]

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.

[old_coot88]

Originally Posted by lnx64

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

The charge will partially recover after the first 'pop'. 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.

[kvflyer]

Originally Posted by old_coot88

The charge will partially recover after the first 'pop'. 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!

[Geoff Bourquin]

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.

[Down Under]

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.

[Reece]

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.

[old_tv_nut]

Originally Posted by Reece

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 ...

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

[old_coot88]

Originally Posted by Down Under

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!

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.

[lnx64]

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

[old_coot88]

Originally Posted by lnx64

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.