Safe way to discharge a CRT?

[shadow460]

Is there a safe way to discharge a cathode ray tube? I've got a couple of upcoming project that involve them, and I'd like to know if the tube can be discharged without damaging it, the wiring, or anything else.

 

Or should I just wrap electrical tape around my fingers and hope for the best?

 

(ok that last line was a joke)

 

EDIT: Found it. A link for tomshardware suggests using 10 100K resistors in series with each other and the shorting probe. Another guide I read says nothing about the resistors at all. It all sounds similar to discharging the HV cap in a microwave oven.

Edited May 21, 2008 by shadow460

[CPUWIZ]

Screwdriver and a 1MOhm resistor to ground.

[A.J. Franzman]

You do want to have resistance in the ground lead, to prevent excess current from damaging the conductive coating in the tube (aquadag) in the vicinity of the anode lead connection. The reason for using multiple resistors is that your typical 1/4 watt resistor is only rated to 250 volts, and theoretically the charge stored in the CRT (which could be as high as 30 kV or so) might arc across it, which would then be essentially the same as not having a resistor in the line. Higher wattage resistors usually have higher voltage ratings too, so if you happen to have a big 1M resistor that's good to 30 kV, use it if you'd like to reduce the parts count in your discharge lead.

Edited May 25, 2008 by A.J. Franzman

[supercat]

Higher wattage resistors usually have higher voltage ratings too, so if you happen to have a big 1M resistor that's good to 30 kV, use it if you'd like to reduce the parts count in your discharge lead.

 

More to the point: the only high-wattage resistors that are available in higher resistances have higher voltage ratings, because otherwise there'd be no point to having a higher-wattage resistor. At lower resistances, there are many higher-wattage resistors without particularly high voltage ratings.

 

Consider that the only way a 1 meg resistor which never gets more than 500 volts across it can't possibly dissipate more than 1/4 watt. It would be pointless to have a 1/2 watt or 1-watt (or larger) one meg resistor that couldn't handle a higher breakdown voltage.

 

On the other hand, there probably isn't much need for most 100 ohm 1-watt resistors to be able to withstand even 500 volts. While there are certainly some applications where it would be necessary (dealing with brief voltage spikes), putting 500 volts across such a resistor would dissipate 2500 watts for the duration of the voltage.

 

In any event, series strings of resistors generally work fine though I would recommend using at least one or two more resistors than you should 'need'. If your voltage gets high enough that the weakest resistor arcs over, that will increase the voltage on the other resistors; if one of them arcs too, there could be a cascading reaction. If you have some extra resistors in series, though, you should be safe even if one of the resistors arcs over for no good reason.

[Dave Neuman]

I have always discharged (with one hand in my pocket) through my high voltage probe ... similar to this one.

[shadow460]

I purchased ten 100K 1/2 watt resistors, but I haven't built the probe yet. I wonder if 1.2 megs total would be too much resistance?

 

BTW, I did finish one of the projects already. I had to work with the input on a 27 inch RCA television. Perhaps I should have discharged the CRT anyway, but it seemed to me that the HV line was very well insulated and out of the way of what I needed to do. Even so, I worked with only one hand the whole time (it's kind of a throwback from my time of working with marine electricity). One thing I did notice is that the HV anode lead's insulation seemed like it was stuck to the CRT with some kind of glue.

 

The next project, a Vectrex, may involve re-seating a yoke and possibly repairing some ring cracks on the HV board. I want to be able to discharge the CRT for that, but if the "suction cup" is glued down, I'm too chicken to peel it off at a loss as to how to expose it safely for discharging.

Edited May 25, 2008 by shadow460

[Gregory DG]

You guys are braver than me. I wouldn't touch one of those tubes if you paid me.

[NeitherSparky]

Screwdriver and a 1MOhm resistor to ground.

 

This is what I had read elsewhere, but the only 1MOhm resistor I could find is only 2w...is that too small? I want to make sure I'm prepared to drain capacitors if needed; I have some projects coming up.

[Dave Neuman]

Before I purchased my High Voltage Probe, I used a device similar to this and never bleed the discharge through a resistor without incuring any damage. Bob Roberts has a good website with advice on Arcade related repairs. His discharge kit does not use any bleed resistors. Randy Fromm's website is also a good resource. I have a couple of his instructional video tapes and he never uses a bleed resistor when discharging. Here is a link to his technical department.

[classics]

If you use a screwdriver just be careful not to scratch the back of the CRT when you go under the cup, the glass is nowhere near as thick as it is in the front.

 

Steve

[shadow460]

one of those probes is very similar to what I had in mind. I had thought of using a hose clamp to secure the wire to a Craftsman screwdriver shaft, and hooking that to the resistor bank. As it is now, I have a pair of clamps and could likely just clamp to the shaft, but I'd rather have a solid connection.

 

I never thought of grinding down the plating. Good idea there.

 

Next question...did the Vectrex have a built in bleeder? If that's still working, then I might not know when I've actually contacted the anode wire under the cup.

Edited May 27, 2008 by shadow460

[+madman]

Next question...did the Vectrex have a built in bleeder? If that's still working, then I might not know when I've actually contacted the anode wire under the cup.

With the Vectrex, I've found I only get a pop noise if I discharge within a few minutes of having had the Vectrex powered on. If I discharge a few hours later, I get no audible sound.

[ckrtech]

Wow. Fancy stuff. We always discharged the CRTs in our machines by tying low gauge speaker wire around the CRT metal chassis and wrapping the other end around a screwdriver. That loud POP! scared me every time.

[channelmaniac]

I don't know about the Vectrex....

 

On arcade games you can discharge them to the chassis on old monitors without any issues. On newer ones with EEPROM chips or on older all-in-one computer systems I would HIGHLY recommend using the resistor.

 

On the old Mac, Mac +, Mac SE, and Mac SE/30 all-in-one systems there were a couple of 74xx parts that would consistently fry if you discharged the CRT without the resistor... On some of the newer monitors you can fry the digital circuitry.

 

RJ