Need to Discharge Broken CRT?

[wesleyfurr]

I have a Mac SE FDHD whose CRT was broken prior to my acquiring it. Broken as in (from memory) the neck of the tube being broken...you can see specks on the front of the screen where it presumably imploded and something impacted it and damaged the phosphor coating. If you turn it on, it flickers and crackles from the back of the neck.

So here's the question...I know CRT's can be quite dangerous if not properly discharged. But - does a broken CRT hold a charge?

I am thinking of selling off the presumably good parts, disposing of the CRT, and building a Raspberry Pi or some sort of custom computer into the case of it. But obviously don't want to fry myself in the process!

Thanks,

Wesley

 

[Timo W.]

But - does a broken CRT hold a charge?

No, but caps on the analog board still can. So it does not hurt to still do the discharge procedure.

 

[wesleyfurr]

No, but caps on the analog board still can. So it does not hurt to still do the discharge procedure.

I certainly worry less about caps than the jolt I understand a tube can give you. Is there an official procedure for discharging?

 

[VERAULT]

its fine. your making an issue where there isnt one. tske it apart. this whole conversation seems needlessly ridiculous. the tube is broken. your fine..

 

[wesleyfurr]

its fine. your making an issue where there isnt one. tske it apart. this whole conversation seems needlessly ridiculous. the tube is broken. your fine..

I beg to differ...not wanting to get a serious shock from a CRT is a real concern. Now that I have confirmation that a broken tube can't hold a charge, no problem. Caps? If there's a big one that doesn't self-discharge, I would like to know that too. In my experience, seems like nearly all of them do self-discharge given a little time, but I did recently get a good jolt from a non-computer device that was holding on to a healthy charge. I don't know about you, but that's the type of surprise I can do without.

 

[VERAULT]

From a Mac SE analog board?! Have you ever worked on one? Thats the thing to make you throws your hands in the air for excessive danger? I am sorry but since the CRT is out of the equation Im not seeing a problem.

The SE analog board has the removeable system PSU chassis. Inside the PSU chassis youll mostl likely have severely leaking caps as thats pretty common on that model.

When was it powered up last?

The air quality in your home is probably more dangerous at this point.

 

[jlang]

Always discharge a CRT before handling. Even a broken one.
Breaking the neck of a CRT does not guarantee the capacitor formed by the coatings is discharged.
What do you call a CRT with a broken neck ... A leyden jar.

joe

 

[mR_Slug]

Most big caps are supposed to have a bleed resistor. If they dont they can hold a charge for an hour or more. You can use a high-watt resistor to discharge them safely, hold it in some insulated pliers. Some people just short them with a screwdriver, but this can damage the caps I'm told.

 

[jlang]

Most big caps are supposed to have a bleed resistor. If they dont they can hold a charge for an hour or more. You can use a high-watt resistor to discharge them safely, hold it in some insulated pliers. Some people just short them with a screwdriver, but this can damage the caps I'm told.

Direct short is OK for HV anode discharge. (It's seldom over 500pf.) On large Electrolytic caps yes it is bad for them. It can be bad for the screwdriver too.

joe

 

[wesleyfurr]

Always discharge a CRT before handling. Even a broken one.
Breaking the neck of a CRT does not guarantee the capacitor formed by the coatings is discharged.
What do you call a CRT with a broken neck ... A leyden jar.

joe

Thank you Joe, I appreciate the information.

 

[twolazy]

From a Mac SE analog board?! Have you ever worked on one? Thats the thing to make you throws your hands in the air for excessive danger? I am sorry but since the CRT is out of the equation Im not seeing a problem.

The SE analog board has the removeable system PSU chassis. Inside the PSU chassis youll mostl likely have severely leaking caps as thats pretty common on that model.

When was it powered up last?

The air quality in your home is probably more dangerous at this point.

I think you and I are old school and it never stopped us. I've been jolted by many things in my life, if you think that Mac is going to kill you OP, unless you have other issues, it isnt going to. I've had worse from car ignition systems. Sheesh.

 

[Gary C]

I suppose working with and on 400kV systems that carry 1000A focuses you more on the real risks of High Voltage as that equipment not only has enough energy to kill but to actually vaporise you.

CRT's, still a healthy respect. Wont 'discharge' by shorting them but I will put a short on to prove dead before touching the HV parts and generally give it time to discharge.

The only recent belt I have had is a 300V DC from a BBC master supply capacitor that should have been dead, but was useful as it diagnosed a failed resistor that should have discharged it

 

[Timo W.]

if you think that Mac is going to kill you OP, unless you have other issues, it isnt going to.

Such "wisdoms" should never be given to anyone - let alone on a public forum.

The point is, it CAN be dangerous and it CAN kill you. You don't know anything about the health state of him or any other person asking. What can be dangerous, should ALWAYS be considered so and handled with care. Not doing so is the main reason for accidents. I hope you don't handle guns the same way you handle CRTs.

And on a side note: in many cases, not the electric shock itself is deadly, but what happens as a result of that (e.g. falling back and hitting something hard with your head).

 

[twolazy]

LOL I have lived life... I assemble race engines for a living. I've been shocked many times over stupid things. Thinking a Mac psu is going to kill you bahhahahhahaha. And on top that a cracked CRT bahahahahaHHA MAY I repeat... bahhahahhahahahhahaa. Do you even understand the voltage on how these things work? 12vdc is gonna kill you? Bahahhahahahhaha. Go touch yer car battery, bet you dont die! 12vdc aint gonna do crap to ya. Good f'n lord peeps . If the crt is arching its already grounded. And juice left after a few seconds with bleed resistors, nothing is going to be left. Never thought I see the day peeps be afraid to open a Mac cuz you might die. LOL.

 

[vwestlife]

With a "necked" CRT there is more danger from broken glass than anything else. And this is small potatoes compared to the people using a microwave oven transformer to do fractal wood burning because they've seen videos of it on TikTok and think it looks cool. 30+ deaths so far of people getting electrocuted while doing it.

Here's LowEndMac's advice about the dangers (real or imagined) of CRTs in vintage compact Macs:

The Truth About CRTs and Shock Danger | Low End Mac

 

[channelmaniac]

Some people just short them with a screwdriver, but this can damage the caps I'm told.

On the Mac SE it can damage that 74xx chip on the analog board if you use just a screwdriver. BITD when I worked at a grey market Apple repair depot we would use a gator clip wire with a 1M resistor inline to discharge the CRT.

 

[channelmaniac]

The only recent belt I have had is a 300V DC from a BBC master supply capacitor that should have been dead, but was useful as it diagnosed a failed resistor that should have discharged it

ROFL! I had scars on my hand that took 16 years to fade away from a #@#$#$ Dynacomp Apple II series power supply. The Astec version had 100K (or was it 150K) resistors across the mains caps to discharge them. The Dynacomp didn't. I learned that the hard way when the supply fell off the bench and I reached down to pick it up.

 

[oldpcguy]

LOL I have lived life... I assemble race engines for a living. I've been shocked many times over stupid things. Thinking a Mac psu is going to kill you bahhahahhahaha. And on top that a cracked CRT bahahahahaHHA MAY I repeat... bahhahahhahahahhahaa. Do you even understand the voltage on how these things work? 12vdc is gonna kill you? Bahahhahahahhaha. Go touch yer car battery, bet you dont die! 12vdc aint gonna do crap to ya. Good f'n lord peeps . If the crt is arching its already grounded. And juice left after a few seconds with bleed resistors, nothing is going to be left. Never thought I see the day peeps be afraid to open a Mac cuz you might die. LOL.

Not 12 volts, try 12,000 volts. Advice to be cautious is excellent advice.

 

[vwestlife]

Not 12 volts, try 12,000 volts. Advice to be cautious is excellent advice.

At extremely low current. As the article I linked to above explains, a stun gun is at least 10 times more powerful than the discharge from a compact Mac's CRT would be.

Most of the danger from electrical shock when working on consumer electronics is not from the shock itself, but rather the surprise causing you to bump into something or knock something over and cause injury that way.

 

[channelmaniac]

but rather the surprise causing you to bump into something or knock something over and cause injury that way.

Which is why people in the know tell you to keep one hand in your pocket when working on high voltage circuits. Serves 2 purposes... 1) to keep you from touching something and having the current path cross your chest... and 2) less things to move around when you're jumping back. :D