Riddled TV Repair Forum

mtorbin · Joined: 09 Oct 2011 Posts: 2 Location: Philadelphia, PA

I bought three Macintosh SE's (picture below) over the past year, two of which haven't been plugged in in months and the other in about a year. I want to gut them and use the shell for something so the guts are of no value to me. Here are my questions:

1) How dangerous is it to deal with this CRT tube?
2) I've done a fair amount of computer repair work but have not worked with a CRT. Is this something I should reasonably leave to an expert?

Thanks,

- MT

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JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

If you can change light bulbs in a multi-lamp chandelier without breaking the burnt out bulb, the new bulb or the glass tube that goes over each bulb, a CRT is actually (other than the neck area) less fragile.

Most CRT sets since the 80s usually had a automatic discharge feature (bleeder resistor).

mtorbin · Joined: 09 Oct 2011 Posts: 2 Location: Philadelphia, PA

So as long as I take the necessary precautions to attempt to discharge the CRT, I should be fine? Do these things / did these things ever carry enough punch to kill someone when unplugged?

kuhurdler · Joined: 28 Dec 2006 Posts: 2879 Location: Overland Park, KS

Yes, they store many thousand volts, which could easily kill you. All CRTs do. Make sure you discharge them safely... several times.

JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

Now come on. You're fretting about this unnecessarily.

From: http://lowendmac.com/tech/crt_danger.html

In the end, it boils down to this: Getting shocked by a CRT is certainly unpleasant, but as long as you're in good enough health that you don't go into cardiac arrest when someone surprises you with a "boo!", you're not going to have a problem with a CRT discharge. There's no need to be terrified of the CRT. You won't be vaporized.

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kuhurdler · Joined: 28 Dec 2006 Posts: 2879 Location: Overland Park, KS

hey, you may be right. They probably won't die. However, I don't think I'm being overly dramatic by trying to play it safe.

I also still put on my seatbelt, wear eye protection, and I don't disable the safety triggers on all my pneumatic nail guns.

Let me put it another way. Would you willingly let your kid take one of those jolts? I wouldn't. I've been zapped once THROUGH the insulation on my pliers... and it hurt like heck. I'd guess that my shock wasn't anywhere near the full potential, and I still deeply fear/remember/respect it.

So as I see it, the argument boils down to:
It takes 10 seconds to discharge it correctly -OR- it MIGHT not kill you. Don't be stupid, just discharge it.

kuhurdler · Joined: 28 Dec 2006 Posts: 2879 Location: Overland Park, KS

Additionally, the article you posted compared them to taser shocks.

People die from taser shocks all the time.

JTS1957 · Joined: 21 Jan 2009 Posts: 2395 Location: Far, Far Away

I assume you meant "stun gun" :

We're talking about 0.05 joules from a CRT. That's 1/84,000 of a calorie. Twelve millionths of a calorie - does that sound dangerous?

Now, a rhetorical question is not proof (but the calculation is pretty revealing), so let's look at another calibration point: The "stun gun". Designs vary, but a typical stun gun generates perhaps 50kV and delivers about 0.5J per pulse. And it typically delivers 10-20 such pulses each second.

Look at those numbers carefully. The voltages are perhaps 5 times higher, the energy per pulse 10 times higher, than the corresponding CRT discharge numbers. And many pulses are typically delivered from a stun gun. The total energy delivered is hundreds of times greater than what a CRT discharge provides
The reason a stun gun is non-lethal is that it delivers its high voltage as high-frequency AC (each of those 10-20 stun pulses per second is actually many cycles of a high-frequency AC waveform), rather than as DC. There's a very well-known phenomenon called the "skin effect", which describes the fact that the current flowing in a conductor tends to concentrate on the surface at high frequencies.

If the frequency is high enough, the interior carries very little. So if energy is delivered to your body at a sufficiently high frequency, it doesn't penetrate to your heart. And that's the key. You might have heard the old saying, "it's not the volts that kill, it's the amps." Well, more precisely, it's the current that flows through your heart that matters (and its frequency and waveshape). If the discharge is fast, hardly anything flows there. So there are two reasons not to fear CRT discharge: The energy itself is embarrassingly puny to begin with, and the discharge occurs so fast that little of that minuscule energy makes it to the heart..

You might counter with news reports of some deaths after stun guns are used, but these deaths are associated with compounding factors, such as dr@g use, ill health, and physical restraints that inhibit breathing. And again, we're talking about stun guns whose energies are hundreds of times greater than that delivered by a CRT. Unless you work on your CRT-based Mac on serious dr@gs while heavy law enforcement officials sit on your chest, you don't have to worry.

kuhurdler · Joined: 28 Dec 2006 Posts: 2879 Location: Overland Park, KS

I'll stand by my original statement. And I'll go ahead and pull rank to make that the official stance for this website.

CRT discharge is not high frequency. It's a transient pulse. When it happened to me it made my arm very sore, and my ears ring for several hours. In my opinion, if the path of discharge includes your heart or any other vital organs, it would be quite dangerous.

If you plan to work on your TV/Monitor, please discharge your CRT.
If you aren't confident that you're capable of doing that, please don't work on your TV.

I'll now close this thread.