Welcome
Hello Electron Eaters!
As you know here at Retro Recipes, we love nothing more than restoring these old classic machines
And in the last couple of episodes, I've been restoring this Macintosh Plus
But when I showed myself this Discharging Cathode Ray Tube
By connecting a screwdriver to this ground lock
A few eagle-eyed viewers thought they spotted something a bit strange about my video
The power cable, and therefore the ground, was disconnected while I discharged it
And this led to one or two comments..
Well.. three
Ah. Well.. six.
Okay. There were 57
But wait.
I've seen videos by Jan Beta or The 9-Bit Guy
Where they, too, discharge a CRT with no cable connected
Just a screwdriver connected to the chassis, as I did
And I can see how that looks counterintuitive and why so many comments said I should have left the mains cable connected
So, are we doing it wrong?
Yes.
Yeah.. I mean no.
And actually to leave the power connected would be dangerous
But don't take my word for it
As always with my videos
I've teamed up with the world's pre-eminent expert with the highest credentials before I bring you this information
Bjork
Wait. Bjork.
Hello. It is Christmas time and I'm sitting here by my TV
I've been watching it very much lately because I'm on a holiday and I've been seeing all those programs
About all sorts of things
About Icelandics being reallly..
Happy about Christmas, really gay.. but now I'm curious
I've.. I've.. I've switched the TV off and now I want to see how it operates
So.
I'm not convinced the sound guy that understands the concept of television
This is what it looks like
This looks like a city, like a little model of a city
You know, the houses, which are here and streets
This is maybe an elevator to go up up there
See I told you she was an expert
And here are all the wires
These wires, they really take care of all the electrons when they come through here
They they take care of that. They are powerful enough to get all the way through here
I read that in a Danish book this morning
Back to our little city
Man, if Bjork doesn't give you ASMR nothing will
But although Bjork showed us one possible way to discharge a CRT there by nearly tazing herself
Discharging it into her fingers
She didn't tell us the correct way
Eee! Don't touch that!
But she does at least go on to make another great point
This is a millions and millions of little screens. So all that's on TV
It just goes directly into your brain and you stop judging if it's right or not
So you just swallow and swallow
This is what the nice poet told me once and I became so scared to television but then later on I stopped being afraid
Because I read the truth
The scientifical truth
Which is much better, you shouldn't let poets lie to you
Now, I'm not saying the comments I got weren't a form of poetry
But at the end, of the day the only thing that matters with any subject
Is that scientifical... scientific truth
Because that's what will keep you safe
And on that point, I'm sure it goes without saying but as with any video on YouTube
Obviously engaged common-sense, you try this stuff at your own risk, and I'm not responsible if you kill Bjork.
So don't try this at home.
Try it at Walmart.
Security Aisle 14 please
Yeah
So anyway, here's why it is best to discharge a CRT with no ground connected
You see a Cathode Ray Tube, and it's circuit, is basically a big capacitor
Here's a normal capacitor
And as you may know, the capacitors job is to store energy between metal plates inside it
This can help smooth and regulate power flow on the board
And it does this by collecting a positive charge on one plate and a negative charge on the other plate
And being Retro Recipes, we love plates!
So with this 250V capacitor, for example
And to simplify things down
One side stores a positive charge and the other side stores a negative charge
That's why capacitors have polarity with a negative pin always marked in this way
And a CRT is like a big capacitor and likewise stores energy
So back to a regular capacitor for this example
Let's say we need to discharge this capacitor to safely work around it after we disconnected the power
And if there was no discharge resistor
Well, the accepted method is to simply short-circuit the positive and negative terminals
So let's demonstrate this with the help of my friend Adam in his lab from the channel
Adam's Lab
And a large car audio capacitor
And now you can see exactly why you don't need any cable connected to ground
When you short-circuit the positive and negative terminals
Energy rushes out of the terminals and is converted into light and heat
And that's the spark you see here
It doesn't go to ground, it goes into the ether
How atmospheric!
And so it is with a CRT
And let's assume has built up a positive charge of static electricity inside the tube
This means the metal chassis of the screen including the handy ground line I showed
Is the negative terminal that connects to the cathode
Again, a positive and a negative a bit like a capacitor
So to discharge the CRT and make it safe
You simply short-circuit between the positive and the negative
In this case, we use a screwdriver
And just like with the capacitor
Sometimes you'll hear a little zap sound indicating that it's been done
And that's it, here goes your energy safely converted into light and heat
Then what ever dangerous voltage was inside there becomes zero
We didn't connect that capacitor to ground when we discharged it and we don't need to connect this either
Then you can remove the anode from the tube
And if you've done everything correctly as I demonstrate here
In theory, it should be completely safe to touch
Ow!
Just kidding!
Forgive my little artistic license there.
Here's the unedited clip
Oh, but speaking of getting zapped
Have you seen the new Kickstarter for next year's Zzap! 64 Annual?
I'm a proud backer and you never know and might even have been asked to feature in the annual itself
Either way, it's always incredible to me that these things are still even being published
So feel free to show your support for them
Check out the description for the Kickstarter link
But don't take my word for it
or Bjork or Neo's
Of why you should discharge with the power disconnected
One of the best Macintosh repair guides out here is Mac Repair Notes
And in it, Thomas H Lee of Stanford University issues this warning
He also goes into a lot of handy detail on repairing the PCB
Speaking of which, did you know that I recommend PCBWay
Speaking of flashes, they've got a flash sale right now with up to 30% off
Cuz as we all know PCB stands for Prevent Cathode Bangs
Doesn't it?
Okay, so hopefully you can see why there's no reason to ground the chassis
It literally adds nothing to the equation
But why not ground yourself or the chassis, anyway, just to be safe. I hear you ask
Well two lethal reasons.
1. If you connect to a mains ground your safety is at risk
Split neutrals and bad earth wiring is unfortunately all too common in household electrical wiring
Particularly older houses
So when you touch the chassis, even with the live switched off, could actually be touching up to 240V
And #2. You definitely shouldn't ground yourself. Why?
Well, I'll just leave this here
But there is another even easier and safer, for you if not the floppy drives, way to discharge a CRT
Simply turn up the brightness while the machine is on and yank out the plug
The useful tidbit again there from Apple Repair Specialist, Thomas Lee
So there it is the correct and safe way to discharge any Cathode Ray Tube display
Powered, as always here at Retro Recipes, by science
and a Facebook poll
Well, that's all for now, but if you learned something today
Feel free to subscribe, check out my Patreon
Or any of these things that helped me keep making these videos for you
Now, I'll be back soon with the finale of that Mac repair
Hopefully..
Until then, you are now discharged.
So, comment below and Cheerio!
This looks like a city, like a little model of a city