Jump to content
Alex

Discharging a CRT - video

Recommended Posts

If you are thinking about working on a compact Mac, it's always safer to work on if the CRT has been discharged.

 

There is a very good video on just that. I should mention that Bruce has countless other videos on all things related to fixing Macs as well, he even does recapping as a service. Be sure to check out his video.

 

Cheers!

—Alex

 

 

 

Share this post


Link to post
Share on other sites

Great vid - thanks for sharing @Alex.  I have always been looking for the official tool that I used to use in the Apple Canada service shop, but they are hard to find.  This video takes away some of the apprehension of building my own.  Cheers!

Share this post


Link to post
Share on other sites

Also, if memory serves, the longer the CRT sits disconnected from power, the more it discharges naturally.  So, when possible, if you can let it sit for a few days or longer, there will be less of a charge to discharge, which makes it a little less scary when doing the actual discharge whether with a homemade tool or an official one.

Share this post


Link to post
Share on other sites
5 minutes ago, dcr said:

the longer the CRT sits disconnected from power, the more it discharges naturally.

related: I watched a video which implied the original 128k, 512k models' PSU-come-analog-board didn't have a bleed off for the CRT, which was introduced in the Plus model. Can someone confirm that?

Share this post


Link to post
Share on other sites
2 hours ago, dcr said:

Also, if memory serves, the longer the CRT sits disconnected from power, the more it discharges naturally.  So, when possible, if you can let it sit for a few days or longer, there will be less of a charge to discharge

Yep. Usually what I do is wait for whatever replacement part to arrive in the mail, and during that waiting period, I do not switch on the Macintosh.

Share this post


Link to post
Share on other sites

Great video!!

 

If I'm going to work on a Plus or earlier and need to work with the CRT, I prefer to leave things unplugged for two weeks. In addition to keeping the Mac off, I'll actually unplug it. This should get rid of almost any buildup in there, but to err on the side of caution, discharge the CRT.

 

The self-discharging bleed came in the SE and later. I know the SE and Classic lines have it and can confirm the 12" RGB (the LC topper monitor) does too. I don't believe any of the Plus boards had it, even the platinum switch ones. Apparently some early SEs don't have it, but I seem to remember a self-discharge in a 1987 SE with the old rat cage fan.

Share this post


Link to post
Share on other sites
On 12/30/2019 at 4:49 PM, dcr said:

Also, if memory serves, the longer the CRT sits disconnected from power, the more it discharges naturally.  So, when possible, if you can let it sit for a few days or longer, there will be less of a charge to discharge, which makes it a little less scary when doing the actual discharge whether with a homemade tool or an official one.

This is not true, a CRT can build up a charge over time even when disconnected. This is what I remember from my Apple Certification Training course of nearly 10 years ago.

 

It's easy to discharge an CRT, the video just shows a Mac but they all follow the same discharge procedure.

Share this post


Link to post
Share on other sites

CRTs make good capacitors, and they will pick up charge from the atmosphere.  I would discharge it if you have to touch the analog board, if you don't, just leave well enough alone.

Alex is correct in that they can build up charge.

Share this post


Link to post
Share on other sites

I'm guessing I may have gotten the information from this article:

 

https://lowendmac.com/2007/the-truth-about-crts-and-shock-danger/

 

Says that: "If you want to minimize the chance of getting an unpleasant jolt, just let the thing sit overnight."

 

But then there's this article that basically says "Run for your life!"

 

http://www.peachpit.com/articles/article.aspx?p=759704&seqNum=2

 

 

I'm not claiming any expertise on any of this.  After reading the first article, I'm a bit concerned now about recapping the power supply I have that needs to be recapped.  After reading the second article, well, CRTs are scary again.

Share this post


Link to post
Share on other sites

It might give you a bad shock & burn, but there are no examples of death (until now).

Refer to authorized technician only. 

Share this post


Link to post
Share on other sites

Someone here asked if confirmation can be established on whether the original 128K and 512K models have a resistor bleeder.

 

Personally I don't know but if someone does please share. I want to make an important point however, the resistor bleeder can be faulty and therefore fail to bleed charge from the CRT. * The best way to guarantee personal safety is to safely discharge the CRT *. By now, all products that employ a CRT have aged considerably enough to encourage anyone working on them to play it safe and simply discharge a CRT.

 

In the meantime, if someone out here who can read schematics, it would be interesting to learn if the aforementioned Mac models had resistor bleeders.

 

Kindest regards

—Alex

Share this post


Link to post
Share on other sites
On 12/30/2019 at 7:44 PM, Dog Cow said:

Yep. Usually what I do is wait for whatever replacement part to arrive in the mail, and during that waiting period, I do not switch on the Macintosh.

When working on the Mac at some point you will need to plug it for testing and when doing so you will have charged up the CRT. If the issue you are attempting to resolve remains unresolved, you will need to continue working and therefore need to discharge the CRT so although the CRT may lose charge while waiting for parts, as a practical matter, when working on the machine you will need to discharge should the issue remain unresolved.

 

So what I am trying to say is that discharging is easy and safe, it will keep you safe, whether it is partially charged or fully charged, it should be done each and everytime you work around a CRT. You can never be too careful, these are extraordinarily large charges that can cause serious bodily harm or even death.

 

The moral of the story is, discharge each time to provide safety.

Share this post


Link to post
Share on other sites
On 12/30/2019 at 8:16 PM, Scott Baret said:

If I'm going to work on a Plus or earlier and need to work with the CRT, I prefer to leave things unplugged for two weeks. In addition to keeping the Mac off, I'll actually unplug it. This should get rid of almost any buildup in there, but to err on the side of caution, discharge the CRT.

I like the cautionary note on discharging, yes this really must be your ultimate goal after popping the case. With all due respect, waiting 2 weeks is unnecessary as a discharge will bleed *all charge* out the CRT, even after it has been plugged in minutes before the discharge procedure. You can do a discharge procedure 2 or 3 times if it makes you feel safer, as a precaution but there is no need to wait two weeks. There really isn't.

 

Here is a practical example. Let's say I wait those two weeks, maybe because I am truly frightened and believe that allowing this time to go by makes me feel safer. I am not here to argue anyone's fear of something, it's an instinct and natural but in practical terms … let's say I do wait those 2 weeks. My next step would be to remove the case and discharge the CRT. Now I begin work, recapping or whatever, then I reassemble the machine and find that I have not resolved the issue. I will need to switch off the machine, unplug the chord for the outlet, remove the logic board and discharge the CRT. Discharge because I want to avoid any bodily harm and or damage to the computer. I won't wait 2 weeks before proceeding.

 

Discharging is safe and is in fact a safety step when working on any hardware that has a CRT. If you follow the video closely you will be safe. I can assure you that this is one of the best videos on the topic that I have seen.

 

I can't remember if it was mentioned in the video, forgive me if it was but remove any jewelry, keep one hand behind your back, don't wear synthetic clothing, do not ground yourself, and when possible, have someone in the room when discharging a CRT.

 

Here is why:

If something goes wrong, having your hand behind your back will force the charge to travel away from your chest where your heart can really get messed and potentially cause a heart attack. Using cotton versus synthetic clothing will prevent burns, synthetic material is very good at holding a charge and you should never wear synthetics to begin with when working on electronics, it can hold and release enough to zap electronics. Removing jewelry is a precaution incase something does go wrong, the charge would be enough to attract the charge to those metals causing potential burn marks to the body. Having someone in the room is recommended in the event something goes wrong and you are knocked out, that person would be there to call an ambulance.

 

These sound like extreme precautions but CRTs are not toys.

 

When at AppleCare, the desktop technician exam was made up of two parts, safety and technical. If a single answer was incorrect in the safety portion of the exam it was enough to fail.

 

Hope this helps.

 

Let me tell you a story about my own fears. The first machine that I opened up was an eMac and I had to get near the CRT. I had no idea how to discharge the CRT but I knew that I was dealing with a potentially dangerous repair especially because I was conscious that there was an obvious knowledge gap. I was as scared as can be but I managed to stay clear of any of the high voltage areas. This was years before I passed the apple desktop technician exam. When at AppleCare, I had to discharge a CRT when practicing. The probe was not a screw driver as in Bruce's video, it was actually a very long probe, it was not an official Apple part, why I don't know but it was so long that one would be a few feet away when discharging. I found it to be cumbersome because you actually end up having a hard time getting under the anode cap.

 

Sorry for the long message but I want anyone coming across this post to be safe and know how to be safe because it helps to feel confident about the procedure and that is always a good thing in my book.

 

Cheers

Share this post


Link to post
Share on other sites
15 hours ago, Alex said:
Quote

Yep. Usually what I do is wait for whatever replacement part to arrive in the mail, and during that waiting period, I do not switch on the Macintosh.

 

When working on the Mac at some point you will need to plug it for testing and when doing so you will have charged up the CRT. If the issue you are attempting to resolve remains unresolved,

I don't need to do any testing while I wait for the CRT part to arrive in the mail. I already did my testing, determined what part(s) I needed, and my Mac remains powered off while they arrive. I install the part(s), then power on the Mac again and it is fixed.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now

×