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Alex

Discharging a CRT - video

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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

 

 

 

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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!

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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.

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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?

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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.

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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.

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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.

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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.

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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.

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

Refer to authorized technician only. 

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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

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