Compact Mac CRTs- Safe to touch and after how long?

[Theretrogamingroom]

I am currently restoring my 128K (which is throwing a RAM error), and I have the machine disassembled after months of it last being turned on. I was wondering if I could take out the PSU and CRT without needed to discharge anything.

Also: what are the "danger zones" on the CRT and PSU?

Thanks

 

[wilykat]

If it hasn't been plugged in for many weeks, it's probably fine.  The danger is the spot on the CRT after you remove the thick anode wire. People who forgets to discharge it may get a shock when touching that spot and drop the CRT, destroying it.

 

[Elfen]

It should take a week for the caps and transformer to charge down without a bleeding resistor in the transformer. It should take about 10 minutes for the caps and transformer to charge down fully if it has a bleeder resistor in the transformer.

The head of the snake is at the metal clip inside the suction cup on the CRT, as this is where the 1-12 KV is at. Then there is the circuit around the transformer on the top of the circuit board which has a lower voltage but at a high amperage. When dealing with the Analog board, only hold it by the edges, and never touch the traces unless you are working on it and only after you discharge it.

Many early Macs like your 128K have transformers without a safety bleeder resistor. To discharge them, read: http://www.biwa.ne.jp/~shamada/fullmac/repairEng.html#DischargeCRT

 

[unity]

I just turn the brightness all the way up then unplug it (do not turn off). Seems to do a pretty good job. Either way the shock is more annoying than anything.

 

[Theretrogamingroom]

By "do not turn off", I guess you mean unplug it and turn it off, not JUST turn it off? Or does the position of the Power Switch actually matter when dealing with a compact mac?

 

[jsarchibald]

By turning the brightness all the way up, and then yanking the power cord, you are removing the power source for that monitor, and hopefully sucking up any residual power.  It seems to be the best way to drain as much excess power out of your compact mac as possible.

Just turning it off still leaves the mac energised, but by pulling the cord, it is an instant loss of power coupled with a high-draw device - the CRT.

 

[Scott Baret]

I have never discharged a compact Mac CRT with the procedure outlined in many service manuals, and have always played by a few basic rules:

--Classics and Classic IIs have a self-discharging CRT. If I need to yank a CRT from them, I usually wait only a few minutes.

--The same goes for most SEs and SE/30s, but some of the earlier ones don't self-discharge. If the analog board is a newer revision, I treat them like a Classic or Classic II. Otherwise, I use the Plus/earlier procedure.

--For early Macs, I will unplug the computer and let it sit for a week.

The trouble is, as others have said, the red anode cap. To play it safe, I never remove it with my bare hands. Instead, I use a small and very well-insulated pair of pliers to squeeze the clips together, prying it out. I have used this same method to re-attach the anode cap but usually will bare-hand it.

Over the years, I have done this procedure over 100 times on various compact Macs and have yet to have a problem with the removal or installation of the cap. I also haven't been shocked, so I must be doing something right (although, to be fair, I've worked on more Classics and SEs than anything).

For me, a bigger challenge these days is finding someone who will take bare CRTs to recycle. Currently, I have a pile of them and need to find a recycler who will accept them. (Before anyone calls me out for getting rid of Mac parts, please note all of these have some sort of issue--burn-in, scratches, or a cracked neck).

 

[techknight]

Its a bit of a misnomer. The power switch is hooked directly in line with the power cord. Lol

 

[snuci]

This is a great thread.  I've always been too chicken to even discharge a CRT.  Had to do that in a Lisa and it was already discharged.  I need to do this on a couple of other monitors (a PET 64 and another Lisa soon).

Thanks for asking this question.

 

[Elfen]

I just turn the brightness all the way up then unplug it (do not turn off). Seems to do a pretty good job. Either way the shock is more annoying than anything.

Don't kid your self on the shock danger of a charged CRT. It only takes 12V @ 4 amps is enough power to stop your heart, and that's nothing. P(ower) = V(oltage) * A(mperage), so if the Power remains the same but the voltage is in the 1,000V area, it take very little amperage to stop your heart.

The thing is, that shock has to go across your body (ie: across you chest) to stop your heart.

Getting shocked, there's an entrance point and exit point, and in many cases it's only in inches or fractions of an inch distance the shock travels through your body. But if your right hand gets shocked as the entrance point and your left hand is grounded and becomes an exit point because you're leaning on your desk against it or your feet as they are grounded to the floor to become another exit point, guess what? That shock just traveled through your body and across your chest, stopping your heart. And YOU CAN'T GIVE YOURSELF CPR!!!

Just because you got shocked hundreds of times by a charged CRT, like I have in the past, does not mean you are going to survive your next shock. Electricity is to be respected, and high voltage electricity is to be feared, neither are to be played with like some pet or toy because it will bite you severely. Enough to kill you if you are not careful.

Do not kid yourself into thinking that the CRT is discharged because you pulled the plug, because it is not. The caps are still charged, the transformer is still transforming, and one wrong slip of that anode chip onto you, and it will bite you. Its like a gun, if you find one, always assume that it is loaded.

Now I stated as to how long a CRT should discharge itself, about a week or so. It should be safe then, but I always discharge the anode cap against the screw when I remove it from the CRT. Better to be safe than sorry. Then I go do what I have to do in terms of repairs.

 

[adnb]

Last I heard, the movement of the Earth causes CRTs to be deathly dangerous to touch, even if left unplugged for a while. If you must work on a CRT, discharge it properly if you know how. Otherwise, take it to a TV repair shop and have them do it for you.

 

[wilykat]

When I was dealing with potentially dangerous power, I've always used my right hand only and made sure my left hand and arm didn't touch anything.  While I've never been zapped by the CRT (and I've worked on the huge projection TV that can have over 80,000v) I did get zapped by live 110v a few times and once 440v line.  No injuries fortunately other than scared out of my wits.

Still if you're nervous, find someone who knows electronics and CRT, share beer and pizza and let him work on it.

 

[Elfen]

Last I heard, the movement of the Earth causes CRTs to be deathly dangerous to touch, even if left unplugged for a while. If you must work on a CRT, discharge it properly if you know how. Otherwise, take it to a TV repair shop and have them do it for you.

Never take a classic Mac to a TV Repair Shop for monitor/crt work. 1) They will charge you an arm, a leg and your first born just to look at it. 2) Tell you that they have to special order parts which will come from Nowhereneccastan! 3) they do not know how to work on a Mac Classic Monitor, they are different from a typical CRT TV set like the Model T Ford is different like a Lexus Q80.

It is better to do it yourself or get a Mac Pro like Uniserver or TechKnight to help you out. Electricity is dangerous but if you respect it, it will respect you.

Also... the movement of the Earth has nothing to do with CRTs to be deathly to touch. Its the caps on the analog board still being charged when you turn it off and when something touches the CRT Anode Wire clip, electricity likes to go to the quickest path to ground - usually through your body if you're that unlucky.

 

[Paralel]

P=VA does not apply with relation to human physiology. Increasing one does not mean you need less of the other to cause trouble. There are independent voltage and amperage thresholds you have to worry about before it becomes a problem with regard to cardiac depolarization.

Last I heard, the movement of the Earth causes CRTs to be deathly dangerous to touch, even if left unplugged for a while. If you must work on a CRT, discharge it properly if you know how. Otherwise, take it to a TV repair shop and have them do it for you.

...You're being facicious, correct?

 

[trag]

Actually, the video circuitry in the Plus and earlier is nearly identical to early 9" B&W security monitors. So to say that it is analogous to a Lexus is stretching it quite a bit. It's more like a circuit someone found laying around and stole for use in the Mac (or used a reference circuit for the CRTs they had chosen).

I found a small stash of replacement Mac CRTs several years ago which were originally sold as service parts for security monitors. The attached circuit board is very similar to the Mac analog board.

 

[rsolberg]

Useful info trag. I'll file that one in the memory banks.

 

[unity]

Its a bit of a misnomer. The power switch is hooked directly in line with the power cord. Lol

Nope. Look again at a board.

 

[techknight]

I have, and the schematics.  it goes power cord, network line filters/suppressors, fuse, and power switch. 

the power switch is ALWAYS in the line circuit on a compact. period....

 

[uniserver]

i always keep a small screwdriver and a alligator jumper handy… every time I power on an A/B, I don't care what it is really. I discharge it before I do anything, witch can get annoying when you are in the fixing stage, but its not worth the risk.