Questions about discharging a CRT

[jlfreund]

Hi,

I just got a Makvision 29" tri-sync CRT which has no plastic housing.  It has no plastic enclosure and the steel chasis, PCB and other parts are all exposed.

I'm appalled at the lack of discussion about safety in the manual that came with it.  I've watched some YouTube videos like the one below:
and poked around the forums here, and I still have many unanswered questions.  I was wondering if anyone can answer the following:

1) What parts of the monitor are dangerous to touch?  Is the entire 2" air gap around the entire monitor a death trap while it's on?  Is it just the metal chasis that's dangerous?  Just the bowl-shaped back of the CRT?  Just the PCB, etc?  

2) What times are different areas dangerous?  Are some parts only dangerous to touch when the monitor is on?  Other parts only when it's plugged in?  Other parts, for some time afterwards (before discharging)?

3) If I don't do any discharge procedure, how long will any portion of the exposed parts of the monitor be dangerous after being unpugged?  If I unplug and wait some amount of time (48 hours or whatever) is it fully safe to touch any part, without discharging?

4) I heard from a YouTube video that newer CRT's often have a "bleeder resistor" that automatically discharge the monitor when it's off.  Does the Makvision have that, and if so, how does it work?  Does it automatically discharge after turning off the monitor?  Or after unplugging it?  How long does it take to fully discharge after that?  Is it still necessary to use a probe to discharge after that?

5) I've seen videos using an alligator clip and a screwdriver.  The alligator clip attaches to the chassis.  But the Makvision has  PCB attached to the chasis.  Is that safe, or will the discharge blow the circuitry of the PCB?

6) In the procedure, you stick a screwdriver under a rubber suction cup thing on the back of the monitor.  Then, without any specific indication (a spark, sound, or apparent bumping of the screwdriver tip to some metal inside the suction cup), the person doing the demonstration declares the monitor is discharged.  What exactly does the screwdriver need to touch under the suction cup in order to complete the discharge?

7) After the screwdriver trick, the demonstration usually removes the suction cup with unprotected hands.  What is that step for?  If the monitor is already discharged, why remove the suction cup?  How do you replace the suction cup?

8) Is there a danger to the monitor in scraping a metal screwdriver under the suction cup and wriggling it around?  

Thanks,
Jason

[Peja]

I am not sure I can answer all of your questions with %100 percent certainty but since I just first discharged my first monitor I can tell you what I experienced and what I have read.

Unless you know exactly what you are doing, NEVER go poking around with a screw driver around the pcb/monitor area while the machine is on.  You can adjust the brightness and other controls just fine on the board but stick to just that.....don't be touching any caps or the suction cup part with a screw driver while it is on.

When discharging my monitor, I unplugged it from the wall, waited about 5 minutes, and then proceeded with the alligator clip and screw driver.  Mine must have discharged quickly because I didn't even get a fun pop or anything(kinda disappointed).  After touching around underneath the cup for about 30 seconds making sure I hit the metal, I lightly pryed with the screw driver and got the cup to come off.  After that, I pulled my board out, I soldered the new caps that next day at work.  Plugged it back in and everything was working great!!

Underneath that cap are two metal prong things that hold it onto the back of the monitor....that is the metal you want to be touching.  If you touch it and you get a pop or even if you don't, it should be discharged......but poke around there to make extra sure.......also MAKE SURE THE MACHINE IS OFF AND UNPLUGGED!!!  People remove the suction cup because it is attached to their board that they want to work on.....you cant pull that board out without removing the cap.  As long as you use the screw driver flat against the monitor there shouldn't be any problem to the monitor....just don't stab the monitor or try to go deep into the hole the clips clip onto.


A lot of people worry more than they should when discharging.  While it is good to be cautious, it really isn't as bad as it sounds.  And from what others have said, if you do get shocked, its more of a quick numb pain/scare than anything.

Good luck and be cautious........do what that guy in the video did....that is the same one I watched.

[jennifer]

       Normally its pretty safe, the frame shouldn't be charged.... But this is not to say it cant happen, A faulty tube can do some bizarre things.
What I do is ground the frame to the water line in the shop, slide a screwdriver under the rubber cup [contacting the pins] and with another
screwdriver touch the shaft of the first screwdriver and the grounded frame. Doing all this while its unplugged for obvious reasons.
       There should be a spark, not always..... And sometimes a friggin bolt of lightning shoots out and  zaps you.
After the initial discharge its still not safe.... residual charge can build up and the monitor is ready for another zap. reach under the rubber
cap [when you think its safe] with a needle nose pliers, I like to use one without serrated teeth but that's just me. And squeeze the two wires
together and its over..... You should get hold of a manual and familiarize yourself with the hazards BEFORE attempting this.

[grantspain]

discharge to the earth dag(braid) around the tube
anyway even if you get a whack it won't kill you,you are more likely to cause damage by your reaction
i get them from time to time when rushing a repair,just feels like a slap on the hand to me

[Q*Bert_OP]

Use caution when powered on:

Don't touch anything except the adjustment pots as needed, and be careful when making adjustments not to touch anything else. Do not adjust any pot that has glue or silicone on it.

Why do you need to discharge this monitor? If there is nothing wrong with it, don't!

But, to discharge, with the power cord unplugged, slide a grounded flat blade screwdriver under the anode cap. The screwdriver should be grounded to the metal frame of the monitor, and it should have a plastic or rubber handle.