🚀 go-pugleaf

In article <1991Jun13.160527.4094@wsrcc.com>, wolfgang@wsrcc.com 
(Wolfgang S. Rupprecht) writes:
>> If possible, short out the anode 
>> cable to the chassis ground first. People do funny things when hit with
>> this kind of voltage ( I didn't, but I'm a Tech ).
> 
> If possible????  You are kidding, right?  BY ALL MEANS ground the HV
> capacitor to if you are planning on messing with the HV section.
> Especially if you are planning on wiping the HV lead with a damp
> cloth!  Keep the shorting strap in place during the whole time you
> work on the set.  Charge will bleed back from the glass parts of the
> CRT onto the internal capacitor plates.

I highly agree with your basic point -- that you should ground the HV lead --
but I'm not sure what "capacitor" you're talking about here.  The CRT in a tv
set *is* the HV capacitor.  The second anode coating on the inside of the
"bell" of the CRT, plus a similar, grounded coating on the outside, forms the
capacitor.  (Not much capacitance, but extremely high voltage and low leakage.
Those who remember the static electricity parts of Physics 101 will note the
resemblance to an old-fashioned Leyden jar.)  

In very old sets (those that use a tube for the high voltage rectifier) the CRT
could hold a dangerous charge for weeks!  In modern sets, there is enough
leakage back through the HV rectifier diode to drain the charge to a safe level
in "only" a day or two...

The usual procedure for grounding the HV lead is to use a screwdriver with a
long plastic (not wood) handle.  Attach a clip lead to the blade shaft and to
chassis ground, hold it by the end of the handle, and gently poke the blade
under the HV connector's "skirt".  You will hear a snap as the charge arcs to
the blade.  You're not done yet; push on until the blade firmly touches the
connector, and leave it there while you work on the set. 

In a similar vein... when working on photoflash units you will find that the
capacitor holds a hefty charge at several hundred volts.  After reading the 
above, you may be tempted to short the capacitor with a screwdriver blade in
order to avoid a nasty shock.  Don't do it!  Photoflash capacitors can be 
damaged if they're discharged into a dead short.  Use a 10K resistor instead.  

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@dcs.simpact.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
Uucp:  ...{crash,scubed,decwrl}!simpact!cmkrnl!jeh