----- Original Message ----
> From: Doug McNutt <[email protected]>
> To: [email protected]
> Sent: Wed, August 4, 2010 9:46:14 PM
> Subject: Re: Monitor Question
>
> At 16:22 -0700 8/4/10, glen wrote:
> >Generally, I have no problem taking anything apart including the Dell.
> >
> >Should I assume the same danger applies to LCD's as CRT monitors regarding
> >electric shock from the charge stored in the capacitors? If so, any safe
> >way
>to
>
> >discharge them? Probably won't try a fix anytime soon, too many more
> >pressing
>
> >tasks to do.
> >
>
> No. The real danger in a CRT device is the picture tube itself. It
> requires
>some 25,000 volts or more to accelerate the electrons that impinge on the
>face
>of the tube to make light by florescence in an internal coating. The outside
>of
>the tube is coated with an electrically conducting surface that acts as a
>capacitor - well it's a Leiden jar - that can store charge for hours.
>
> Flat panel monitors operate with voltages that are not harmful. If they plug
>directly into a wall without a wall-wart or in-line power converter they will
>have some 360 volts on capacitors that are part of the isolated low voltage
>power converter. That can wake you up and is especially dangerous because
>that
>part of the circuitry is connected directly to the household power lines.
>It's
>usually easy to avoid them in a special enclosure provided by the
>manufacturer.
>The capacitors there discharge fairly quickly when the unit is unplugged. If
>you can afford one, an isolation transformer is a good thing to have on your
>workbench.
>
>
> >FYI, I'm 63 and in reference to old TV's, I remember as a child of maybe 10
> >years, when the TV (a relatively new technology at that time) would go in
> >the
>
> >fritz and start rolling, usually while watching a favorite show, my dad
> >would
>
> >frantically go to the back of the TV to "adjust the horizontal hold" -- his
> >words.
> >
>
> Gotcha beat. I'm 75.
>
>
> >Apparently the really old TV's had some sort of knob on the backside you
>could
>
> >turn with a screw driver to the fix the problem. On the rare occasion when
>Dad
>
> >failed to fix it, he would call the TV repairman. They made next day house
>calls
>
> >in those days;, ahh the 1950's ;0 --glen
> >
>
> He was tuning the frequency of the horizontal oscillator. It has to match
> the
>rate at which the transmission is sent, about 15 kHz was standard NTSC
>television. There is also a vertical oscillator which was once 30 Hz. Modern
>CRT monitors are "multiscan" and can operate over a big range like 25 kHz to
>over 75 kHz.. But they still have that adjustment. It's usually a ferrite
>core
>in a wound inductor and you need a special tool to twist it.
>
> There are capacitors in the flat panel displays that can be a problem. In a
>great effort to make things smaller we have managed to standardize on little
>cylinders that are aluminum electrolytic capacitors. The insulator is
>aluminum
>oxide which is electroformed in an acid solution after the capacitor is
>built.
>The result is a device that is full of acid that can produce gas and explode.
>There are even little Xs formed into the aluminum case to make them leak gas
>without showering acid all over the place. You can often identify the bad
>ones
>by looking for Xs that have expanded into a dome.
>
> It's amusing that really old capacitors, like the ones in that TV set don't
>have the same problems. The aluminum is thicker and the oxide layer is
>thicker.
>Continuous usage keeps the oxide formed because the applied voltage does that
>for you. But they're a few inches high and an inch or so in diameter and the
>modern public won't accept that.
> --
Great reply Doug,
Answered my specific question and provided so much more technical/historical
information that answered many of my unasked questions. Really appreciate it,
thanks --glen
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