I need to replace a faulty capaitor trimmer on my T.V
I now that the chasis of T.V.'s is live so does this mean
that the T.V componenets are rated for 220V. Does
this mean I cannot use a 50V cermaic capacitor to
replace the trimmer? (it belongs to the colour osc)
> I need to replace a faulty capaitor trimmer on my T.V
> I now that the chasis of T.V.'s is live so does this mean
> that the T.V componenets are rated for 220V. Does
> this mean I cannot use a 50V cermaic capacitor to
> replace the trimmer? (it belongs to the colour osc)
"live" is relative. Any point within any circuit is relative to another,
although they both may be ground referenced. For example, if
two points are different by just 5V, then you need a component,
a resistor or capacitor say, that is rated for just 5V, even though
it may have 1000V on one terminal and 995V on the other. All
that generally matters is the voltage drop or potential difference
across the component
In all seriousness, "DO NOT OPEN UP A TV SET". The extremely high voltages
required for the CRT will be present long after the TV is turned off and
unplugged. THEY CAN KILL YOU. DEAD!
>> I need to replace a faulty capaitor trimmer on my T.V
>> I now that the chasis of T.V.'s is live so does this mean
>> that the T.V componenets are rated for 220V. Does
>> this mean I cannot use a 50V cermaic capacitor to
>> replace the trimmer? (it belongs to the colour osc)
As Bill (or Pookie?) said - a television can be dangerous or deadly if
you don't know what you are doing. And (no offence intended) you
obviously don't. It would be wiser to find someone locally who knew a
little (or a lot) more to assist you.
That said, the TV's component voltage ratings are mostly unrelated to
the local mains voltage. The TV has its own power supplies and
voltages will be what the designer finds appropriate. Many will be
less than 220v and some will be higher.
If the circuit uses a trimmer then it is HIGHLY likely that a trimmer
is needed to adjust the circuit's operation during setup. As a rule
TVs don't use a dear component where a cheaper one will do. A 50v
capacitor will QUITE POSSIBLY do OK voltage wise.but may not.
If you MUST play inside a TV.
- Turn off power at switch at TV and at wall.
- Unplug from wall.
- Leave for a day or so.
- Then regard everything as possibly live and act accordingly.
The above may well be overkill, but always remember your overkill
factor is 1 - you only get 1 (physical) death (unless you are a cat).
Even Pookie only gets 1 death.
In all seriousness, "DO NOT OPEN UP A TV SET". The extremely high voltages
required for the CRT will be present long after the TV is turned off and
unplugged. THEY CAN KILL YOU. DEAD!
I read this a lot, and I believe it. But I was always curious how the experts
discharge these caps?
------------------------------------------------------------------------------
The really nasty "cap" of biggest concern here is the CRT. The design of a CRT
creates a fairly high value capacitor which becomes charged to 2nd anode
voltage during operation. This could be as high as 40 kV, and is almost always
above 15 kV. It can retain this charge for a LONG time. It is DEADLY.
I use a homemade shorting wand. An 18 inch long piece of 1 inch dia. PVC pipe
With a 1 meg ohm resistor epoxied into the end. Leave the protruding resistor
lead full length. The opposite lead is connected to an 18 inch length of hookup
wire with a clip attached.
In use, the ground clip is attached to chassis ground (make sure that it is in
common with the conductive coating on the rear of the CRT), then use an all
plastic tool (Plexiglas,lexan, etc.) to lift the edge of the plastic anode cap
until the anode connector is visible, then touch the anode connector with the
shorting wand probe. Hold it in place for at least a minute.
I agree with other posters, anyone contemplating working inside a TV for the
first time should go find some experienced help. These things can be dealt with
safely, BUT if proper procedures are not followed they can KILL you.
At 11:05 AM 5/16/2005, Richard E. Teague wrote:
>
>
>-----Original Message-----
>From: EraseMEpiclist-bouncesspam_OUTTakeThisOuTMIT.EDU [piclist-bouncesspam_OUTMIT.EDU] On Behalf Of
>Lindy Mayfield
>Sent: Monday, May 16, 2005 8:33 AM
>To: Microcontroller discussion list - Public.
>Subject: RE: [EE] replacing a faulty capacitor on a T.V
>
>
>
>I read this a lot, and I believe it. But I was always curious how the experts
>discharge these caps?
The thing is called a "jesus stick", though it mostly gets that name
when used on even larger systems.
It's simply a device to slip in under the anode cap, and connect that
point to ground, accompanied by a nice "pow".
It needs to be very well insulated, of course.
In large systems, the discharge is accompanied by some vaporized
metal, and the operator's enthusiastic invocation of the local deity.
There are supposed to be safety resistors to bleed these charges off
over a period of a few minutes, but they almost always fail due to
the heat that they dissipate. So, you can't ever count on them, which
makes their existence in the first place, somewhat pointless.
At 11:05 AM 5/16/2005 -0500, you wrote:
>
>
>-----Original Message-----
>From: @spam@piclist-bouncesKILLspamMIT.EDU [KILLspampiclist-bouncesKILLspamMIT.EDU] On Behalf Of
>Lindy Mayfield
>Sent: Monday, May 16, 2005 8:33 AM
>To: Microcontroller discussion list - Public.
>Subject: RE: [EE] replacing a faulty capacitor on a T.V
>
>
>
>I read this a lot, and I believe it.
I'm rather dubious myself. The energy stored in the capacitance of a 27" or
smaller
CRT will likely be well under 10 joules, and hence not likely to kill
anyone in normal
health. It might make you drop the set on your foot. The fat electrolytic
power supply
capacitors are probably more dangerous, and VERY deserving of proper
respect. They
store enough energy to burn the skin, especially if your skin is unusually
conductive
(wet/sweaty).
Has anyone making these claims pointed to a single cite of such a fatality?
I'm sure
plenty of people have gotten a shock. I probably have, but I can't
remember. ;-)
> But I was always curious how the experts
>discharge these caps?
Ideally, through appropriately rated resistors (both voltage and current). The
old-fashioned method of shorting to the chassis can damage modern sets.
DO NOT use a regular resistor, it will most likely arc and can damage the set.
I have not been inside a TV for many years but back when that was my bread &
butter, I simply discharged it with a clip lead. Of course, you need to
ground the lead first! And I clip the business end to a small insulated
screwdriver.
BTW, the CRT construction is about the best Leyden Jar you might envision. I
have seen many CRT's that were stored accumulate enough of a charge to draw
an arc. Discharge it EVERY time you might come in contact with it.
At 11:54 AM 5/16/2005, John Ferrell wrote:
>I have not been inside a TV for many years but back when that was my
>bread & butter, I simply discharged it with a clip lead. Of course,
>you need to ground the lead first! And I clip the business end to a
>small insulated screwdriver.
I wasn't so trusting.. :)
>BTW, the CRT construction is about the best Leyden Jar you might
>envision. I have seen many CRT's that were stored accumulate enough
>of a charge to draw an arc. Discharge it EVERY time you might come
>in contact with it.
There is a phenomenon called "soak" where a cap can be shorted, and
after the short is removed, it recharges to some level.
I've seen this in CRTs, where I definitely shorted them out, and
later, without being powered, they were able to spark again.
I've never seen or heard anyone killed by them, but I have seen a few
injuries caused when muscle contractions draw your hand back
forcefully, removing it from the cage of razor-sharp stamped metal at
high speed..
Still, I treat anything like this as if it is lethal, and so far, I'm
still around to tell cautionary tales.
> I read this a lot, and I believe it. But I was always curious how the
> experts discharge these caps?
With an insulated power resistor that has two sharp probes. And wear
safety glasses when you do this, the probes don't always go (or stay)
where intended.
On Mon, 16 May 2005 12:50:22 -0500, Dave VanHorn wrote:
> There is a phenomenon called "soak" where a cap can be shorted, and
> after the short is removed, it recharges to some level.
> I've seen this in CRTs, where I definitely shorted them out, and
> later, without being powered, they were able to spark again.
The first colour television transmission I ever saw was in the Science Museum in London when I was 15. It was
a "Trade Test Transmission" and it was all about working on high voltage circuits - it demonstrated, by a
short fictional story, why the acronym SIDE is important: Switch off, Isolate, Dump, Earth. The Dump part
is using the resistor probe that people have been discussing, the Earth part is to counteract the situation
you mention above - leaving the earthing stick (or whatever) connected to dissipate any returning charge.
I'll never forget the part where the engineer was distracted by a ringing phone so made a mistake in the
circuit (which was big, walk-around style, and very high voltage) and later when it didn't work (and he'd done
the SIDE thing) was about to touch something that should have been earthed but wasn't, thought about it and
reached for an earthing stick, and was thrown backwards by the huge Zap that it caused! It was a very
effective lesson, and one I can remember in detail over thirty years later - it may even have saved my life
(since I have always followed it, and haven't had the sort of accident it was designed to prevent).
>-----Original Message-----
>From: Dave VanHorn [spamBeGonepiclist-bouncesspamBeGoneMIT.EDU]
>Sent: 16 May 2005 18:50
>To: Microcontroller discussion list - Public.; Microcontroller
>discussion list - Public.
>Subject: Re: [EE] replacing a faulty capacitor on a T.V
>
>
>There is a phenomenon called "soak" where a cap can be shorted, and
>after the short is removed, it recharges to some level.
>I've seen this in CRTs, where I definitely shorted them out, and
>later, without being powered, they were able to spark again.
"Dieletric absorption" is the technical name for this AFAIK.
Regards
Mike
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