Hi, how is the safe and proper way to fully unload all the electricity in a
TV ( to earth it??? ). I need to go over the circuit board for dry
joints......

Thanks Shaun B.

Presume you mean discharge the CRT?? Clip lead attached to a plastic handle
flat screwdriver. Ground the loose lead to the ground strap attached to the
outside of the CRT. Carefully slide the metal blade of the screwdriver under
the rubber anode cap at the top, centre of the CRT. Gold the screwdriver
only by the plastic handle. Sliding the screwdriver into the anode
connection will discharge any residual voltage from the CRT.
May want to do a bit of reading regarding safety concerns when servicing
these devices since you need to ask this particular question!! IMHO.
"Shaun B" wrote in message
...
Hi, how is the safe and proper way to fully unload all the electricity in
a
TV ( to earth it??? ). I need to go over the circuit board for dry
joints......

Thanks Shaun B.

"Shaun B" wrote in message
...
Hi, how is the safe and proper way to fully unload all the electricity in
a
TV ( to earth it??? ). I need to go over the circuit board for dry
joints......


It's probably not practical to discharge every capacitor on the PCB. By far
the most dangerous part of a TV is the mains rectifier smoothing capacitor.
By dangerous I mean lethal. It can be easily found- it's usually the
largest, fattest capacitor in the TV, and is often rated at 400V or 450V.

Check the voltage across this cap before you work on the TV, it should be
very low, eg less than 15V to be safe. If it shows no signs of dropping
after the TV has been unplugged for a few minutes, it means there is no
bleeder resistor fitted across it. You can safely discharge it by using a
normal household bulb across the terminals, (at least here in Europe with
230/240V bulbs!) or a large wattage resistor of suitable resistance. Make
sure the voltage has indeed dropped to a safe value afterwards, never assume
anything.

You should also discharge the CRT to remove its residual charge if you
intend on removing the chassis. Electrically, a CRT is amonsgt other things
a HV capacitor, and can pack enough of a wallop to cause very painful and
potentially injurous muscle spasms.

To discharge the CRT, just take a screwdriver, attach one end of a croc lead
to it and the other end to the metal ground of the CRT (*NOT* a circuit
board ground!) The metal grounding braid stretched over the CRT is a good
place. Slide the screwdriver under the anode cap until it touches the metal
terminal. You may hear a sharp crack as the charge is grounded. Take care
not to scratch the CRT with the screwdriver.

Once these staeps are taken, the TV should be safe. However, still be
cautious and be prepared for a tingle off the CRT if you touch the bare
anode terminal- the charge has a habit of creeping back a little! I always
try to keep a grounded screwdriver in contact with the anode as I'm removing
the cap just in case, and avoid touching the anode wherever possible.

You might want to check the voltage across some of the higher voltage
electrolytics if you want total peace of mind, but I find these are usually
shunted by the circuitry and do not pose much of a hazard on modern TVs. No
guarantees though, YMMV as they say! Just don't touch anything you
absolutely don't have to, and certainly avoid touching the PCB or its
components with both hands at once. That way there can be no electrical path
across your body. IOW, if you must put your hand near potentially charged
caps, keep the other hand in you pocket!

Dave

"Dave D" wrote in message
...

"Shaun B" wrote in message
...
Hi, how is the safe and proper way to fully unload all the electricity in
a
TV ( to earth it??? ). I need to go over the circuit board for dry
joints......


It's probably not practical to discharge every capacitor on the PCB. By
far the most dangerous part of a TV is the mains rectifier smoothing
capacitor. By dangerous I mean lethal. It can be easily found- it's
usually the largest, fattest capacitor in the TV, and is often rated at
400V or 450V.

Check the voltage across this cap before you work on the TV, it should be
very low, eg less than 15V to be safe. If it shows no signs of dropping
after the TV has been unplugged for a few minutes, it means there is no
bleeder resistor fitted across it. You can safely discharge it by using a
normal household bulb across the terminals, (at least here in Europe with
230/240V bulbs!) or a large wattage resistor of suitable resistance. Make
sure the voltage has indeed dropped to a safe value afterwards, never
assume anything.

You should also discharge the CRT to remove its residual charge if you
intend on removing the chassis. Electrically, a CRT is amonsgt other
things a HV capacitor, and can pack enough of a wallop to cause very
painful and potentially injurous muscle spasms.

To discharge the CRT, just take a screwdriver, attach one end of a croc
lead to it and the other end to the metal ground of the CRT (*NOT* a
circuit board ground!) The metal grounding braid stretched over the CRT
is a good place. Slide the screwdriver under the anode cap until it
touches the metal terminal. You may hear a sharp crack as the charge is
grounded. Take care not to scratch the CRT with the screwdriver.

Once these staeps are taken, the TV should be safe. However, still be
cautious and be prepared for a tingle off the CRT if you touch the bare
anode terminal- the charge has a habit of creeping back a little! I always
try to keep a grounded screwdriver in contact with the anode as I'm
removing the cap just in case, and avoid touching the anode wherever
possible.

You might want to check the voltage across some of the higher voltage
electrolytics if you want total peace of mind, but I find these are
usually shunted by the circuitry and do not pose much of a hazard on
modern TVs. No guarantees though, YMMV as they say! Just don't touch
anything you absolutely don't have to, and certainly avoid touching the
PCB or its components with both hands at once. That way there can be no
electrical path across your body. IOW, if you must put your hand near
potentially charged caps, keep the other hand in you pocket!

Dave


Dave

I used to work for Rediffusion years ago, and they were always really into
safety research and training for engineers. One thing that they came up with
was that it was very dangerous to zero-ohm a CRT cavity connector, as
momentarily, a huge current flows. This, they said, caused micro-cracks in
the bowl glass, and could lead to substantial reductions in the intrinsic
strength of the CRT structure. I seem to remember a microscopic photograph
of this appearing in the company magazine.

Soon after this, all workshops around the country, were issued with pukka
discharging wands, which were made of a sort of hard cardboard tube, with
some resistors inside, a filed down threaded rod at the business end, and a
croc lead for grounding at the other - a bit like a typical EHT multiplier
probe for your AVO back in the 70's.

I was never sure that I really believed it, and continued to discharge tubes
in the ' traditional ' way by sticking a pair of crossed screwdrivers across
the cavity to the 'dag. To counter the charge backwash effect, where the
substantial charge retained by the CRT runs away from the discharge point to
the corners, then quietly creeps back to have you just as you lift the tube
off its mountings, I had an old EHT lead with poppy connector on one end,
and croc clip on the other. You could clip this to the end of the 'dag
grounding spring, to keep the tube tamed ...

Arfa

"Arfa Daily" wrote in message
...

I used to work for Rediffusion years ago, and they were always really into
safety research and training for engineers. One thing that they came up
with was that it was very dangerous to zero-ohm a CRT cavity connector, as
momentarily, a huge current flows. This, they said, caused micro-cracks in
the bowl glass, and could lead to substantial reductions in the intrinsic
strength of the CRT structure. I seem to remember a microscopic photograph
of this appearing in the company magazine.

Soon after this, all workshops around the country, were issued with pukka
discharging wands, which were made of a sort of hard cardboard tube, with
some resistors inside, a filed down threaded rod at the business end, and
a croc lead for grounding at the other - a bit like a typical EHT
multiplier probe for your AVO back in the 70's.

I was never sure that I really believed it, and continued to discharge
tubes in the ' traditional ' way by sticking a pair of crossed
screwdrivers across the cavity to the 'dag. To counter the charge backwash
effect, where the substantial charge retained by the CRT runs away from
the discharge point to the corners, then quietly creeps back to have you
just as you lift the tube off its mountings, I had an old EHT lead with
poppy connector on one end, and croc clip on the other. You could clip
this to the end of the 'dag grounding spring, to keep the tube tamed ...

Arfa


That's very interesting, and I suppose it makes sense if you think about it.
You'd never discharge the mains smoothing cap by putting a screwdriver
across it- it'd damage the capacitor, as well as send chunks of white hot
metal flying around, so I suppose theoretically a CRT could be damaged by
directly shorting the anode.

However, one has to look at the sheer number of techs who use a direct short
to discharge CRTs, and the lack of CRTs suffering any visible effects.
Perhaps Rediffusion were being a little over cautious! I suppose it would be
a bad thing to do daily, but I expect the few times a TV usually gets its
CRT discharged is unlikely to have that much of an effect.

Dave

"Dave D" wrote in message
...

"Arfa Daily" wrote in message
...

I used to work for Rediffusion years ago, and they were always really
into safety research and training for engineers. One thing that they came
up with was that it was very dangerous to zero-ohm a CRT cavity
connector, as momentarily, a huge current flows. This, they said, caused
micro-cracks in the bowl glass, and could lead to substantial reductions
in the intrinsic strength of the CRT structure. I seem to remember a
microscopic photograph of this appearing in the company magazine.

Soon after this, all workshops around the country, were issued with pukka
discharging wands, which were made of a sort of hard cardboard tube, with
some resistors inside, a filed down threaded rod at the business end, and
a croc lead for grounding at the other - a bit like a typical EHT
multiplier probe for your AVO back in the 70's.

I was never sure that I really believed it, and continued to discharge
tubes in the ' traditional ' way by sticking a pair of crossed
screwdrivers across the cavity to the 'dag. To counter the charge
backwash effect, where the substantial charge retained by the CRT runs
away from the discharge point to the corners, then quietly creeps back to
have you just as you lift the tube off its mountings, I had an old EHT
lead with poppy connector on one end, and croc clip on the other. You
could clip this to the end of the 'dag grounding spring, to keep the tube
tamed ...

Arfa


That's very interesting, and I suppose it makes sense if you think about
it. You'd never discharge the mains smoothing cap by putting a screwdriver
across it- it'd damage the capacitor, as well as send chunks of white hot
metal flying around, so I suppose theoretically a CRT could be damaged by
directly shorting the anode.

However, one has to look at the sheer number of techs who use a direct
short to discharge CRTs, and the lack of CRTs suffering any visible
effects. Perhaps Rediffusion were being a little over cautious! I suppose
it would be a bad thing to do daily, but I expect the few times a TV
usually gets its CRT discharged is unlikely to have that much of an
effect.

Dave


That was pretty much my feeling also. However, I did one day get caught
kicking off the neck of a B+W tube that I'd just replaced. I wasn't wearing
any of the workshop safety equipment provided - facemask, apron, dustman's
gloves. Worse, the person who caught me was the regional chief engineer, a
person about 2 points to the left of God himself ... With him was my branch
chief engineer. Worst of all, I was the branch safety officer .... !!

To say that I got the bollocking to end all bollockings, is the epitomy of
understatement.

Oh happy days ! How did we ever survive without all the safety legislation
that there is in the workplace now ??

Arfa