Adjusting pots on tube of monitor while switched on

[Jeff Liebermann]

Everything went fine with the picture changing until I moved around to
have a better look at the screen and the point of the screwdriver
slipped out of the pot and onto a live pin on the board ....
next thing I knew I was sat on the floor on the other side of the room
with a sore arm and a blackened screwdriver

Amazing. That was also one of my earlier introductions to electricity
when I was about 13 or 14 years old. I deduced that anything that
powerful was worth understanding.

I kinda miss the good old daze of tube radios and televisions. After
getting zapped a few times, one automatically developes a healthy
respect for high voltage. In the 2way radio business in the 1960's,
it was mostly tube radios. In Smog Angeles, we would kill off about 1
or 2 technicians each year, mostly from high voltage related
accidents. Darwin would be honored as the unworthy and careless
eliminated themselves.

My wakeup call was holding a cast zinc grounded microphone in one
hand, while probing around the high voltage cage with an NE-2 neon
lamp. The last thing I remember was a bright purple flash from the
NE-2 when I hit the plate cap.

I guess the only high voltage challenges left are CRT's, broadcasting,
and power transmission.

I did get an electrical engineering degree a few years later ......

Yep. I also managed to graduate college without getting killed or
drafted into the army. I even learned a few things along the way.

 

[Tom Horne]

Apologies if I underestimated you, but there are plenty of people who
apparently really believe that an electric shock will "throw you across
the room". I've no idea how they think it does this, and probably
neither have they, but they've heard it somewhere so it must be true
and will tenaciously defend this viewpoint against all attempts to
correct it.

Rod.

I have to explain that one to apprentices all the time. What throws you
across the room is your own muscles. When an electric shock causes all
of them to contract the stronger ones win the argument and their violent
contracting does the rest. If the shock does not kill you your own body
might throw you off a ladder or into a higher energy shock.

Yes I know that many of you new that but for those that didn't...

 

[Derek ^]

Greater susceptibility to noise on received sugnals, which would cause
raggedness of vertical lines due to variations in line timing. There's a
reason for using what was called "flywheel sync". A shorter time constant
effectively made it work with a smaller "flywheel" - OK for clean locally
generated video signals, but not good for off-air material.

I've only just realised reading this that it's *years* since I saw
ignition interference on my TV, there was a time when everything from
an electric razor upwards could flatten the received signal.

DG

 

[Dave D]

i thought that some posts to a very recent thread said that HT was not
actually so very dangerous but would throw you across a room (the main
danger being how you landed).

To clarify- Mains areas inside a monitor or TV are potentially lethal
because there is enough current for the full voltage to be presented across
the body, or from hand to earth and it is either low frequency AC or DC from
the mains smoothing caps- very nasty. There is more than enough available
current to kill- IIRC it only takes around 100mA to do the job! Luckily,
unless one is sweaty or wet, there's a good chance the body's internal
resistance will keep the current well below that and make it a nasty,
frightening experience, rather than meaning your family going out to pick a
coffin!

The EHT, or anode voltage, inside a TV monitor is a much higher voltage, but
as the supply is high impedance the available current is very small (a
couple of milliamps or so), so as soon as it is presented across your body
it loads the supply and the voltage dips dramatically. Also, very high
voltages have a habit of preferring to travel across the skin rather than
through your body. The same thing applies to high frequency pulsed DC like
we have in TVs and monitors, so the effect is probably even more pronounced.

AIUI the most lethal voltage range is surprisingly low- 200V to 2000V or
thereabouts if my memory serves.
EHT at high currents (power grid etc) tends to cause a corona around the
victim and massive burns, but it is not unheard of to survive power line
shocks at several KVs!

Dave

 

[Tom MacIntyre]

Amazing. That was also one of my earlier introductions to electricity
when I was about 13 or 14 years old. I deduced that anything that
powerful was worth understanding.

I kinda miss the good old daze of tube radios and televisions. After
getting zapped a few times, one automatically developes a healthy
respect for high voltage. In the 2way radio business in the 1960's,
it was mostly tube radios. In Smog Angeles, we would kill off about 1
or 2 technicians each year, mostly from high voltage related
accidents. Darwin would be honored as the unworthy and careless
eliminated themselves.

My wakeup call was holding a cast zinc grounded microphone in one
hand, while probing around the high voltage cage with an NE-2 neon
lamp. The last thing I remember was a bright purple flash from the
NE-2 when I hit the plate cap.

I guess the only high voltage challenges left are CRT's, broadcasting,
and power transmission.

Don't try a MWO capacitor...apparently the worst hazard in consumer
repair...never tried it.

Tom

 

[Jeff Liebermann]

AIUI the most lethal voltage range is surprisingly low- 200V to 2000V or
thereabouts if my memory serves.

Lower. See:
http://en.wikipedia.org/wiki/Electric_shock
"It is believed that human lethality is most common with AC
current at 100-250 volts, as lower voltages can fail to overcome
body resistance while with higher voltages the victim's muscular
contractions are often severe enough to cause them to recoil
(although there will be considerable burn damage). However,
death has occurred from supplies as low as 32 volts."

 

[Sam Goldwasser]

To clarify- Mains areas inside a monitor or TV are potentially lethal
because there is enough current for the full voltage to be presented across
the body, or from hand to earth and it is either low frequency AC or DC from
the mains smoothing caps- very nasty. There is more than enough available
current to kill- IIRC it only takes around 100mA to do the job! Luckily,
unless one is sweaty or wet, there's a good chance the body's internal
resistance will keep the current well below that and make it a nasty,
frightening experience, rather than meaning your family going out to pick a
coffin!

The EHT, or anode voltage, inside a TV monitor is a much higher voltage, but
as the supply is high impedance the available current is very small (a
couple of milliamps or so), so as soon as it is presented across your body
it loads the supply and the voltage dips dramatically. Also, very high
voltages have a habit of preferring to travel across the skin rather than
through your body. The same thing applies to high frequency pulsed DC like
we have in TVs and monitors, so the effect is probably even more pronounced.

High voltages don't in thermselves prefer the surface. It's the high
frequencies of Tesla coils and to some extent TV and monitor horizontal
deflection that do this.

HV DC will take the path of least resistance.

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Important: Anything sent to the email address in the message header above is
ignored unless my full name AND either lasers or electronics is included in the
subject line. Or, you can contact me via the Feedback Form in the FAQs.

 

[Tom MacIntyre]

Lower. See:
http://en.wikipedia.org/wiki/Electric_shock
"It is believed that human lethality is most common with AC
current at 100-250 volts, as lower voltages can fail to overcome
body resistance while with higher voltages the victim's muscular
contractions are often severe enough to cause them to recoil
(although there will be considerable burn damage). However,
death has occurred from supplies as low as 32 volts."

And, if I remember right, depending upon the person involved,
somewhere in the 50mA-150mA range is where the heart's rhythm gets
disrupted...I may be wrong.

Tom

 

[Jeff Liebermann]

And, if I remember right, depending upon the person involved,
somewhere in the 50mA-150mA range is where the heart's rhythm gets
disrupted...I may be wrong.
Tom

Yep. The point where fibrillation occurs is where you have problems.
Instead of pumping at 50-70 times per minute, the heard sorta vibrates
at a much higher rate. There's not enough time for blood to go in and
out of the various heart valves and chambers, so blood flow
effectively stops. Giving the heart a good blast of the right
frequency pulses gets it back into sync. I don't know the current
range required to induce fibrillation, but it's not very much.

Anyway, my point was that the most dangerous voltage range is common
household AC power.

 

[Jeff Liebermann]

High voltages don't in thermselves prefer the surface. It's the high
frequencies of Tesla coils and to some extent TV and monitor horizontal
deflection that do this.

Yep, sorta. The common explanation of why you don't get electrocuted
with RF frequencies is because skin effect causes it to flow on the
surface, where there's little chance of heart and muscle damage.
However, there are those that suggest that the reason RF frequencies
have no effect on human muscles is that the frequency response of
those muscles is insufficiently high for RF to have an effect. At
relatively low frequencies (i.e. 50/60Hz), the muscles respond quite
nicely and violently. At much higher frequencies, they just sit
there.

HV DC will take the path of least resistance.

DC is also bad because it makes the muscles contract, causing one to
be unable to let go of whatever is causing the shock. Many years ago,
I learned that if you wanna use your hand to check for voltage, use
the back of the hand. If you use the fingers or palm side, you could
easily end up grabbing the wire and not being able to let go.

Incidentally, based on atmospheric potential differences, we have
about 200VDC between our head and our toes. So, why don't we get
electrocuted?

 

[Dave D]

High voltages don't in thermselves prefer the surface.

I stand corrected!

Dave

 

[SpamTrapSeeSig]

Don't try a MWO capacitor...apparently the worst hazard in consumer
repair...never tried it.

You wouldn't be posting if you had!

Considering the energy stored and the voltage, it would be almost
certainly fatal if it discharged through the body (instead of across
your hand to the chassis). Nowadays many (most?) of them have a high
value (10 M Ohm?) parallel resistor built into the package so that they
self discharge, but not all of them!

I was highly relieved to find that, when ours started misbehaving, it
was a microswitch, and I didn't have to play with the HT side...


Regards,

Simonm.

 

[Richard Lamont]

Don't try a MWO capacitor...apparently the worst hazard in consumer
repair...never tried it.

What is an MWO capacitor?

 

[Tony Quinn]

What is an MWO capacitor?

I assume Microwave Oven .......

 

[Jeff Liebermann]

What is an MWO capacitor?

Multi-Wave Oscillator.
http://www.altered-state.com/index2.htm?/lmwo/lmwo.htm
http://www.altered-states.net/barry/newsletter161/index.htm
http://www.braintuner.com/mwo.htm
http://www.copenlabs.com/copenmwo.htm
Cures cancer. Satisfaction guaranteed if you live to collect your
money back.

 

[Ashley]

You wouldn't be posting if you had!

Considering the energy stored and the voltage, it would be almost
certainly fatal if it discharged through the body (instead of across
your hand to the chassis). Nowadays many (most?) of them have a high
value (10 M Ohm?) parallel resistor built into the package so that
they self discharge, but not all of them!

I was highly relieved to find that, when ours started misbehaving, it
was a microswitch, and I didn't have to play with the HT side...


Regards,

Simonm.

Mine's just packed up and it's the EHT side!

Found the 550mA 5kV fuse blown. Can't find any shorts so it might just
be the fuse. It's about £7 for a new fuse. Quite a lot if it just blows
again!

 

[Pat Horridge]

Mine's just packed up and it's the EHT side!

Found the 550mA 5kV fuse blown. Can't find any shorts so it might just
be the fuse. It's about £7 for a new fuse. Quite a lot if it just blows
again!

You know the answer.
replace the fuse with a solid short.
Power up in the garden on a long mains lead.
If it goes with a big bang then whatever was at fault will be clear to see
and you can thorw it all away.
If it works then replace the short with the new fuse.
Most likely it's either the capacitor or the magnatron which will make it
cheaper to replace then repair.

 

[SpamTrapSeeSig]

You know the answer.
replace the fuse with a solid short.
Power up in the garden on a long mains lead.
If it goes with a big bang then whatever was at fault will be clear to see
and you can thorw it all away.
If it works then replace the short with the new fuse.
Most likely it's either the capacitor or the magnatron which will make it
cheaper to replace then repair.

Won't comment on the technique (!) but I found spares etc. fairly easy
to obtain on the net. True if it's the Magnetron it'll not be worth it,
but the HT caps weren't that bad, neither were the diodes. Mine's a
Panasonic, YMMV.

Regards,

Simonm.