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Adjusting pots on tube of monitor while switched on

Chris Vowles hath wroth:

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.

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

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann
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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...

-- Tom of the sparks and arcs

"This alternating current thing is just a fad. It is much too dangerous for general use." Thomas Alva Edison

Reply to
Tom Horne

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

Reply to
Derek ^

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

Reply to
Dave D

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

Tom

Reply to
Tom MacIntyre

"Dave D" hath wroth:

Lower. See:

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"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."

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
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.

HV DC will take the path of least resistance.

--- sam | Sci.Electronics.Repair FAQ:

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Reply to
Sam Goldwasser

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

Reply to
Tom MacIntyre

Tom MacIntyre hath wroth:

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     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

Sam Goldwasser hath wroth:

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.

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?

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

I stand corrected!

Dave

Reply to
Dave D

In article , Tom MacIntyre writes

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.

--
simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay
SIMON MUIR, BRISTOL UK                                      www.ukip.org
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Reply to
SpamTrapSeeSig

What is an MWO capacitor?

-- Richard Lamont

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Reply to
Richard Lamont

I assume Microwave Oven .......

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Reply to
Tony Quinn

Richard Lamont hath wroth:

Multi-Wave Oscillator.

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Cures cancer. Satisfaction guaranteed if you live to collect your money back.

--
Jeff Liebermann     jeffl@comix.santa-cruz.ca.us
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558
Reply to
Jeff Liebermann

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! :)

--
Ashley
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Reply to
Ashley

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.

Reply to
Pat Horridge

In article , Pat Horridge writes

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.

--
simonm|at|muircom|dot|demon|.|c|oh|dot|u|kay
SIMON MUIR, BRISTOL UK                                      www.ukip.org
EUROPEANS AGAINST THE EU                     www.members.aol.com/eurofaq
GT250A'76  R80/RT'86  110CSW TD'88  www.kc3ltd.co.uk/profile/eurofollie/
Reply to
SpamTrapSeeSig

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