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Adjusting pots on tube of monitor while switched on

D

Dave D

Jan 1, 1970
0
I assume that this was mains voltage rather than EHT, otherwise you
probably wouldn't be around to tell the tale.

The EHT in a TV (anode voltage) is *far* less of a threat to life than the
mains.


Dave
 
M

Michael Kennedy

Jan 1, 1970
0
In the late sixties I worked with a BBC Electronic Services engineer who
regularly used to check whether EHT was present on a faulty monitor by
removing the plug from the tube and sticking his thumb on it.

Ouch! That hurts.. I got zapped from the neckboard on my ms pacman cocktail
table while reaching around inside with it on.. That was a mistake that I
don't plan to repeat any time soon.. ;-)

- Mike
 
M

Mark Carver

Jan 1, 1970
0
Chris said:
In the late sixties I worked with a BBC Electronic Services engineer who
regularly used to check whether EHT was present on a faulty monitor by
removing the plug from the tube and sticking his thumb on it.

My grandfather's Christmas party piece was to open up the tin box that
housed the EHT rectifier, and with a damp wooden handle screwdriver,
draw an arc from the anode terminal.
 
R

Roderick Stewart

Jan 1, 1970
0
My grandfather's Christmas party piece was to open up the tin box that
housed the EHT rectifier, and with a damp wooden handle screwdriver,
draw an arc from the anode terminal.

I remember one of those translucent orange handled screwdrivers would do
quite well too. I've no idea what the resistance of the handle would be, but
I'd trust it to be a bit more consistent than a wooden spoon.

Slightly safer now of course, though not quite so much fun, because voltage
triplers are the usual thing - not such a big spark.

Rod.
 
A

Ashley

Jan 1, 1970
0
Roderick said:
I remember one of those translucent orange handled screwdrivers would
do quite well too. I've no idea what the resistance of the handle
would be, but I'd trust it to be a bit more consistent than a wooden
spoon.

Slightly safer now of course, though not quite so much fun, because
voltage triplers are the usual thing - not such a big spark.

Rod.

My father had an eht voltmeter that consisted of a calibrated spark
gap. You turned a knob bringing the balls closer together until the
spark jumped the gap. Then you read off the voltage on a scale.
 
C

Clive

Jan 1, 1970
0
Ashley said:
My father had an eht voltmeter that consisted of a calibrated spark
gap. You turned a knob bringing the balls closer together until the
spark jumped the gap. Then you read off the voltage on a scale.

How accurate would this have been ?

Wouldn't humidity / air quality have a measurable an effect on a spark gap ?

//Clive.
 
T

tony sayer

Jan 1, 1970
0
Mark Carver said:
My grandfather's Christmas party piece was to open up the tin box that
housed the EHT rectifier, and with a damp wooden handle screwdriver,
draw an arc from the anode terminal.

We used to do that to test for EHT, firm we worked for in those days was
too bl**dy tight to buy one.

Had the odd belt of many a CRT that wasn't discharged, but by far and
away were aerial isolator plates that had been bypassed and that wasn't
much fun if you we working up on a roof.

Metal ladder, damp soil, live aerial..Nasty!.....
 
M

Mark Carver

Jan 1, 1970
0
Clive said:
How accurate would this have been ?

Wouldn't humidity / air quality have a measurable an effect on a spark gap ?

Were they not sealed devices ?
 
S

Sam Goldwasser

Jan 1, 1970
0
Roderick Stewart said:
I remember one of those translucent orange handled screwdrivers would do
quite well too. I've no idea what the resistance of the handle would be, but
I'd trust it to be a bit more consistent than a wooden spoon.

Slightly safer now of course, though not quite so much fun, because voltage
triplers are the usual thing - not such a big spark.

The anode terminal of the HV rectifier tube was connected to the flyback
which was 15.7 kHz AC - similar to the output from a small Tesla
coil. So, as long as the screwdriver had some capacitance, the conductivity
of the handle probably didn't matter much.

With modern flybacks, the output is DC filtered by the CRT capacitance,
so the effects are quite different.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
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R

Roderick Stewart

Jan 1, 1970
0
The anode terminal of the HV rectifier tube was connected to the flyback
which was 15.7 kHz AC - similar to the output from a small Tesla
coil. So, as long as the screwdriver had some capacitance, the conductivity
of the handle probably didn't matter much.

I was referring to the fact that most modern EHT circuits will only have 8kV
pulses instead of 25kV pulses, because they use a diode/capacitor multiplier and
a lower voltage transformer.
With modern flybacks, the output is DC filtered by the CRT capacitance,
so the effects are quite different.

That happens anyway doesn't it? The coatings on the inside and outside of the
bowl of the CRT constitute the EHT smoothing capacitor.

Rod.
 
S

Sam Goldwasser

Jan 1, 1970
0
Roderick Stewart said:
I was referring to the fact that most modern EHT circuits will only have 8kV
pulses instead of 25kV pulses, because they use a diode/capacitor multiplier and
a lower voltage transformer.


That happens anyway doesn't it? The coatings on the inside and outside of the
bowl of the CRT constitute the EHT smoothing capacitor.

What I was trying to say is that in a modern CRT TV or monitor, there is
no place to find the full HV pulses - 5 kV or 25 kV - since the only
output of the flyback goes directly to the CRT anode and its capacitance.

In old once, the flyback output went to the 1B3 (or whatever) anode and
was AC at that point.

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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
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R

Roderick Stewart

Jan 1, 1970
0
What I was trying to say is that in a modern CRT TV or monitor, there is
no place to find the full HV pulses - 5 kV or 25 kV - since the only
output of the flyback goes directly to the CRT anode and its capacitance.

If you mean the multiplier, then yes, it would incorporate the EHT rectifier which
would otherwise have been a separate diode. The output terminal of the multilplier
unit would be the cathode of the diode, so no AC at that point.
In old once, the flyback output went to the 1B3 (or whatever) anode and
was AC at that point.

Yes, a whacking great pulse equal to the full EHT potential onto the anode of a
separate diode. Treat with extreme respect!

Rod.
 
A

Andy

Jan 1, 1970
0
They are convergence pots. They alter the relationship of the
geometries between the beams. The red, green and blue images which
make up the colour picture on your monitor must overlap as close to
perfectly as possible, otherwise you will see red, green or blue
fringes on different parts of the image. They should only ever be
altered by someone who understands how to set them up- they can be
very difficult to get back if you don't know what you're doing.

If the picture looks OK, you've got away with it.

Phew!



At least you didn't alter the static convergence rings on the CRT
neck- that really would have caused problems.

Hmmm. Let me have a look and see if I can move .... :)
 
A

Alex Coleman

Jan 1, 1970
0
I assume that this was mains voltage rather than EHT, otherwise you
probably wouldn't be around to tell the tale.


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).
 
S

Sam Goldwasser

Jan 1, 1970
0
Andy said:
Hmmm. Let me have a look and see if I can move .... :)

Everyone loves to diddle with those magnet rings.... :)

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
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| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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
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R

Roderick Stewart

Jan 1, 1970
0
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).

Electricity doesn't "throw you across a room", so if somebody's
description included this statement you know how much you can trust the
rest of it.

If you find yourself somewhere else in the room after an electric shock,
it will have been your own muscles that put you there.

Rod.
 
A

Alex Coleman

Jan 1, 1970
0
Electricity doesn't "throw you across a room", so if somebody's
description included this statement you know how much you can trust
the rest of it.

If you find yourself somewhere else in the room after an electric
shock, it will have been your own muscles that put you there.

Steady on Roderick! Now that's a wee bit pedantic.
Are you not happy with ellipsis? Otherwise we will have to sort out
things like:

Water does not satisfiy your thirst, it satisfies your body's thirst.

A car does not run on pertrol, a car's engine runs on petrol.

You do not watch television, you watch images on the tube of the
television.

PHEW!
:-0
 
D

David Nebenzahl

Jan 1, 1970
0
Alex Coleman spake thus:
Steady on Roderick! Now that's a wee bit pedantic.
Are you not happy with ellipsis? Otherwise we will have to sort out
things like:

Water does not satisfiy your thirst, it satisfies your body's thirst.

A car does not run on pertrol, a car's engine runs on petrol.

You do not watch television, you watch images on the tube of the
television.

PHEW!

Thank you for de-pedantifying that.


--
Pierre, mon ami. Jetez encore un Scientologiste
dans le baquet d'acide.

- from a posting in alt.religion.scientology titled
"France recommends dissolving Scientologists"
 
S

Sam Goldwasser

Jan 1, 1970
0
Alex Coleman said:
Steady on Roderick! Now that's a wee bit pedantic.
Are you not happy with ellipsis? Otherwise we will have to sort out
things like:

Water does not satisfiy your thirst, it satisfies your body's thirst.

A car does not run on pertrol, a car's engine runs on petrol.

You do not watch television, you watch images on the tube of the
television.

No, the images and sound are usually a waste of time. Watching the
electronics is more educational. :)

--- sam | Sci.Electronics.Repair FAQ: http://www.repairfaq.org/
Repair | Main Table of Contents: http://www.repairfaq.org/REPAIR/
+Lasers | Sam's Laser FAQ: http://www.repairfaq.org/sam/lasersam.htm
| Mirror Sites: http://www.repairfaq.org/REPAIR/F_mirror.html

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.
 
R

Roderick Stewart

Jan 1, 1970
0
Steady on Roderick! Now that's a wee bit pedantic.
Are you not happy with ellipsis?

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.
 
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