Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a floating
ground?

There seems to be 30V ac with reference to the scope ground when I use
a VOM to measure voltage. Will it blow or damage the scope? Anyway to
prevent sending live current down the scope ground and blowing it up?
Thanks.

rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a floating
ground?

There seems to be 30V ac with reference to the scope ground when I use
a VOM to measure voltage. Will it blow or damage the scope? Anyway to
prevent sending live current down the scope ground and blowing it up?
Thanks.


The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope ground.
Second you learn about the potential (in both senses) dangers associated
with TVs

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:
rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a floating
ground?

There seems to be 30V ac with reference to the scope ground when I use
a VOM to measure voltage. Will it blow or damage the scope? Anyway to
prevent sending live current down the scope ground and blowing it up?
Thanks.


The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope ground.
Second you learn about the potential (in both senses) dangers associated
with TVs

--

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"rogersk8ter" wrote in
ups.com:


N Cook wrote:
rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a
floating ground?

There seems to be 30V ac with reference to the scope ground when I
use a VOM to measure voltage.

Don't forget DCV;the chassis can be as much as 170VDC difference from
earth ground for a 120VAC line supply.


Will it blow or damage the scope?
Anyway to prevent sending live current down the scope ground and
blowing it up? Thanks.


The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope
ground. Second you learn about the potential (in both senses) dangers
associated with TVs

--

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?


It's a MUST. Repeat;a MUST.

TV chassis can be electrically hot WRT earth ground.
Just look at any switching power suppply schematic.
There's no ground,instead "common".

Hook your scope ground to it and you blow the TV and/or your scope,make
lots of sparks and smoke.

Or your scope case becomes electrically hot and you touch it and a real
ground,you get electrocuted.(that's why you don't use groundbusters.)

--
Jim Yanik
jyanik
at
kua.net

rogersk8ter wrote:
snip

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?

If you have to ask, it's MANDATORY - NO EXCEPTIONS !!!

GG

Jim Yanik wrote in message
...
"rogersk8ter" wrote in
ups.com:


N Cook wrote:
rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a
floating ground?

There seems to be 30V ac with reference to the scope ground when I
use a VOM to measure voltage.

Don't forget DCV;the chassis can be as much as 170VDC difference from
earth ground for a 120VAC line supply.


Will it blow or damage the scope?
Anyway to prevent sending live current down the scope ground and
blowing it up? Thanks.


The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope
ground. Second you learn about the potential (in both senses) dangers
associated with TVs

--

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?


It's a MUST. Repeat;a MUST.

TV chassis can be electrically hot WRT earth ground.
Just look at any switching power suppply schematic.
There's no ground,instead "common".

Hook your scope ground to it and you blow the TV and/or your scope,make
lots of sparks and smoke.

Or your scope case becomes electrically hot and you touch it and a real
ground,you get electrocuted.(that's why you don't use groundbusters.)

--
Jim Yanik
jyanik
at
kua.net

Could you explain to a Limey what a groundbuster is and I'll add to this "2
nations divided by a common language" file

UK / USA Tool Terminology Translator
http://www.divdev.fsnet.co.uk/tool_terms.htm

N Cook wrote in message
...
Jim Yanik wrote in message
...
"rogersk8ter" wrote in
ups.com:


N Cook wrote:
rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the probe's
ground? Over the common ground on the PCB board? Isn't that a
floating ground?

There seems to be 30V ac with reference to the scope ground when I
use a VOM to measure voltage.

Don't forget DCV;the chassis can be as much as 170VDC difference from
earth ground for a 120VAC line supply.


Will it blow or damage the scope?
Anyway to prevent sending live current down the scope ground and
blowing it up? Thanks.


The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope
ground. Second you learn about the potential (in both senses) dangers
associated with TVs

--

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?


It's a MUST. Repeat;a MUST.

TV chassis can be electrically hot WRT earth ground.
Just look at any switching power suppply schematic.
There's no ground,instead "common".

Hook your scope ground to it and you blow the TV and/or your scope,make
lots of sparks and smoke.

Or your scope case becomes electrically hot and you touch it and a real
ground,you get electrocuted.(that's why you don't use groundbusters.)

--
Jim Yanik
jyanik
at
kua.net

Could you explain to a Limey what a groundbuster is and I'll add to this
"2
nations divided by a common language" file

UK / USA Tool Terminology Translator
http://www.divdev.fsnet.co.uk/tool_terms.htm



I take it from Googling that a ground buster / groundbuster is an adaptor
that simply converts a 3 prong AC plug into a 2 prong.
In the UK they don't exist as such , the third earth/ground pin is needed to
open the safety sliding shrouds covering the powered sections of the socket.
"wall warts" for the UK always seem to have plastic earth/ground pins which
have to be there as dummies so you can physically plug into UK power
sockets.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

"N Cook" wrote in
:

N Cook wrote in message
...
Jim Yanik wrote in message
...
"rogersk8ter" wrote in
ups.com:


N Cook wrote:
rogersk8ter wrote in message
ups.com...
Im trying to scope a TV signal. Where should I ground the
probe's ground? Over the common ground on the PCB board? Isn't
that a floating ground?

There seems to be 30V ac with reference to the scope ground
when I use a VOM to measure voltage.

Don't forget DCV;the chassis can be as much as 170VDC difference
from earth ground for a 120VAC line supply.


Will it blow or damage the scope?
Anyway to prevent sending live current down the scope ground
and blowing it up? Thanks.


The first thing you do is obtain an isolation transformer
110/110V or 240/240V that allows TV to float up or down relative
to the scope ground. Second you learn about the potential (in
both senses) dangers associated with TVs

--

Can I just ground t to the TV chasis? Where the springs is? Or a
isolation transformer is a "must"?


It's a MUST. Repeat;a MUST.

TV chassis can be electrically hot WRT earth ground.
Just look at any switching power suppply schematic.
There's no ground,instead "common".

Hook your scope ground to it and you blow the TV and/or your
scope,make lots of sparks and smoke.

Or your scope case becomes electrically hot and you touch it and a
real ground,you get electrocuted.(that's why you don't use
groundbusters.)

--
Jim Yanik
jyanik
at
kua.net

Could you explain to a Limey what a groundbuster is and I'll add to
this
"2
nations divided by a common language" file

UK / USA Tool Terminology Translator
http://www.divdev.fsnet.co.uk/tool_terms.htm



I take it from Googling that a ground buster / groundbuster is an
adaptor that simply converts a 3 prong AC plug into a 2 prong.

Correct.

In the UK they don't exist as such , the third earth/ground pin is
needed to open the safety sliding shrouds covering the powered
sections of the socket. "wall warts" for the UK always seem to have
plastic earth/ground pins which have to be there as dummies so you can
physically plug into UK power sockets.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/




But one could still cut the ground WIRE either at the plug or inside the
scope to disable the ground(to "float the scope").
At least one scope offered a SWITCH to do this,others solder in diodes to
"isolate" the ground.Bad idea,as you have no idea if the diodes are always
functional and not shorted or open.

Tektronix used to offer a pamphlet about scope measurements and ground
safety.I used to include one with every scope I repaired that had the
ground pin cut off or otherwise disabled.It explained why various methods
of "floating" a scope are unsafe.
IIRC,TEK now has something similar on their website.

--
Jim Yanik
jyanik
at
kua.net

Jim Yanik wrote in
:

Tektronix used to offer a pamphlet about scope measurements and ground
safety....It explained why various
methods of "floating" a scope are unsafe.

IIRC,TEK now has something similar on their website.

Yep, here it is:

http://www.tek.com/Measurement/cgi-bin/framed.pl?
Document=/Measurement/App_Notes/Technical_Briefs/tds3000-
float/eng/limitations.html&FrameSet=oscilloscopes

Quoting from the web page,

"Floating" a ground referenced oscilloscope is the technique of defeating
the oscilloscope's protective grounding system - disconnecting "signal
common" from earth, either by defeating the grounding system or using an
isolation transformer. This allows accessible parts of the instrument
such as chassis, cabinet, and connectors to assume the potential of the
probe ground lead connection point. This is dangerous, not only from the
standpoint of elevated voltages present on the oscilloscope (a shock
hazard to the operator), but also due to cumulative stresses on the
oscilloscope's power transformer insulation. This stress may not cause
immediate failure, but may lead to future dangerous failures (a shock and
fire hazard), even after returning the oscilloscope to properly grounded
operation!

Not only is floating a ground-referenced oscilloscope dangerous, but the
measurements are often inaccurate. This results from the total
capacitance of the oscilloscope chassis being directly connected to the
circuit under test at the point where the common lead is connected.

WARNING

Never attempt to defeat the protective grounding system of your
oscilloscope by using an isolation transformer or disconnecting the
ground connector on the power plug. Failure to follow safety warnings can
result in serious injury or loss of life.