"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