Can an oscilloscope be safley used to check a TV set?

B

Blue News

Guest
Hi,

cant remember where I read this but I thought there was a danger in using an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can one
use a scope for the purpose?

thanks
Sean
 
"Blue News" <ss@ss.com> wrote in message news:<FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk>...
Hi,

cant remember where I read this but I thought there was a danger in using an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can one
use a scope for the purpose?

thanks
Sean
Click to expand...
Not just o'scopes, any test device. Use an isolation transformer if
there is ANY doubt, and even if there isn't.

GG
 
"Blue News" <ss@ss.com> wrote in message news:<FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk>...
Hi,

cant remember where I read this but I thought there was a danger in using an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can one
use a scope for the purpose?

thanks
Sean
Click to expand...
10 minutes ago, I said to use an isolation transformer if there is any
doubt as to the safety. I didn't say that the isolation transformer is
to be used on the TV, not the scope. Isolating the scope would allow
you to touch the scope ground to the TV BUT anything else (soldering
iron, computer, stereo... ) WOULD STILL BE HAZARDOUS. If you're not
aware of these issues, perhaps the TV is not the best thing to be
repairing.
GG
 
Agreed! One of the basic safety issues that most tech automatically do when
diagnosing any tele.
"Glenn Gundlach" <stratus46@yahoo.com> wrote in message
news:acb22b57.0402182005.6447b166@posting.google.com...
"Blue News" <ss@ss.com> wrote in message
news:<FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk>...
Hi,

cant remember where I read this but I thought there was a danger in
using an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can
one
use a scope for the purpose?

thanks
Sean

10 minutes ago, I said to use an isolation transformer if there is any
doubt as to the safety. I didn't say that the isolation transformer is
to be used on the TV, not the scope. Isolating the scope would allow
you to touch the scope ground to the TV BUT anything else (soldering
iron, computer, stereo... ) WOULD STILL BE HAZARDOUS. If you're not
aware of these issues, perhaps the TV is not the best thing to be
repairing.
GG
Click to expand...
 
"Blue News" <ss@ss.com> schreef in bericht
news:FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk...
Hi,

cant remember where I read this but I thought there was a danger in using
an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can
one
use a scope for the purpose?

thanks
Sean
Click to expand...
Sean,

Back in the days of the old electron tubes, the TV was often powered
directly by the mains. So all of its inside was connected to the mains and
repair without insulating transformer was always dangerous. Connecting a
non-insulated scope (or any other test device connected to protective
ground) caused an outbreak of fireworks.

These days TVs are powered using an SMPS. This SPMS contains a transformer
that insulates its mains part from the other parts of the TV circuit. So you
can do your measurements relatively safely in this parts of the TV. I write
*relatively* because a TV has dangerous high voltages inside even when
insulated from the mains.

But... The SMPS can be cause of the problem quite often. Touching the
non-insulated part of the SMPS - even shortly by accident - may still start
the fireworks. So most of the times an insulating transformer is still
necessary. Remind, even then there are dangerous voltages.

petrus



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Well after reading through all the responses, I guess I will just say NO! I
have been in normal low voltage electronics for years but have always
avoided TVs or monitors due to their high voltages and I have just been
reminded of the reasons again. So, thank you all for your inputs and stay
safe and I'll leave the TVs to the experts.

"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message
news:YGqZb.3168$A7.2702@amsnews05.chello.com...
"Blue News" <ss@ss.com> schreef in bericht
news:FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk...
Hi,

cant remember where I read this but I thought there was a danger in
using
an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can
one
use a scope for the purpose?

thanks
Sean



Sean,

Back in the days of the old electron tubes, the TV was often powered
directly by the mains. So all of its inside was connected to the mains and
repair without insulating transformer was always dangerous. Connecting a
non-insulated scope (or any other test device connected to protective
ground) caused an outbreak of fireworks.

These days TVs are powered using an SMPS. This SPMS contains a transformer
that insulates its mains part from the other parts of the TV circuit. So
you
can do your measurements relatively safely in this parts of the TV. I
write
*relatively* because a TV has dangerous high voltages inside even when
insulated from the mains.

But... The SMPS can be cause of the problem quite often. Touching the
non-insulated part of the SMPS - even shortly by accident - may still
start
the fireworks. So most of the times an insulating transformer is still
necessary. Remind, even then there are dangerous voltages.

petrus



---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.572 / Virus Database: 362 - Release Date: 27-1-2004
Click to expand...
 
On Sat, 21 Feb 2004 19:23:56 GMT, "Blue News" <ss@ss.com> Gave us:

Well after reading through all the responses, I guess I will just say NO! I
have been in normal low voltage electronics for years but have always
avoided TVs or monitors due to their high voltages and I have just been
reminded of the reasons again. So, thank you all for your inputs and stay
safe and I'll leave the TVs to the experts.
Click to expand...

Yeah... bone up on Usenet too, as top posting is just as retarded
as hooking up electronic instruments to HV sources.

Sorry so succinct... NOT!
 
stratus46@yahoo.com (Glenn Gundlach) wrote in message news:<acb22b57.0402182005.6447b166@posting.google.com>...
"Blue News" <ss@ss.com> wrote in message news:<FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk>...
Hi,

cant remember where I read this but I thought there was a danger in using an
oscilloscope to check a TV sets signal due to the chassis being directly
connected to mains or something like that - is that right, if so how can one
use a scope for the purpose?

thanks
Sean

10 minutes ago, I said to use an isolation transformer if there is any
doubt as to the safety. I didn't say that the isolation transformer is
to be used on the TV, not the scope. Isolating the scope would allow
you to touch the scope ground to the TV BUT anything else (soldering
iron, computer, stereo... ) WOULD STILL BE HAZARDOUS. If you're not
aware of these issues, perhaps the TV is not the best thing to be
repairing.
GG
Click to expand...
Not sure if my earlier post in sed got through (ISP news server has
problems), so excuse possible duplication:

Can you clarify how exactly the use of an isolation transformer helps
with the sort of measurements I sometimes want to make on 240 V mains
circuits please? My 'scope return probe remains grounded to mains
earth, so if I apply both probes across a component (even if fed via
an isolation transformer), it seesm to me that still risks shorting a
hot main-level node to earth. For example, I might be looking at the
voltage across an SCR or triac in a dimmer.

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
"Terry" <terrypin@dial.pipex.com> schreef in bericht
news:21688f52.0402220805.669f950e@posting.google.com...
stratus46@yahoo.com (Glenn Gundlach) wrote in message
news:<acb22b57.0402182005.6447b166@posting.google.com>...
"Blue News" <ss@ss.com> wrote in message
news:<FMRYb.102510$cL.2835@news-lhr.blueyonder.co.uk>...
Hi,

cant remember where I read this but I thought there was a danger in
using an
oscilloscope to check a TV sets signal due to the chassis being
directly
connected to mains or something like that - is that right, if so how
can one
use a scope for the purpose?

thanks
Sean

10 minutes ago, I said to use an isolation transformer if there is any
doubt as to the safety. I didn't say that the isolation transformer is
to be used on the TV, not the scope. Isolating the scope would allow
you to touch the scope ground to the TV BUT anything else (soldering
iron, computer, stereo... ) WOULD STILL BE HAZARDOUS. If you're not
aware of these issues, perhaps the TV is not the best thing to be
repairing.
GG

Not sure if my earlier post in sed got through (ISP news server has
problems), so excuse possible duplication:

Can you clarify how exactly the use of an isolation transformer helps
with the sort of measurements I sometimes want to make on 240 V mains
circuits please? My 'scope return probe remains grounded to mains
earth, so if I apply both probes across a component (even if fed via
an isolation transformer), it seesm to me that still risks shorting a
hot main-level node to earth. For example, I might be looking at the
voltage across an SCR or triac in a dimmer.

--
Terry Pinnell
Hobbyist, West Sussex, UK
Click to expand...

Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can choose wh
ich part of the circuit under test - if any - you connect to ground. I guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.

petrus




---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.593 / Virus Database: 376 - Release Date: 20-2-2004
 
"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message news:<nO5_b.13029$A7.3125@amsnews05.chello.com>...
Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can choose wh
ich part of the circuit under test - if any - you connect to ground. I guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.
Click to expand...
Thanks petrus. To make sure I really have this straight, could you
take a look at the illustration I've drawn at
http://www.terrypin.dial.pipex.com/Images/Isolation.gif
and tell me if that setup looks OK please?

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
"Terry" <terrypin@dial.pipex.com> schreef in bericht
news:21688f52.0402230117.2a4fdf91@posting.google.com...
"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message
news:<nO5_b.13029$A7.3125@amsnews05.chello.com>...
Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can
choose wh
ich part of the circuit under test - if any - you connect to ground. I
guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.

Thanks petrus. To make sure I really have this straight, could you
take a look at the illustration I've drawn at
http://www.terrypin.dial.pipex.com/Images/Isolation.gif
and tell me if that setup looks OK please?

--
Terry Pinnell
Hobbyist, West Sussex, UK
Click to expand...

Terry,

That's the way I've set my 'scope and insulation transformer.

petrus


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.593 / Virus Database: 376 - Release Date: 20-2-2004
 
Terry wrote:
"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message news:<nO5_b.13029$A7.3125@amsnews05.chello.com>...

Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can choose wh
ich part of the circuit under test - if any - you connect to ground. I guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.

Thanks petrus. To make sure I really have this straight, could you
take a look at the illustration I've drawn at
http://www.terrypin.dial.pipex.com/Images/Isolation.gif
and tell me if that setup looks OK please?
Click to expand...

At a test bench I (in the US) always have a solid mechanical ground
drawn directly to the bench. We use a star system for power
distribution in the US while in UK the common system is to
have a "ring".

The schematic you've provided clearly has a "ground loop" in
it and that could be a significant problem. For some
background take a look at this web page:

http://www.epanorama.net/documents/groundloop/index.html

and/or go a google search for "ground loop" (be sure to
use the double quotes.)
 
"Terry" <terrypin@dial.pipex.com> schreef in bericht
news:21688f52.0402230117.2a4fdf91@posting.google.com...
"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message
news:<nO5_b.13029$A7.3125@amsnews05.chello.com>...
Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can
choose wh
ich part of the circuit under test - if any - you connect to ground. I
guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.

Thanks petrus. To make sure I really have this straight, could you
take a look at the illustration I've drawn at
http://www.terrypin.dial.pipex.com/Images/Isolation.gif
and tell me if that setup looks OK please?

--
Terry Pinnell
Hobbyist, West Sussex, UK
Click to expand...



Terry,

Third time I post this reply. Hope it comes through this time.

That's the way I've set my 'scope and insulation transformer.

petrus




---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.593 / Virus Database: 376 - Release Date: 20-2-2004
 
"Bill Vajk" <bill9north@hotmailDITCHTHIS.com> schreef in bericht
news:7Z6dnXVmVMzpjqfdRVn-hg@comcast.com...
Terry wrote:
"petrus bitbyter" <p.kralt@hccnet.nl> wrote in message
news:<nO5_b.13029$A7.3125@amsnews05.chello.com>...

Terry,

Almost glad I'm not the only one whos provider has problems with news
servers :)

A insulating transformer is supposed to deliver mains voltage without a
galvanic connection to the mains. So on the secondary side you can
choose wh
ich part of the circuit under test - if any - you connect to ground. I
guess
your 'scopes frame has been connected to its common and to protective
ground, so you can connect the common of the circuit under test to the
'scopes frame. No need to say the voltages are still dangerous.

Thanks petrus. To make sure I really have this straight, could you
take a look at the illustration I've drawn at
http://www.terrypin.dial.pipex.com/Images/Isolation.gif
and tell me if that setup looks OK please?


At a test bench I (in the US) always have a solid mechanical ground
drawn directly to the bench. We use a star system for power
distribution in the US while in UK the common system is to
have a "ring".

The schematic you've provided clearly has a "ground loop" in
it and that could be a significant problem. For some
background take a look at this web page:

http://www.epanorama.net/documents/groundloop/index.html

and/or go a google search for "ground loop" (be sure to
use the double quotes.)
Click to expand...
I fail to see a ground loop in this setup. Maybe if one of your mains wires
has been connected to protective ground like seems to be done on the west
side of the pond. When that's the case you have two wires from the 'scope to
the mains earth. Even then I doubt it to give a problem but I have no
experience. In Europe the neutral is never connected to protective ground
but in the distribution station. Protective ground on the other hand is usua
lly "earthed" near the place the mains cable enters the house.

petrus


---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
Version: 6.0.593 / Virus Database: 376 - Release Date: 20-2-2004
 
"petrus bitbyter" <p.kralt@hccnet.nl> wrote:


Terry,

Third time I post this reply. Hope it comes through this time.

That's the way I've set my 'scope and insulation transformer.

petrus
Click to expand...
Yes thanks, received at least twice! You sound as if you're having as
much trouble as me with Usenet. Demon's service was down for 2 full
days. Finally got back to something like normal this morning.

--
Terry Pinnell
Hobbyist, West Sussex, UK
 
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