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:6LRYb.102503$cL.9597@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...

Your right, the metalwork of the TV or whatever will probably be at mains
potential whilst the scope metalwork case, probe return etc will be at
ground potential and there will be a bang when they meet. What you do is
isolate the test item from "live" mains by using an isolating transformer.
don't cut corners here the scope must be correctly earthed.
you might have read about it here http://www.repairfaq.org/ if you
didn't, you should have. so do it BEFORE you blow the fuses.
Pete
 
The case of the scope will most likely be true ground. The TV set may be a
hot chassis type. The TV must be run on an isolation transformer before
using any type of test instrument on it, that is especially connected to the
AC mains.

If the ground of the scope meets a hot chassis TV set, there may be a short,
and thus cause damage to both the scope and the TV set.

Scopes are an essential instrument for troubleshooting most equipment at in
debt levels. We use scopes for evaluating the various pulses and waveforms
in TV equipment, as well as many other types.

Care must be taken for high voltages that can damage the front end of the
scope. I have seen many times where a tech blew the front end of the scope.
There are special high voltage type probes for scopes for when working in
circuits that put out very high amplitudes of voltages to be tested. A very
dangerous area is the horizontal output stage, switching drivers, and high
voltage drive circuits. The focus volts to the CRT is another area that may
damage the scope.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
=========================================


"Blue News" <ss@ss.com> wrote in message
news:6LRYb.102503$cL.9597@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
 
"Blue News" <ss@ss.com> wrote in message
news:6LRYb.102503$cL.9597@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...
Get an isolation transformer to power the TV, they're available cheaply from
vendors that sell parts for arcade games as most older arcade monitors
require one. Should cost you about $10.
 
Thank you all for your informative replies.
I thought the newer type of TVs (ones with video input or scart) didn't have
a mains potential on the chassis, otherwise we would almost have the same
problem? so am I correct in assuming newer TVs would be a safe bet?

Sean


"Blue News" <ss@ss.com> wrote in message
news:6LRYb.102503$cL.9597@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...
 
And somewhere around the time of 02/18/2004 14:33, the world stopped and
listened as Blue News contributed the following to humanity:

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...
You need to use an isolation transformer to isolate the TV from the
line. This only applies to modern transformerless TVs that use
switching power supplies.

--
Daniel Rudy

Remove nospam, invalid, and 0123456789 to reply.
 
Thank you!
I thought the new TVs were isolated due to their video input etc. thought it
was only the old TVs needed isolating?

"Daniel Rudy" <dcrudy@invalid.pacbell.nospam.net.0123456789> wrote in
message news:_CjZb.15076$3H6.10017@newssvr29.news.prodigy.com...
And somewhere around the time of 02/18/2004 14:33, the world stopped and
listened as Blue News contributed the following to humanity:

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



You need to use an isolation transformer to isolate the TV from the
line. This only applies to modern transformerless TVs that use
switching power supplies.

--
Daniel Rudy

Remove nospam, invalid, and 0123456789 to reply.
Click to expand...
 
Thank you!
I thought the new TVs were isolated due to their video input etc. thought it
was only the old TVs needed isolating?
Click to expand...
Most of them are isolated, but not all. Some of the lower priced sets still use
a hot chassis. You can tell by checking the resistance with a meter between the
CATV input or AV inputs and the tuner. If they are directly connected, you
don't have a hot chassis and it's safe to use the scope. If there's a high
resistance measured, you need an isolation transformer to do the job safely.
Ron
 
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