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

B

Blue News

Jan 1, 1970
0
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
 
J

John Woodgate

Jan 1, 1970
0
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?
Feed the TV though a proper safety isolating transformer.
 
S

Stefan Heinzmann

Jan 1, 1970
0
Blue said:
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?

It depends on the TV set whether the chassis is live or not. If you're
unsure you would measure the chassis first. If it is live then
connecting the scope probe's ground to the chassis will have unwelcome
consequences.

Possible workarounds are:
o Using an isolation transformer with the TV set
o Using differential probing
o Using isolating probes
o Lifting the scope's ground or using it with an isolation transformer
(Caution *dangerous*! The scope case will be live!)
 
P

Philip A. Marshall

Jan 1, 1970
0
It depends on the TV set whether the chassis is live or not. If you're
unsure you would measure the chassis first. If it is live then
connecting the scope probe's ground to the chassis will have unwelcome
consequences.

Possible workarounds are:
o Using an isolation transformer with the TV set
o Using differential probing
o Using isolating probes

o Lifting the scope's ground or using it with an isolation transformer
(Caution *dangerous*! The scope case will be live!)

A grad student around here isolated his scope that way to make
measurements on a three-phase power system. Everything was fine until
he made the mistake of hooking up his serial printer (which he
neglected to isolate) to the scope to make a printout of one of the
traces...
 
J

Jan Panteltje

Jan 1, 1970
0
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
It is right that in most OLDER TV sets the chassis was 'live', usually there
was a bridge rectifier and the minus connected to it.
Other configurations existed.
Then later came the TV sets with video input etc.., these have a separate
mains supply unit, that separates the set from the mains.

In a workshop you should ALWAYS use a 1:1 mains separation
transformer of ENOUGH watts, and that maybe as much as several hundred for
old sets.

I admit to doing it without on occasion, but also got alomost killed that way.

So, ground your scope, use a separations transformer.

You should make sure the secondairy does not drop if loaded with a set,
some old sets, for example the Blaupunkt thyristor ones, chopped up so bad that
even a 350W transformer started to smell..
This is because the losses are i^2 x R, and rectifiers and other stuff draw only
current in a small part of the wave, and then much more then it would be if
it was a purely resistive load.
JP
Copyright Jan Panteltje 2004 All Rights reserved.
 
M

Martin Riddle

Jan 1, 1970
0
Philip A. Marshall said:
A grad student around here isolated his scope that way to make
measurements on a three-phase power system. Everything was fine until
he made the mistake of hooking up his serial printer (which he
neglected to isolate) to the scope to make a printout of one of the
traces...

I ceases to amaze me that very few know how to use a scope in differential mode.
ie; invert and add.

Cheers
 
P

Philip A. Marshall

Jan 1, 1970
0
I ceases to amaze me that very few know how to use a scope in differential mode.
ie; invert and add.

Honestly, I've never been taught that in any lab >.< So score one for
those of you who complain that they don't teach you anything useful in
college... hah

but now that you bring it up, it makes perfect sense. so, just to be
clear:
-channel 2 invert
-view channel 1 + channel 2 (math addition)
-hook up the positives of both channels and float the ground

that will effectively be one oscilliscope channel except you don't
have to worry about ground potential differences between what you're
measuring and the scope? (i realize there are still limits on the
amount of difference from earth ground that you can measure, but at
least you're not shorting part of the circuit to earth)

thanks for the useful information!
 
T

Terry Pinnell

Jan 1, 1970
0
I ceases to amaze me that very few know how to use a scope in differential mode.
ie; invert and add.

On the occasions that I've been forced to use this method, it always
seemed very limited in terms of the amplitudes you could measure, and
also gave a rather unsteady and somewhat 'noisy' display.
 
T

Terry Pinnell

Jan 1, 1970
0
John Woodgate said:
Feed the TV though a proper safety isolating transformer.

Could you amplify on the set up you use please? My 'proper safety
isolating transformer' has a normal 3-pin mains plug, and mains earth
is therefore present on both sides. So do all the other 'isolating'
transformers I've come across.
 
S

Stefan Heinzmann

Jan 1, 1970
0
Martin said:
I ceases to amaze me that very few know how to use a scope in differential mode.
ie; invert and add.

This method has very bad common mode rejection, unfortunately. This may
be the reason why it is not used more widely. A big improvement in this
respect are differential probes or differential vertical amplifiers (if
you have a scope mainframe with plugins).
 
P

Paul Hovnanian P.E.

Jan 1, 1970
0
Philip A. Marshall said:
A grad student around here isolated his scope that way to make
measurements on a three-phase power system. Everything was fine until
he made the mistake of hooking up his serial printer (which he
neglected to isolate) to the scope to make a printout of one of the
traces...

Here's one place that my old scopemeter shines. Just unplug the wall
wart and run off batteries. If I need to print, the serial interface is
optical.

Do you really need a state of the art bench scope to work on TV sets?
100 MHz bandwidth should be plenty.
 
B

Blue News

Jan 1, 1970
0
Thanks for all your inputs. From the last post (Jan Panteltje) am I correct
to assume one does not necessarily need a isolation transformer with newer
TVs i.e. the ones with scart and video input?
I haven't got an isolation transformer and have never used a scope to
repair/test a TV but I was recently asked by a friend to look at his TV
which is not an old one. so is there any reasonably safe way of doing such
thing with out a transformer on newer TVs?

Sean
 
S

Stefan Heinzmann

Jan 1, 1970
0
Blue said:
Thanks for all your inputs. From the last post (Jan Panteltje) am I correct
to assume one does not necessarily need a isolation transformer with newer
TVs i.e. the ones with scart and video input?
I haven't got an isolation transformer and have never used a scope to
repair/test a TV but I was recently asked by a friend to look at his TV
which is not an old one. so is there any reasonably safe way of doing such
thing with out a transformer on newer TVs?

It begs the question how you're going to find a fault in a device you
are unfamiliar with. You'll most probably need a schematic drawing. In
newer devices most of the functions have disappeared in ICs.

Also you probably are aware that even with a chassis that has no mains
connection you are going to encounter some rather high voltages in the
device, for example for acceleration. Higher than your and/or your
scope's absolute maximum rating anyway. Dig in there only if you are
confident that you understand what you're doing.

If the TV set is mains isolated, as is most probably the case, you may
probe around in the circuit without using an isolation transformer, as
the isolation will be provided by the mains transformer of the TV set.
The primary side *is* live though and needs to be treated with respect.

It is prudent to measure the voltage against ground of various exposed
metal parts of the opened TV set before touching them or connecting the
scope ground to it.

And make sure somebody is nearby who knows how to quickly interrupt the
mains if something goes seriously wrong.
 
J

Jan Panteltje

Jan 1, 1970
0
Could you amplify on the set up you use please? My 'proper safety
isolating transformer' has a normal 3-pin mains plug, and mains earth
is therefore present on both sides. So do all the other 'isolating'
transformers I've come across.
I go to the shop, get a 240 to 240 V (or whatever voltage you have there)
350 W transformer, connect main to one end, and TV to other end.
Any transformer factury has them or can make one for you.
JP
 
J

Jan Panteltje

Jan 1, 1970
0
Honestly, I've never been taught that in any lab >.< So score one for
those of you who complain that they don't teach you anything useful in
college... hah

but now that you bring it up, it makes perfect sense. so, just to be
clear:
-channel 2 invert
-view channel 1 + channel 2 (math addition)
-hook up the positives of both channels and float the ground

that will effectively be one oscilliscope channel except you don't
have to worry about ground potential differences between what you're
measuring and the scope? (i realize there are still limits on the
amount of difference from earth ground that you can measure, but at
least you're not shorting part of the circuit to earth)

thanks for the useful information!
It is not so usefull, try measuring a first IF output at around 40MHz in a TV
with live chassis this way.
So yo uare measuring mV using perhaps a 1:1 probe, and the common mode
of that diff input is not enough, in fact the scope likely will evaporate.
DON'T do it for TV service.
And it is against EVERY safety regulation for the workplace.
 
J

Jim Yanik

Jan 1, 1970
0
It begs the question how you're going to find a fault in a device you
are unfamiliar with. You'll most probably need a schematic drawing. In
newer devices most of the functions have disappeared in ICs.

Well,you can make a list of the ICs and websearch for the data sheets,then
print them out for reference.
Also you probably are aware that even with a chassis that has no mains
connection you are going to encounter some rather high voltages in the
device, for example for acceleration. Higher than your and/or your
scope's absolute maximum rating anyway. Dig in there only if you are
confident that you understand what you're doing.

If the TV set is mains isolated, as is most probably the case, you may
probe around in the circuit without using an isolation transformer, as
the isolation will be provided by the mains transformer of the TV set.
The primary side *is* live though and needs to be treated with
respect.

It is prudent to measure the voltage against ground of various exposed
metal parts of the opened TV set before touching them or connecting
the scope ground to it.

And make sure somebody is nearby who knows how to quickly interrupt
the mains if something goes seriously wrong.

Without service data it will be hard to tell what is a safe ground to
connect to for measurements.
 
J

Jan Panteltje

Jan 1, 1970
0
Thanks for all your inputs. From the last post (Jan Panteltje) am I correct
to assume one does not necessarily need a isolation transformer with newer
TVs i.e. the ones with scart and video input?
If one knows what one is doing and is 100% concious all the time, and only
scoping the part of the set NOT connected to the mains, perhaps.
But I say NO USE A SEPARATION TRANSFORMER unles of cause you
are eager to get electrocuted or blow something up with a grounded scope.
JP
 
N

N. Thornton

Jan 1, 1970
0
doable yes, but safe is a relative term. Even a portable TV will
typically have 19,000 volts in it, and sometimes a more vicious 7kV
line, as well as the usual several hundred volt lines for assorted
electrodes. And of course flyback pulses here there and everywhere,
plus live mains on board and a nice switched mode power supply
complete with assorted lethal Vs. So I'm not sure I'd describe TVs as
safe to work on. You'd probably be OK, but you might not.


Regards, NT
 
V

Vlad

Jan 1, 1970
0
I know that it is illegal to help a person to commit suicide so, don't
do it the way I do.

I isolate the ground pin of the oscilloscope with an adapter .

But remember that your scope will be at the same potential as the TV
chassis.
If the tip of your soldering iron is grounded you may have fire works.
The same applies to any other piece of equipment that connects to the
TV chassis.

In reality your scope behaves like if it is operated by batteries..
Not very safe.

I have been around for more then 75 years so if I get electrocuted, I
have less to lose then most of you

Vlad
 
J

Jan Panteltje

Jan 1, 1970
0
I know that it is illegal to help a person to commit suicide so, don't
do it the way I do.

I isolate the ground pin of the oscilloscope with an adapter .

But remember that your scope will be at the same potential as the TV
chassis.
If the tip of your soldering iron is grounded you may have fire works.
The same applies to any other piece of equipment that connects to the
TV chassis.

In reality your scope behaves like if it is operated by batteries..
Not very safe.

I have been around for more then 75 years so if I get electrocuted, I
have less to lose then most of you

Vlad
It is the unexpected, just that not concentrated moment that does get you.
Combination of factors.
I had live chassis in left hand, then took arial plug (right hand, safe,
grounded !) and was stuck. (wanted to plug it in).
Found out I could still move my legs, and jumped, that broke the contact.
That is how I found out there was no isolation transformer in that
workplace (was not mine).
JP
 
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