# Newbie scope question

#### UnnamedUser159

Joined May 3, 2016
501
hi there.
i have 1202x-e. Have used it for a very little before years.
Now want to look if audio chip has a normal output. i think it was 7378.
Just power the scope and put the probe on output ? should i connect somewhere the sided crocodile ?

thank you

Last edited:

#### MisterBill2

Joined Jan 23, 2018
6,007
hi there.
i have 1202x-e. Have used it for a very little before years.
Now want to look if audio chip has a normal output. i think it was 7378.
Just power the score and put the probe on output ? should i connect somewhere the sided crocodile ?

thank you
For those of us who lack a mind reading ability you will need to give us more information. I guess that a 1212xe is some sort of oscilloscope, which I am not familiar with. I do know about using scopes but not specific ones.
And your intended use, to check the operation of some audio IC device that I have no information at all about, while it may be correct, I have no information about the connections to that device or the named function of the specific pint that you mention.
What I can tell you relative to your question is that to get a useful scope signal the probe "ground" connection should be connected to the circuit common point, often called ground. This would be the input common point, if you have the circuit available to look at.

#### MrChips

Joined Oct 2, 2009
21,044
The GROUND clip on your oscilloscope probe is connected to EARTH GROUND via the ground pin on the 3-pin AC power cord.
If you attempt to connect the clip to some where in the external circuit you could:
1) blow the circuit
2) blow the oscilloscope
3) blow both the circuit and the oscilloscope

When the probe is connected to the circuit and the GROUND clip is left disconnected you might see:
1) the correct signal
2) a nasty looking 60Hz AC waveform

In both cases, the correct connection of the GROUND clip to the circuit is a point that is called circuit COMMON if you can verify that such COMMON is either "floating" or connected to EARTH GROUND same as your oscilloscope.

#### UnnamedUser159

Joined May 3, 2016
501
I guess that a 1212xe is some sort of oscilloscope
It is not some sort of osciloscope. its Siglent SDS1202X-E.

However thank you

#### MrSoftware

Joined Oct 29, 2013
1,775
Watch this guys videos:

#### UnnamedUser159

Joined May 3, 2016
501
its me again - as usual i am confused.

If the device use 2 pin plug to the wall i dont connect this probe to the circuit ground or i do connect it ?

to be informed if i want to analyze device which is powered with 3 pin cable to wall how to proceed?

The audio device is :
i-trigue 3330

#### MisterBill2

Joined Jan 23, 2018
6,007
The GROUND clip on your oscilloscope probe is connected to EARTH GROUND via the ground pin on the 3-pin AC power cord.
If you attempt to connect the clip to some where in the external circuit you could:
1) blow the circuit
2) blow the oscilloscope
3) blow both the circuit and the oscilloscope

When the probe is connected to the circuit and the GROUND clip is left disconnected you might see:
1) the correct signal
2) a nasty looking 60Hz AC waveform

In both cases, the correct connection of the GROUND clip to the circuit is a point that is called circuit COMMON if you can verify that such COMMON is either "floating" or connected to EARTH GROUND same as your oscilloscope.
Evidently MC has not used a scope on circuits powered by a supply totally isolated from the outside world. Earth ground is seldom the best choice.
With any reasonably isolated power supply the scope signal return connection is to the common line within that circuit. Of course, that circuit MUST be isolated from the AC mains, which most audio circuits are. And of course the user is obligated to understand enough about the circuit being investigated to be aware of it's level of isolation. But we woukld not be expecting somebody walking in off the street with no knowledge or insight to be using a scope, or other piece of test equipment on some electronic device, would we?

#### MrChips

Joined Oct 2, 2009
21,044
No point in trying to confuse "somebody walking in off the street".
Of course I know when and how to use an isolation transformer. Done it many times.

TS just needs to understand why a nasty looking 60Hz AC waveform will be seen on the oscilloscope and how to remedy that.
A lot of "people off walking in off the street" do not understand the waveform they see just by touching the tip of the oscillator probe.

#### bertus

Joined Apr 5, 2008
20,528
Hello,

You couls also have a look at this Tektronix document about oscilloscopes.

Bertus

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#### UnnamedUser159

Joined May 3, 2016
501
so so...
i always was "happy" when some internet dude is attacking me that i am stupid and ugly.
but i think the forums was to be helpfull. i can say bad words too but then i will not receive usefull answers.

i will take a look maybe at the videos and pdf from thread.

one concrete question - my case - the board is powered with 220/12 AC adapter. i can attach a photo of my board. should i connect the ground aligator somewhere or not?

thank you normal and not offensive people.

#### MisterBill2

Joined Jan 23, 2018
6,007
Because the product is powered by an adapter it is adequately isolated from the mains, and thus the fears of sparking destruction are unfounded. And I am not familiar with the "siglent" brand of test equipment, and the "Siglent SDS1202X-E. ", and so my comment was not intended to be at all dismissive. I understand about test equipment but I am not familiar with what is used in many other parts of the world. And I do not know all of the brand names nor what all of the quality levels are, nor do I research every one that is mentioned.
A photo of the board should allow some of us to see just what is being discussed, and possibly be able to offer some useful advice about where to connect to it.

#### UnnamedUser159

Joined May 3, 2016
501

I have checked power supply to tda7378 and the two U schemes. it`s good. Have replaced the input Capacitor because its value was very decreased. Have checked 4 of maybe 6 bipollar transistors.

thank you

#### MrChips

Joined Oct 2, 2009
21,044
Do you understand what it means when a unit is said to be "floating"?

Does your oscilloscope probe have a x1 - x10 switch? Set it to x10
Set the CHAN 1 input scale to 2V/DIV.
Set the HOR scale to 10ms/DIV.
Go ahead and connect the tip of the oscilloscope probe to any point on the circuit.
You will notice a nasty looking 60Hz AC waveform. This is normal. Nothing is wrong.

We will take it from there.

#### MrSoftware

Joined Oct 29, 2013
1,775
I think the challenge is that it's not possible to explain how to safely use an oscilloscope in a few sentences. So I personally think the best approach is to go ahead and learn about the oscilloscope by watching some videos and reading a bit.

To over simplify; you must connect the ground clip to something or your readings will be bad. The big danger is if you clip it to the wrong thing you could cause a short and damage the device you're measuring and/or your scope. If you are unsure what to clip it to, the easiest way to make it safer is to power the device that you're measuring through an isolation transformer, or powering it from a battery if you have that option. Now clipping the ground clip to the wrong thing isn't nearly as dangerous.

#### martyrcomms

Joined Nov 24, 2015
1
hi there.
i have 1202x-e. Have used it for a very little before years.
Now want to look if audio chip has a normal output. i think it was 7378.
Just power the scope and put the probe on output ? should i connect somewhere the sided crocodile ?

thank you
Hi
Just reading the posts here on this subject a simple observation might just clarify the point to connect the crock clip. First thing to check is what type of power supply it is for example "Switch Mode " or not this is relatively easy to identify. Look at the incoming mains to the board if you follow it through from the I/P it will eventually lead to some form of bridge rectifier it might be 4 diode close to each other or a encapsulated one with 4 connections, If so also lookout for a transformer usually with yellow tape around it this is the switch mode transformer an this will indicate that you need to be very careful as the equipment hotside will be at half mains input votage (125v) so the audio ic ground will be at a different potential. So you would the place your clip (eth) on the output side the easiest way is to locate the Neg end to the smoothing Electrolytic caps supplying the audio ic that you mentioned this is achieved by soldering a neg wire on the board and connecting it to the clip tack care not to touch the chassis as it will blow the fuse and possibly the switch mode transistor into orbit. Just a little reminder DO NOT make contact with the chassis metal and the neg side of the board as this will cause you to light up because you would be across half the mains supply "ouch" the little reason that the board doesn't go bang with to different negative 's is that there will be a opto isolated from each other. I hope this might help it's a lot easier when you can point out it to people. Hope this helps Ray

#### MisterBill2

Joined Jan 23, 2018
6,007
Hi
Just reading the posts here on this subject a simple observation might just clarify the point to connect the crock clip. First thing to check is what type of power supply it is for example "Switch Mode " or not this is relatively easy to identify. Look at the incoming mains to the board if you follow it through from the I/P it will eventually lead to some form of bridge rectifier it might be 4 diode close to each other or a encapsulated one with 4 connections, If so also lookout for a transformer usually with yellow tape around it this is the switch mode transformer an this will indicate that you need to be very careful as the equipment hotside will be at half mains input votage (125v) so the audio ic ground will be at a different potential. So you would the place your clip (eth) on the output side the easiest way is to locate the Neg end to the smoothing Electrolytic caps supplying the audio ic that you mentioned this is achieved by soldering a neg wire on the board and connecting it to the clip tack care not to touch the chassis as it will blow the fuse and possibly the switch mode transistor into orbit. Just a little reminder DO NOT make contact with the chassis metal and the neg side of the board as this will cause you to light up because you would be across half the mains supply "ouch" the little reason that the board doesn't go bang with to different negative 's is that there will be a opto isolated from each other. I hope this might help it's a lot easier when you can point out it to people. Hope this helps Ray
Back in post #10 the TS tells us that the device is powered by an external power module. That means that it is electrically isolated from the mains, and thus no hazard of mains voltage in the device exists. AND, very likely it does not even have a metal chassis to connect to.

#### ggrise

Joined Nov 7, 2016
1
A lot of discussion here on ground hazards here. I generally start with a meter and measure the voltage between the ground on the scope and the ground on the circuit to be probed. If the circuit is floating or already at earth ground, good to go. If floating, circuit could be acting as an antenna. In that case. Use a jumper with a 10k resister between the 2 grounds in that case to see voltage/current state.
If there is a voltage /current differential between the 2 grounds, you could damage the circuit under test by tying the 2 grounds together. You need an isolation transformer on the circuit under test to remove the hazard. Check again after isolation.
Measurements without a scope ground can be misleading as the circuit under test and probe are acting as an antenna

#### MisterBill2

Joined Jan 23, 2018
6,007
A lot of discussion here on ground hazards here. I generally start with a meter and measure the voltage between the ground on the scope and the ground on the circuit to be probed. If the circuit is floating or already at earth ground, good to go. If floating, circuit could be acting as an antenna. In that case. Use a jumper with a 10k resister between the 2 grounds in that case to see voltage/current state.
If there is a voltage /current differential between the 2 grounds, you could damage the circuit under test by tying the 2 grounds together. You need an isolation transformer on the circuit under test to remove the hazard. Check again after isolation.
Measurements without a scope ground can be misleading as the circuit under test and probe are acting as an antenna
Evidently a few folks live under an intense fear aura and it affects everything they say and do. A careful evaluation of a situation is a much better approach. and much healthier than all of that fear.