How to use an Oscilloscope

[Netguru5]

Hi,

I finally got ahold of a fairly nice scope and had some newbie questions. I've read quite a bit of info on this scope and others on its use, but not much in practical tests on complex systems like motherboards. Can I damage the scope by poking around with the probe? And where should I connect to ground when checking components on a motherboard like IC's or crystals? how would I check if a cap was blown? I was able to check the frequency of a crystal without the ground clip attached, but what grounding point should I use on the motherboard? Im learning so any advice would be appreciated.

Thanks!

 

[ghostman11]

a good newbie guide to using a oscilloscope and how to test and fault find would be very useful, i too have just got hold of a scope and have found it hard to find much helpful info on useing it.
I know what we should really do is jump right in and play with it but like you after spending that kind of money i dont want to risk breaking it!!

 

[ke5frf]

a good newbie guide to using a oscilloscope and how to test and fault find would be very useful, i too have just got hold of a scope and have found it hard to find much helpful info on useing it.
I know what we should really do is jump right in and play with it but like you after spending that kind of money i dont want to risk breaking it!!

The risk, in most cases, isn't that the "scope" will be damaged...but rather that the scope may damage the circuit being analyzed. Any device connected to a circuit to measure it presents itself as a "load". In other words, the source impedance of the circuit will see a slight change in the load impedance because the measuring circuit "becomes part of" the circuit being measured...so to speak.

Most scopes and meters are designed to have a very high input impedance so that the effect is minimal. But very reactive circuits like high frequency RF resonant and filter circuits can be effected. The probe leads have series resistance. A series resistance by nature in a straight wire has an X-L component. The two leads in parallel have a X-C component. The higher the frequency of the circuit, the more critical this becomes. Therefore, special RF probes are usually used for radio circuits.

Switching power supplies are another example, and this is where damage can really happen. The potentials on the output of a SMPS are isolated from earth ground and floating. An oscilloscope is typically connected to AC mains and is at earth ground potential. This can create very high potential voltages at the output of the power supply with respect to the chassis potential of the earth-grounded scope. Damage to both equipment can occur, effectively shunting voltage in the power supply to earth ground. Isolating the scope with a transformer is one "technique" as is using a battery powered scope, but isolation transformers can be dangerous.

Here's a good tutorial on the loading effects of scopes. Just be mindful of the nature of the scope and the nature of the circuit being analyzed and be careful.
**broken link removed**

 

[royesses]

Try this:
oscilloscope Tutorials.

 

[rogsilk]

try this site ( **broken link removed** )

 

[ghostman11]

thats really helpful........but are there any guides on fault finding techniques with a scope, like wich part of a circuit to start or do u start at the vcc part or ground part? do u start with transitors or the chips. All this stuff probally seems obvious to most of you but i cant be the only newbie/noob thats a bit lost

 

[ghostman11]

infact i was thinking.. i have a web server and as long as people dont mind i might put all the really useful tips and advice and links that have been posted into a subdomain on the server for a referance for noobs.there are many posts on here that contain some real nuggets but can be hard to track down on a forum....however if anyone objects then...........i wont

 

[ke5frf]

thats really helpful........but are there any guides on fault finding techniques with a scope, like wich part of a circuit to start or do u start at the vcc part or ground part? do u start with transitors or the chips. All this stuff probally seems obvious to most of you but i cant be the only newbie/noob thats a bit lost

Think about using a scope or meter to fault find as you would troubleshooting an automechanical problem. Lets say you are experiencing loss of power in your vehicle. It just doesn't have the "go" that it used to have.

Would you start pulling apart the radiator, water pump, or the air conditioner compressor because of such symptom?

Probably not. The symptom isn't related to their function. So you would eliminate them as a likely possibility...though you might consider remote ways they could contribute if other ideas do not work.

You would probably think about the systems that are relevant. Engine, transmission, exhaust, etc. You might consider the electrical problems such as spark plugs, plug wires, fuel pump, injection, and filter, etc.

You would consider the most likely, easy candidates to check first. Is the car ignition system missing (cylinder not firing)? Maybe it isn't the plugs or plug wires. OK, lets consider the fuel pump. Disconnect the fuel line to the injector/carburetor. Turn on the ignition key. Gasoline flowing or not?

All checks out...perhaps the injectors are bad? Or the cylinder is not compressing. Put it on a diagnostic and see what the sensors say.

Etc etc.

Same thing here. Start out at the level of understanding the whole function of the circuit to eliminate what it can't or shouldn't be. Then divide it into systems where potential exists. Think of the most likely candidates for failure...where heat, current exists or where weakness is possible. Diodes, regulators, cmos chips, power supply components, Electrolytic capacitors often go bad, check them etc.

Divide and conquer. No output signal? Is there an input here where there should be? No? What device or voltage supplies the input signal? What waveform do I expect here? IS this DC or AC? Is there ripple? Should there be?

What is my supply voltage? Should I be reading supply voltage across this resistor, or is it dropping multiple places across these series components? Why is this resistor showing supply voltage across it? It should be showing 1.6 volts here, it current limits this device...etc, etc. Ah! R6 is open at the base of this transistor. Lets replace it and see what happens.

Your checks will depend on what SHOULD be there at each component, or an approximation. You might start out checking waveforms with a scope from ground potential...here, here, there, over there, etc. Until you see no voltage or waveform present. At this point you have lost a signal...why? Is this component involved in the circuit at this time, or should it be? Is there a logical reason why no voltage would be present here with respect to ground? Ah, maybe I'm checking the ground side of the component, thats why there is no voltage. What about the other side. Still none? Back up, what is before it in the circuit? Ah, that R6 resistor that current limits it. Why is there voltage on one side of that resistor but not on the other? Why is it full supply voltage?

Check ACROSS the resistor. I see my waveform there. Yes, peak voltage across it, but there are other series components. That shouldn't be. I should be seeing a drop. Ah, R6 is definately bad.

You see how it goes? Divide and conquer, big systems down to components. Logical progression of checks.

Schematics are very useful, sometimes vital.

 

[ommicro]

thanks

thanks for the help

 

[joespaz7]

dont worry about damaging the scope, ive been using the things for years and never broke one, however, when ever u can connect to ground. other then that, you will figure out everything else through trail and error

 

[Reloadron]

In my nutshell opinion a scope is a tool like any other tool. Learning how to use the settings and features of a scope comes first. However, the scope, like any tool is only as good or useful as the person using it.

Troubleshooting anything involves knowing how what you are troubleshooting works. Be it an automotive engine or a complex motherboard. If I notice the CPU fan isn't spinning I can't use a scope to troubleshoot unless I know what to look for and how the fan should work under normal conditions. If I don't know those things the scope has as much value as breast on a bull.

Become familiar with the scope as a tool and learn what it can and more important can't do. Know its limitations.

Example:

Years ago I had a technician replace a 74123 retriggerable multivibrator no less than three times. I finally asked what he was doing? Using a scope he showed me it had pulses in but nothing out. The logic out remained high despite pulses in. Yes, the scope clearly showed that. However, in this case the 74123 was designed as part of an alarm circuit. It was supposed to time out and change output states only when the pulses in were missing pulses. The technician knew how to use the scope but had no clue what he was supposed to see. That incident was about 27 years ago and I have not forgotten it, I also doubt the technician has either, .

Get friendly with your new tool and know what to look for.

Just My Take
Ron