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.