Troubleshooting basics procedures

[danny davis]

What are some troubleshooting procedures

1.) First check the power supply voltage
2.) Check the V+ on IC chips to check if the IC chip is getting power
3.) Check every input and output for each stage of a transistor or IC op amp of the circuit until you find no signal

4.) Can you add any others on the list

A SHORT Will make the circuit have no signal in that stage of the circuit, it will have zero voltage on the PCB board pads in that area of the short, Blocking the path to the next stage of the circuit

An OPEN will make a circuit have no signal in that stage of the circuit, breaking the path to the next stage of the circuit

An OPEN of a component in a circuit can also change the bias or gain of the circuit

If a transistor is shorted internally From either Base, emitter, collector it will change what?
it changes the voltage measurements

If the transistor is open it breaks the path from input to output of that stage so the signal is gone

 

[danny davis]

Rise & Fall Time, Pulse width, Duty Cycle Oscilloscope Tests

What circuits or When have you guys measured

1.) Rise & Fall Time of a circuit
2.) Pulse width of a circuit
3.) Duty Cycle of a circuit

When you did do these types of Rise & Fall time, pulse width, Duty Cycle measurements on the Oscilloscope what were u looking for that was errors in the circuit?

How would I know viewing on an oscilloscope what is BAD or something wrong with the circuit when looking at the Rise & Fall Time, Pulse width, Duty Cycle of a circuit?

 

[(*steve*)]

1.) First check the power supply voltage

Often a place to start. But not only the voltage, but you may want to check the ripple, or the current being drawn, or the voltage drop under load. And there are other things to test too. It all depends on the fault you're seeing.

2.) Check the V+ on IC chips to check if the IC chip is getting power

Maybe. If there are regulators, I would check them next. These may be part of the power supply, or there may be secondary regulators around the board.

You can also stick your finger on an IC and determine if it's drawing *lots* of power. Same thing is true of many components.

3.) Check every input and output for each stage of a transistor or IC op amp of the circuit until you find no signal

You can do it that way, or you can inject a signal starting from the output and moving back until it no longer appears at the output.

Or, if the signal is distorted, you can look at the signal from either end and keep moving toward the other until the problem appears or disappears.

4.) Can you add any others on the list

Look, listen, smell...

Some component faults you can spot just by looking at them. (swolen capacitors, blown fuses, burnt resistors, etc)

If something has let out the magic smoke then you may be able to smell it.

Some faults will make a sound (not many, but high voltage discharge and shorted or badly overloaded switchmode power supplies often will)

As mentioned earlier, feeling for things that are warm or hot (only in low voltage circuits) can help.

A SHORT Will make the circuit have no signal in that stage of the circuit, it will have zero voltage on the PCB board pads in that area of the short, Blocking the path to the next stage of the circuit

Depending on what is shorted, the signal may be affected before the short too.

If it's a power rail, then maybe the entire power rail will droop to some very low value.

A short may not be zero ohms either...

An OPEN will make a circuit have no signal in that stage of the circuit, breaking the path to the next stage of the circuit

Normally, yes. It may also affect the previous stage because some load that is normally present is absent.

An OPEN of a component in a circuit can also change the bias or gain of the circuit

Open, short, and other types of failure (but not all) can do this.

If a transistor is shorted internally From either Base, emitter, collector it will change what?
it changes the voltage measurements

This will typically mean you get exactly the same voltage on the shorted connections. You can short BE or BC and leave the other one unshorted, but you can't short EC and leave the base unaffected.

If the transistor is open it breaks the path from input to output of that stage so the signal is gone

If a transistor is open circuit, there is normally physical damage

Normally they will short first then go open with a bang (or a fizzle)

 

[(*steve*)]

1.) Rise & Fall Time of a circuit

rarely.

2.) Pulse width of a circuit

infrequently

3.) Duty Cycle of a circuit

occasionally

I would measure these things if I thought that they were important for the circuit function and related to the fault.

I assume we're talking digital here.

Other things to look for are:

4) ringing
5) incorrect voltage levels
6) glitches
7) absence of an expected signal
8) presence of an unexpected signal
9) noise
10) signal delay