When a circuit stops working it can be hard to know where to start. Sometimes the problem is simple like a loose connection or a power supply issue.

I usually think it makes sense to check the basics first before replacing any parts.

What do you normally check first when troubleshooting a circuit? Do you start with the power supply and connections or use a multimeter to check the signals?

I’d be interested to hear what works best for others.

 

You don’t begin by replacing parts. You locate the problem first.
There is no single solution to cover all cases.

This is where you become a detective in solving a murder mystery.

You start with a conversation with the owner. Learn about the history of the appliance, age, usage, circumstances, for example, water damage. What were the conditions leading up to the failure.

Next comes a visual inspection, look, smell, touch (while unit is powered, or use a thermal imaging camera to locate hot spots, after you have done the first things below).

As you said, start with the simple things first, fuse, power cord, plugs, switches. A very common failure point is the power cord at the neck of the plug or socket.

The testmeter is used to check for AC voltage coming in and DC voltages at the power supply (assuming that it is DC powered).

 

When a circuit stops working it can be hard to know where to start. Sometimes the problem is simple like a loose connection or a power supply issue.

I generally start with the power supply following a good visual inspection. Obviously start with any fuses or disconnects like breakers. Really depends on what I am troubleshooting.

Just My Take....
Ron

 

an important step is getting familiar with the product. often no schematics is available so one is forced to make a substitute - either hand drawn circuit or mental image of what the circuit is and what it does - by identifying key components.
one does not go blindly and start probing things at random - first you try to understand the circuit, then make a prediction of what values are reasonable at certain points, then measure at those points and compare measurement with prediction.

say you are troubleshooting AC outlet that should measure some 120VAC. you expect 120VAC (that is prediction). then you measure and see 117V or 121V or similar. this is close enough to declare it ok.
but if you measure 69V or 237V, something is way off. one would not know if that is ok unless we had expectation (prediction) what the value is supposed to be.
everything that we measure is based on some prediction (knowledge/expectation or previous experience).
and it goes beyond that. before making measurement, one should be able to also make the next prediction - what could be wrong or what should be measured next if this measurement is not matching prediction.

for example you are about to measure voltage on that 120VAC outlet. you know what to expect (one prediction). but you should also be able to tell what the next step should be (before you measure) what the possibilities or options are if the measurement is too low, or too high.
if the voltage to be measured is too low, chances are that there is a loose connection somewhere causing voltage drop and low reading.
if the voltage to be measured is too high, something is injecting higher voltage into circuit. there must be source of higher voltage present and path for current to leak into whatever circuit you are testing.

this is how you build a path to be followed till root cause is uncovered.

 

Depends on the circuit.

First thing to check is if it's receiving power or not, then a visual inspection, as Ron has already suggested. Then check if it has warm spots. And then comes the interesting part, for instance if it's a digital circuit, is its oscillator doing its thing? ... if it's analog then an oscilloscope is an indispensable tool.

Diagnosing a circuit is more an art than a trade, IMHO

 

Fuse. IF blown - why? What caused it?
If not the fuse, power. Is there power?
Visual. Does there appear to be any visible damage? If so - don't just start replacing things. Something caused the problem which may have manifested itself as a blown component.
FULLY DIAGNOSE THE PROBLEM before you start throwing parts at it.
Once you have eliminated the fuse and power supply you start looking for places where an interrupt may have occurred. Check the solder for fractures or cold solder joints. Frequently enough the problem can be a bad solder joint. I was given a TV with a Horizontal Control issue. I reflowed all the solder joints having to do with HC and that solved the problem. There was nothing visual that lead me to that - other than the horizontal was restricted to 1/3 of the screen.
Beyond the basics what you do depends on the circumstances that has lead to the failure. Does it make a sound? Is there a light? From there it becomes far more focused on the type of circuit you're working on. Tracing down an audio problem in a stereo is different from fixing a kids toy, or a computer. But throwing parts at what you believe is the issue may be a result of a true fault, one you haven't detected yet. That's why you fully diagnose the problem before you start buying and replacing parts.

 

"Is is plugged in, is it turned on, is the brightness|volume|whatever control turned up?"

 

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Let's assume that the unit fell out of the back of a truck (figuratively speaking). You have no history to go on.

Let's examine some of the top reasons for a non-functioning unit.

1) No power. Power switch not turned on. No power from wall socket (for example, wall outlet is controlled by a switch). Bad power plug. Bad power connector. Bad power cord. Bad on/off switch. If it is a soft power switch, bad control circuitry.

If it is battery powered, look for dead battery, corroded contacts, batteries inserted incorrectly.

2) Blown fuse. Don't just replace the fuse. There is a reason the fuse blew. A new fuse will likely blow too.
I use an electronic circuit breaker. I can set the maximum amperage. it will tell me the trip current. Having the right test equipment makes a lot of difference in diagnosing the problem.

 

Certainly verifying the correct power is present is the first step after a visual inspection to look for OBVIOUS problems, such as broken wires or components. At this point it is very useful to know, at least basicly, how a thing works.
Understanding how it should work, and knowing what failed to work, are the basic elements of servicing.
Consider that a resistor normally burns up because of two much current, the need is to know what caused the "too much current" condition. Simply replacing a burned up resistor with a higher wattage rated resistor will usually result in the destruction of other more expensive parts. ONLY the very poorest service folks start by simply replacing parts, since they have no other skills available.

Diagnosing a circuit is more an art than a trade, like was stated much earlier, it usually requires understanding how a circuit, or a machine, is intended to function. Diagnosing a failure in an appliance or a machine is similar, except understanding the function requires more than just circuit understanding. (Probably including machines and appliances is beyond what the TS was intending. Sorry about THAT)

 

You don’t begin by replacing parts. You locate the problem first.
There is no single solution to cover all cases.

This is where you become a detective in solving a murder mystery.

You start with a conversation with the owner. Learn about the history of the appliance, age, usage, circumstances, for example, water damage. What were the conditions leading up to the failure.

Next comes a visual inspection, look, smell, touch (while unit is powered, or use a thermal imaging camera to locate hot spots, after you have done the first things below).

As you said, start with the simple things first, fuse, power cord, plugs, switches. A very common failure point is the power cord at the neck of the plug or socket.

The testmeter is used to check for AC voltage coming in and DC voltages at the power supply (assuming that it is DC powered).

the detective comparison is spot on. knowing what happened before the failure can save a lot of time, and checking the power cord, fuse and supply before going deeper makes good sense. replacing parts without finding the cause can just lead to the same problem coming back.

 

I generally start with the power supply following a good visual inspection. Obviously start with any fuses or disconnects like breakers. Really depends on what I am troubleshooting.

Just My Take....
Ron

That seems a sensible order, especially checking the obvious things before digging into the circuit. the type of equipment makes a big difference too, so having a fixed starting point but adapting from there is probably the best approach.

 

an important step is getting familiar with the product. often no schematics is available so one is forced to make a substitute - either hand drawn circuit or mental image of what the circuit is and what it does - by identifying key components.
one does not go blindly and start probing things at random - first you try to understand the circuit, then make a prediction of what values are reasonable at certain points, then measure at those points and compare measurement with prediction.

say you are troubleshooting AC outlet that should measure some 120VAC. you expect 120VAC (that is prediction). then you measure and see 117V or 121V or similar. this is close enough to declare it ok.
but if you measure 69V or 237V, something is way off. one would not know if that is ok unless we had expectation (prediction) what the value is supposed to be.
everything that we measure is based on some prediction (knowledge/expectation or previous experience).
and it goes beyond that. before making measurement, one should be able to also make the next prediction - what could be wrong or what should be measured next if this measurement is not matching prediction.

for example you are about to measure voltage on that 120VAC outlet. you know what to expect (one prediction). but you should also be able to tell what the next step should be (before you measure) what the possibilities or options are if the measurement is too low, or too high.
if the voltage to be measured is too low, chances are that there is a loose connection somewhere causing voltage drop and low reading.
if the voltage to be measured is too high, something is injecting higher voltage into circuit. there must be source of higher voltage present and path for current to leak into whatever circuit you are testing.

this is how you build a path to be followed till root cause is uncovered.

The part about predicting what you should measure before taking a reading is important. Otherwise, a number on the meter does not tell you much on its own. Planning the next check based on the result gives you a much clearer path to finding the fault.

 

Depends on the circuit.

First thing to check is if it's receiving power or not, then a visual inspection, as Ron has already suggested. Then check if it has warm spots. And then comes the interesting part, for instance if it's a digital circuit, is its oscillator doing its thing? ... if it's analog then an oscilloscope is an indispensable tool.

Diagnosing a circuit is more an art than a trade, IMHO

i like the point about digital and analogue circuits needing different checks. once the basic power checks are done, knowing where to look next depends a lot on how the circuit is meant to work. that is where understanding the design really pays off.

 

Fuse. IF blown - why? What caused it?
If not the fuse, power. Is there power?
Visual. Does there appear to be any visible damage? If so - don't just start replacing things. Something caused the problem which may have manifested itself as a blown component.
FULLY DIAGNOSE THE PROBLEM before you start throwing parts at it.
Once you have eliminated the fuse and power supply you start looking for places where an interrupt may have occurred. Check the solder for fractures or cold solder joints. Frequently enough the problem can be a bad solder joint. I was given a TV with a Horizontal Control issue. I reflowed all the solder joints having to do with HC and that solved the problem. There was nothing visual that lead me to that - other than the horizontal was restricted to 1/3 of the screen.
Beyond the basics what you do depends on the circumstances that has lead to the failure. Does it make a sound? Is there a light? From there it becomes far more focused on the type of circuit you're working on. Tracing down an audio problem in a stereo is different from fixing a kids toy, or a computer. But throwing parts at what you believe is the issue may be a result of a true fault, one you haven't detected yet. That's why you fully diagnose the problem before you start buying and replacing parts.

the point about finding the cause of a blown component is important. replacing it without checking what made it fail can just lead to the same problem again. cracked solder joints can be easy to miss too, especially when there is no obvious sign of damage.

 

Certainly verifying the correct power is present is the first step after a visual inspection to look for OBVIOUS problems, such as broken wires or components. Atthis point it is very useful to know, at least basicly, how a thing works.

understanding how the circuit is supposed to work makes the checks much more meaningful. otherwise, it is easy to miss something unusual because you do not know what the normal behaviour should be.

 

I check my DMM first.

 

I went back to the original post #1 , and now my answers may be a bit different: The very first step, assuming that the device actually had been working prior to not workig, is a quick visual inspection, intended to find any physical damage to whatever has ceased to work. After that, a check for presence of the correct power. At that point an understanding of how an unknown device, or circuit, SHOULD work is important. Testing and measuring to verify the presence of the correct power would then be the next stage.

The process is quite different if it is a circuit or machine that is new and has never been oppertional!
Evaluating a new circuit or a new machine certainly demands understanding how it is intended to operate.
For evaluating a circuit that has not been verified as correct it is required to know what it is supposed to do. Beyond that, quite a bit more than basic circuit theory is needed to understand how it actually would function.

 

First thing to do if a board stops working.
Take a coffee , make note of what's happened .
What's changed ?
Where you doing anything when it stoped ?
Is there a computer involved. Has there been a SW update ?
Are the power supplies still at correct levels
Did any smoke escape
Whos worked on it / the pc since it last worked ?
Have any of the cables come loose / broken

Basically thing what could cause what's happened, and check

Practise , you will learn to prioritise where to look first based upon situation .

 

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actually the opposite can also be true - do not turn off running system... it may not start again.

one such example was call from company making solar panels and of course using robots. complaint was that after scheduled preventive maintenance, some of the robots did not work any more. they displayed error messages that client never saw before.
i came to investigate and found that problem was caused by program changes added couple of years ago. it did not take long to determine that the additional code was incomplete - it did not initialize some of variables. so whoever made the change, manually set the values, tested changes and was satisfied that everything was working. but never performed cold start...
simply turning disconnect switch to off and back to on does reboot but... this uses preconfigured boot type (unless reconfigured or forced by user to use different one).
in warm start memory is retained (similar to computer hibernation) and even instruction pointer is at the same line of code where it was before shut down.
in cold start that is not the case, any uninitialized variable loses value... in this case problem was eventually exposed but MUCH later...