The art of troubleshooting / analyzing cirquits.

[bastouw]

Hello.

I want to improve my analyzing skills and i know the first thing is to examen the power source(s) or supply voltages.
but I would like to know the basic analyzing order for analog electroncs cirquits.
I am a IT person, i use the OSI model as a general point to start from. now i am searching for a simular thing for analog eletronics.

Can someone helpme or point me to methodes or documentation to start from?

[madires]

The OSI model won't help you here ;-) BTW, it's used for networking. Learn about components and basic circuits, then you'll start to see how everything fits together.

[Rigby]

Well, the physical layer will always be the first step.  "Is it plugged in, and not destroyed, physically?"  Then, link layer (Thou shall always check voltages).  Beyond that it falls apart, yes. 

There probably is a less precise layer model that could be applied, but with analog stuff it'll get so fuzzy that the layer definition would change wildly from circuit to circuit.  For digital stuff, the OSI layer (or an analog of it) would apply a bit more.

Anyway, for me, and I'm new at this, it's physical, voltages, then go from there.  If the thing isn't destroyed, it is plugged in, and the voltages are all good, and the thing still isn't operating, it's because I missed something somewhere.  Shorts and open circuits show up in voltage readings, if you have the know-how to know what you should be seeing.

[w2aew]

There is no standardized process.  It all starts with the basics.  You'll need a decent understanding of how the device is supposed to work, and the basic properties of the analog components in the circuit.  Start with making sure that the power supplies to the circuit are all correct.  If they are, then start measuring the DC bias voltage at various nodes in the circuit, and checking to see if these bias voltages make sense with respect to the proper operation of the components connected at those points.

[Rerouter]

On well laid out boards you will generally find all the parts for a specific function packed into one area of the board, thus you can generally break it down into a function block type approach, e.g. power supply, I/O protection, main micro-controller, etc

by doing this you only need to attack a smaller and generally simpler circuit than the whole and can generally work out which function block(s) is playing up

next up is one of 2 methods:

looking for parts that from your own and others experience are likely to have failed in the function block, e.g. 20 year old electrolytic caps have probably dried out in a hot device, tantalum capacitors with no current limiting or that can have there voltage rating exceeded, or even 1980's era high wattage carbon slug resistors, looking for discoloration on the board can be another good way of narrowing down a fault from overheating (resistors can crack there solder joint with thermal cycling.

or throwing the circuit into a simulator, and seeing what to expect at each node,

you can attack with one or both, the second being more so if your not familiar with the way they have done something,

you can probably call this the divide and conquer method, (there is also the brute force way of swapping out every cheap passive at one, but i tend to avoid that unless it has me truly stumped)

 

[mazurov]

"Troubleshooting Analog Circuits" by Bob Pease is a good read -> http://www.amazon.com/Troubleshooting-Analog-Circuits-Series-Engineers/dp/0750694998/ref=sr_1_1?ie=UTF8&qid=1393623828&sr=8-1&keywords=pease+troubleshooting. Books by Jim Williams are very good too. Also, read the comments on Amazon and check suggestions.

 

[AG6QR]

For troubleshooting a device that was known to be previously working, (different from analyzing or designing a new device), you can achieve some level of success by simply knowing what kind of components commonly fail.

• Power supply/fuses are obvious
  
• Any sorts of mechanical devices, like switches, relays, connectors, sockets.  I've fixed many devices by simply reseating boards or connectors.
  
• Electrolytic caps.  Nice to have a basic ESR meter, but even a visual check for bulging or leaking electrolytics is sometimes enough.
• Static sensitive devices, especially those connected to external connectors.

A deeper understanding of a circuit's inner workings, along with a well-equipped bench of test gear, will greatly improve your odds of success, but there are quite a few broken things that can be fixed by just knowing who the usual suspects of failure are.

[vk6zgo]

Often,you can cut your fault finding time by checking the power supplies for the presence of correct voltages,but it isn't always that easy!

In general:

If the thing has multiple stages,have a look at the input section.
If that's OK, look at the next section,& so on.

OR

Start at the output end & work backwards,

Or do a combination of both.

If you are only using a multimeter,check the dc operating voltages of each stage as you go.
This will probably show up most faults,but some more complex ones will require resistance testing of components.

Signal tracing is a lot less tedious,so if you have an Oscilloscope,use it!

If a stage works,you can be pretty sure its operating voltages are OK,but if you need to check its dc conditions Oscilloscopes can do that,too.

In many cases you can troubles-shoot a piece of equipment using a 'scope without needing to turn your DMM on.

[ampdoctor]

Assuming nothing is obviously burned or leaking, I tend to look first at the easiest things to fix even if they're not likely to be the culprit. It just abbreviates the time spent mucking about under the hood. After that I start looking at components that tend to fail with regularity such as e-lytics and tantalums. After that it's down to accurately diagnosing the symptoms or causes of the failure and working through their respective function blocks.

Also, a lot of these things aren't actually done sequentially but rather concurrently. A lot of it comes down to experience and getting a feel for rooting out the cause of a problem. If you consistently work on similar types of equipment, you'll get FAST because it's often the same problems that keep showing up time after time.

Once you've found the offending fault, it's always a good ideal to suss out why that fault occurred in the first place. Save for parts just getting old and tired, or some sort of rough handling/environmental issue, components shouldn't just up and fail. In this case you may need to do some on-the-fly design engineering so you don't wind up fixing the same problem again 2 weeks down the road. This is really a good habit to get into especially if you're attempting to run a service business somewhere down the road. You'll soon get a rep for being the go-to guy that will fix your gear right and not just having a quick turn shop with half-assed workmanship. You can charge more and they'll pay it! 

[pickle9000]

Start with your eyes and nose. They are highly sophisticated instruments if you know how to use them.

[Shock]

The basics:
- Gather the schematics and operation manual if required.
- Develop a safe strategy for investigating and testing the fault to avoid damaging equipment or yourself e.g. testing it under reduced current or voltage and using an isolation transformer.
- Examine the device to make sure safe to work on e.g. check the grounding and power source wiring, shorted outputs, discharge components or circuits if required.
- Perform a physical inspection i.e. check for burnt or damaged components or circuits, drop damage or misuse, disconnected cabling.
- Attempt to reproduce the fault inline with your safety strategy.  Test full operation or perform stress testing if no fault found.
- If you find obvious problems investigate the root cause.  e.g. faulty component (due to age) or faulty connector (because of dust or oxidization).
- Apply fixes to obvious faults to ensure the root cause is taken into account to prevent further failures, misdiagnosis or expense.

If more in-depth troubleshooting is required:
-Google or service logs
-Isolate the fault using the service manual flow diagrams or schematics e.g. supply, control or interface, or output issue.
-Follow troubleshooting procedures in the service manual e.g. measure test points
-Use test and measuring equipment to pinpoint discrepancies.
-If required break your testing down into smaller parts of the circuit.
-Ensure your familiar with how the circuit is operating. Research similar circuits and read up if required.
-Ask for help if required.

Once you have found the fault:
-Apply fixes as mentioned previously taking into account root cause and ensure it's safe fix.
-Attempt to reproduce the fault again to confirm you were correct in diagnosing the fault.
-Perform preventative maintenance if cost effective or if a further failure will likely occur.

[Shock]

It's also handy to know the function of the part of the circuit your focusing on.
Power supply, rectification, regulation, filtering, amplification, oscillation or time based, pulse or waveform, thyristor and triac, analog or digital integrated circuits.
Input, output, control, display, driver, software, communication, mechanical.

[hamster_nz]

I find it is a bit like sudoku - you keep on filling in the blanks (voltages, currents and so on) until you resolve all your unknowns.

And just like sudoku, different puzzles need to be solved in a different order...

[Psi]

As long as the fault is not intermittent it's a straight forward process of eliminating sections of the circuit until you have "backed the fault into a corner" or in this case, a component.

It also really helps to keep thinking "what would cause this sort of fault" and keep that in your head as you troubleshoot parts of the circuit.

Another thing that helps is really understanding the problem first.
When it occurs but just as important when it does not!

For example, if you have a home theater amp which powers up but gives no speaker output don't just take it apart and start looking for a problem inside.

First play around with every input, output and button on the unit and take note of how the fault interacts with them all.

You might find that there's no output when using the CD and TV inputs but the builtin radio work fine.
You might find turning the surround sound modes off by-passes the problem.
You discover that the rear speakers are working and its only the fronts that are not
You might find that you can hear a tiny bit of sound with the volume at max

All of these things tell you something about the fault and give clues as to what it might be.
eg, If you find everything works when you use the headphone jack, instead of speaker outputs, you can be pretty sure the fault is not with the input boards and associated logic. You also know the amplifier board is a good place to start looking for the fault.


When the device under test is very complicated this fault analysis is even more important and realizing the fault only occurs when switch A is OFF can be the key to understanding the problem and saving days of time