A systematic approach to troubleshooting
14 minutes · Intermediate
Simulations and quizzes aren't read aloud; pause when you reach them and try them out.
The difference between a repairer and a parts swapper is method. An experienced repairer finds the fault with a few measurements even on a device they've never seen, because they work systematically, not at random. In this course you'll learn this method and apply it to real cases.
This course's safety limit
The practical work in this course is for low-voltage devices that run from batteries, adapters or USB. Mains-powered devices contain capacitors that hold over 300 V even after being unplugged. Measuring inside these devices requires an isolation transformer, a suitable meter and experience. In the power supplies lesson only the symptoms and the checks that can be done from outside are covered.The eight-step method
1. Pin down the symptom
"It doesn't work" isn't a diagnosis. What exactly happens? Does it not turn on at all, turn on and off, or lack one function? When did it start: after a drop, getting wet, a lightning strike? Ask the user, and see for yourself.
2. Check the outside
Cable, adapter, battery, fuse, switch, connector. A surprising share of faults are outside the device, not inside it. Measuring an adapter's output takes 10 seconds.
3. Use your senses
Look when you open the cover: burn marks, bulging capacitors, broken wires, cracked solder, liquid residue, corrosion. Smell: a burnt smell. Listen: clicking, crackling. If you've powered it, touch carefully (low voltage only): a part that's overheating.
4. Split it into blocks
Divide the device into blocks such as the power supply, control (processor) and output (motor, display, speaker). Look at each block's input and output. The block with a correct input and a wrong output is the faulty one.
5. Supply first
The whole circuit depends on its supply. The first measurement is always the supply voltages: are 12 V, 5 V and 3.3 V correct and ripple-free? If the supply is wrong, the other measurements mean nothing.
6. Halve it, and advance by measuring
Measure in the middle of the signal path, find which half the fault is in, and split that half again. Remember the fault-hunt game from the breadboard course.
7. Confirm the suspect
Test the part before replacing it. Measure it off the board if possible. Replacing parts on a hunch wastes both money and time, and can add new faults.
8. Ask about the root cause
Why did the part fail? A burnt transistor may have been caused by a shorted motor. If you don't fix the root cause, the new transistor will burn out too.
Statistics are on your side
Electronic faults aren't randomly distributed. According to the shared experience of repairers, the parts that fail most often rank roughly like this:
- Mechanical parts: connectors, USB/charging sockets, cables, switches, buttons
- Electrolytic capacitors: especially in hot areas and power supplies
- Power semiconductors: switching transistors, MOSFETs, regulators, bridge rectifiers
- Solder joints: cracks in the legs of large parts that heat up and cool down
- ICs: the last parts to suspect. They usually fail as a result of another fault
Start your search at the top of the list. Looking at the socket and capacitors before replacing an IC often saves hours.
Take notes, take photos
- Take photos before taking the device apart and at every step. Cable positions and screw lengths get mixed up.
- Write the values you measure on paper or on the schematic.
- Push the screws you remove into a sheet of paper in order, or put them in a magnetic tray.
Test yourself
1. What should be measured first when finding a fault?
2. Which parts fail most often?
3. What should be done after replacing a burnt transistor?
4. How do you know a block is faulty?
This was the free first lesson of the course
The remaining 7 lessons of Troubleshooting and Repair Techniques, with their simulations and quizzes, unlock when you buy the course.
See the full course →