I have been working in the electronic field for more than 15 years. I have worked in the Manufacture, Communication, and Avionic field.
Perform a visual inspection of the unit. Look for any sign of damage (nicks, bent corners) or any indication that the unit was drop or hit.
Open the unit and look for loose screws, washers, nuts, etc.
Look for damage components due to heat ( IC, Microprocessors, memories, resistors, etc ).
Sometimes is very obvious to see the burn marks on a board, but sometimes it can be very difficult, specially when working with a double layer board. If you are working with one of this board look for any sign of discoloration on the board. It can be an indication of burn.
Solder bridge can happen with any kind of electronic circuits, but they are more common with Surface Mount circuits. In most cases you will need the aid of a magnify lamp to find it.
I have seen this happen only with Surface Mount circuits, specially units use in avionics ( Airplane, Helicopter, etc ) due to vibration, unit been drop, boards been drop, etc. So look carefully.
Corrosion can happen on a board due to lack of humiseal. Humiseal is use to protect the board again humidity and is more notorious in units use in the avionic field. Look very carefully because sometimes corrosion develops under multi pin connectors which make it very difficult to find.
Solder Joints are shiny and smooth. If you see any dull solder joint in the section of the board you are working with check it.
Look for blown fuses. Remember diodes can be use as fuses.
Crystal Oscillators provide timing for the circuit. So it is a good idea to check the output frequency of it.
Fracture solder joints usually happens with time, specially on boards use in the avionics field due to vibration.
Look for this defect. It can save you a lot of time. It happens anytime you have water damage, due to humidity.
Apply power to the unit and touch the top of the component ( IC, Microprocessor, Memories, etc ) to see if it is overheating.
It happens a lot in the Manufacture field.
Very common in the Manufacture field.
Look for wires making intermittent contact.
You will find components mount on the board using screws, washers, etc like SCR's, Heatsink specially in Power Supplies. Check them carefully. They can get loose.
Make sure the unit you are working with has the right software. Sometimes reloading the software can solve the problem.
You will need the following:
Let say you have a Power Supply that does not power up. First thing you check is to make sure the power cord is in good condition. Go over the theory of operation so you know how it work. Then you want to place the negative cable from your multimeter to ground and take readings with the positive cable. If the power cord is in good condition, I will start by taking voltage readings from the output back to the input. Check for open diodes. Check the voltage regulator's input and output voltage. Look for open fuse. Damage wires, loose connections. Check the input and output of the bridge rectifier. Look for overheating components.
If you have an amplifier where the output is suppose to be 20VAC and you have 5VAC, start from the output to the input. Usually one or more of the power transistors can be open or short. Check for open or short diodes. Check the supply voltage. Check for overheating components.
Check the power cord, fuse, and any damage components due to heat. If all that checks OK, try reloading the software if it applies.
Check inputs and outputs signals. Check voltage levels. Usually if the voltage is low you will have a leakage capacitor. If the voltage reads 0V, you probably have a bad relay or a short component.
Usually when you have this problem the Crystal Oscillator is not putting out the right frequency. Also the CPU could cause this kind of problem.
When the battery gets hot you have a short in the circuit. Check the diodes. 98% of the time that is causing the problem.
Symptom - Unit failed full test, but passed individual test.
Let say you are running a test that has 100 steps and the unit failed test#75 but when you run test# 75 by itself it passed. Open the unit take every board out of the unit and apply heat. Put boards back in the unit and re-test. If that does not fix the problem try applying cold. If neither fix the problem, best thing to do is to find the board causing the failure and replaced it.
Symptom - Unit works on and off.
After the unit is power up, the display flashes on and off. Open the unit and perform a visual inspection. If inspection is ok, reload the software. Let unit on for several hours and re-tested. If there is no software to reload, pull out all the board and clean all the contacts. If you have the option, perform an environmental test cold-hot. If unit still fails, check the inputs going to the display.
Symptom - Unit power up but does not do anything else.
First thing you want to do is check all the voltages. Get the schematic and make sure the voltages are ok. If the voltages are ok check the inputs and outputs to the main chip. (usually the biggest chip in the circuit). Check the memory chip. Make sure is not hot. Also check the inputs and outputs.
Symptom - Unit works for few minutes then stop.
Check for overheating components. Also check for water damage, moisture, corrosion.
Symptom - Unit is plug into the wrong outlet.
If you have an electronic device that works with 12Vdc for example and you plug it into a power source other than 12Vdc (20Vdc or higher). Look for a blown fuse or burn components in the Power Supply section.
It is good to have a spare unit when troubleshooting to compare readings. Sometimes the schematic does not have enough information on it.
Software can also cause problems. Make sure you have the correct software install.
Another thing to do is to test the unit under hot and cold conditions.
Also it is a good idea to reset the boards by pulling them out of the unit.
Remember, if you are working with a unit and you just can find what it is wrong with it. Put it to the side and work with a different unit. Go back to it later and it is a 99% chance you will find the problem.
Well, I hope these tips will help you solve some of the problems you will encounter as an electronic technician.