Diagnosing circuits

[Deven]

Last night SeaShadow and myself fixed a couple broken LCD monitors we had lying around the hackerspace. There were a few questions regarding how to test for common problems that came up. Rather then retyping a ton of information (and potentially getting it wrong), I will post a couple links here to help those looking for info.

Testing Capacitors: Often one of the first things to die. Check for bloating / corrosion / etc. Another site on testing capacitors.

Testing Diodes and Transistors: Aside from Capacitors, probably one of the first things to go bad in many electronics. NOTE: Always check model numbers (just google it); just because it is in a TO-92 or TO-220 package doesn't mean it is a transistor. Images of Transistors and Diodes.

Testing Resistors: Another thing that can go horribly wrong. Usually I look for discoloration of the resistor / pcb around the resistor first. These are signs that the resistor has been WAY too hot, and likely is bad. Another site on testing resistors. Images of Resistors and Resistor Arrays.

There's some very basic steps I follow when testing manufactured circuits.

1) Open the device. Please follow safety procedures; this usually is as simple as unplugging the device and waiting a couple hours. However some devices may require additional steps. You will want to research this (google) as it is well out of the scope of this post.

2) Look for obvious problems: bloating of caps, corrosion, burn marks, exploded pcb traces, etc.

3) Test and replace bad capacitors, resistors, transistors, diodes, etc. See the other sticky threads in this forum for online and local suppliers.

4) If the device doesn't power on at all: check the incoming voltage on the device's circuit board. Again follow safety precautions! Replace fuses, etc as needed. Check the voltage into and out of any transformers.

5) Test the voltage coming out of the power supply(s). Make sure it is within specifications. If you aren't getting the correct voltage from the outputs of the power supply check for bad fuses, bridge rectifiers, etc. Cheaper circuits may have a voltage regulator that you can replace. NOTE: Many crt's, lcd's, computer PSU's, etc have standby power that you can test, but you won't be able to test the full capabilities of that power supply circuit until you can trigger the main circuit on.

6) If you are getting the correct voltage(s), then go back to steps 2 and 3. You may also want to check all physical switches to verify they are working correctly.

Sorry for the semi-unreadable post, I may clean it up in the future.
Feel free to add more, or compose your own tips/tricks/faqs/manuals below.