How do you test a diod in the circuit

After a power surge or thunder storm your TV, DVD, Amplifier or anything with a power supply will not turn on or partially turns on with errors. Your unit may also be many years old and has quit while operating or after shutdown it would not turn on.

If this is the circumstances of your unit you need to first unplug the unit from the mains for safety and remove the cover(s) or back if a TV to access the power supply board.

You will no doubt need to do some sodering so see my tip at http://www.fixya.com/support/r9091711-step_step_procedures_soldering for help with soldering.
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The first thing you should check is the fuse although with switch mode power supplies you do not find blown fuses very often these days. Using a DMM set to the ohms range (preferably continuity check) check for an open fuse. If there is continuity the fuse is good. If the meter reads a high resistance the fuse is open and needs replaced. Always replace with the exact kind and size (amps) of fuse you take out. Generally these days the fuse rating is marked on the circuit board near the fuse.

Restore power to the unit and try turning it on. If your replacement fuse blows you have a short somewhere in the system. Any boards which will not affect the unit turning on should be disconnected and the fuse replaced and try to power it on again. If the fuse does not blow the problem is in one of the circuit boards which are not hooked up to the power supply. If the fuse blows the problem is in any boards still connected or the power supply.

To troubleshoot the power supply look for any charred or burned components. Look at the electrolytic capacitors to see if the ends are bulging. If they are all the electrolytics should be replaced. Electrolytic capacitors look like small cans, usually different colors but they could be black. They will be marked in microfarads and have a dc voltage rating. They should only be replaced with exact units, same values for both parts of the rating. Also they have a polarity and must be installed the same way they are taken out. The negative terminal is marked on the board and on the device.

If all the capacitors look ok or you replace them all (they are cheap) continue checking all diodes for shorts or opens. Out of the circuit a diode should read high resistance one way and when the leads are reversed you should read about 400 to 800 ohms. This is showing a good part. The DMM should have a diode check setting. It is the location with the symble of a diode beside the position. Diodes can usually be checked in circuit to determine a ball park estimate of whether they have a problem. Remove any which seem suspect and check them out of circuit. Make sure they are reinstalled correctly. They have a polarity also. Line the symble on the device up with the one on the circuit board.

After the electrolytic capacitors and diodes all the transistors (mainly power) should be checked for shorts. If they are shorted you will find them in circuit. Any suspicious units can be removed and tested with a DMM. Testing is done similar to a diode except there are two junctions to test. You should have low resistance between two different sets of leads and high resistance between all others (out of circuit only).

The only things left to check are the resistors. The easiest way to test them is with the meter on diode check make sure they are not in a dead short. Some will be low enough resistance they should test as a short.
They should be marked with the values. Resistors with a high resistance can be tested by changing the range or use auto range.

Any parts can be purchased at http://www.encompass.com or http://www.mcmelectronicsinc.com or http://www.mouser.com .

If all the components check out your power supply is ok and you need to do the troubleshooting on the remaining boards.

Thanks for using FixYa and for the great rating.

hardrocko