I am learning how to repair Ubiquity Lite AP 120 radio. Yesterday I wrote note on how to check for bad capacitor on the PCB of the radio. I came across a diode which showed continuity between its terminal. And so today I wanted to write a note on how to check for bad diode on the PCB(printed circuit board) board of the radio.
I think continuity check across a diode is the fastest way to check whether a diode on PCB is faulty or not. Normally there should not be a continuity beep across the diode because a diode allows current to flow in one direction but not the other way. This means that the diode is probably damaged. It could be due to overvoltage.
But there are also situations where a diode may show continuity even though it is healthy. If the diode is soldered on a PCB, other components (like capacitors or parallel paths) might create a conductive path, causing a false beep. The solution to check this case is to desolder one side of the diode and check for continuity.
Another reason for the continuity beep across the diode could be that the diode is Zener diode or Schottky diode with low forward voltage. Such low voltage Zener or Schottky diode can trigger continuity beep. The way we can get rid of this wrong reading is to use the diode mode on the multimeter to measure the actual voltage across the diode.
In my case, I checked the diode in diode mode on the multimeter also and there was OL reading with both ends measured with +ve and -ve probe on the cathode and anode of the diode and when they were swapped. I ensured that this is the case, by testing a known good diode of the same but another Ubiquiti Lite AP radio. So the diode is definitely bad.
This is illustrated in the video below.
Related notes:
