Broken 24V Switched Mode Power Supply

My LED light stopped working this week. I am quite fond of it as it is quite a "quiet" one. By that I mean, it does not produce so much electrical noise as most LED lights do these days. As a keen amateur radio enthusiast, that is important to me. So my son openend the plastic brick and we went in search of the fault.

Switched mode power supplies can store high voltages even after they are unplugged from the mains supply. At 230V there can be DC voltages in excess of 315V stored inside. Be sure you know what you are doing and know how to safely discharge these voltages first, before working on any kind of mains powered devices.

The first thing I do, when fault fixing switched mode power supplies, is to test the transistors, diodes and bridge rectifier components with a test meter set to diode test mode. Fluke published a good article explaining how to do this. If everything tests good, I then go and test all the electrolytic capacitors using an ESR meter. I am looking for shorted components.

A quick look over the PCB showed, that the rectifier that converts 230v AC to DC, had one leg physically broken off; So a new one was ordered and soldered in place.

Power up on a dim bulb test bench supply, showed no power output. So debugging continued. I drew up this version of the PCB in Adobe as a .pdf to help with the debugging process. It shows the bottom of the PCB.

The orange line on the right shows the division between high voltage on the left and low voltage on the right. A quick test, showed no low voltage at all on the right hand side. So the problem must be on the left. The end of the red and black lines should show high voltage DC. This was also not present in the circuit. So the tracing went back to the newly soldered in bridge rectifier at the bottom left of the picture. The middle two pins should have 230V mains AC voltage but there was nothing. So the line voltage was traced from where it came into the PCB (see the blue lines). However, it stopped at the four pin component on the bottom left corner. Turning the PCB over, we can see that this is a choke (top left).

Zooming in, we can see that a leg has broken off.

So, the leg was re-attached to the thin wire of the choke and the component re-attaced to the PCB using hot glue. Finally, the case was glued back together and the LED light is working again.