This professional lighting module came in for repair, the fault was no power. The customer had replaced the fuse and it kept blowing when power was applied. This is usually a good sign as it means that the overload protection is working correctly. This most often means a shorted component somewhere, and it seemed it might be possible to salvage it.
However, you have to bear in mind that for some things that this is not always possible. For instance, if the lighting section has also blown or the motor has shorted on one of its windings then its pretty much game over. Also if it is a logic IC that was flashed with a program, then similarly it is game over.
The trouble is, many products are designed in such a way that they are difficult to repair. The manufacturers do not make spare parts available for the most part, and they keep churning out low quality wares. I have discussed this issue at length previously in this article. This lighting repair was no different. The power supply was difficult to get out and I ended up breaking the motor connection, which was flimsily held on with a tiny dab of solder.
In order to get access to the parts you had to remove cable clips. Unfortunately, rather than use decent connectors, this model used glue. This glue had to be cut off manually to ensure that the clip could be removed without damaging the board.
Another SMPS repair
After finally getting the power supply out after a struggle, it was a case of de-soldering the heat sinks and then investigating whether this could be repaired. I applied power from my bench supply and the light lit up. This indicated that the bulb and surrounded mechanism was intact. Next, I tested the motor coils for resistance, and then applied a voltage. The motor juddered to life, indicating that there was a power supply failure.
I got some strange readings on some components on the hot side of the coil. I started to unsolder all the suspect components so I could test them.
In all, there were 5 bad components:
Internal fuse, mosfet, resistor, bridge rectifier, and suppression capacitor. The fuse was open, the resistor, and capacitor, were out of their tolerable ranges. The bridge rectifier measured 1.2 ohms on all pins and the mosfet measured 1.7 ohms between drain and source, indicating catastrophic short circuit. All in all I had to rebuild the primary side of the power supply pretty much!
To compound matters, the parts used were not generic and I had to source equivalents. When replacing components you must make sure you spec them as close as possible. For instance, you need to match voltage and current ratings, as well as behaviours such as maximum reverse current on bridge rectifiers. Luckily I was able to find datasheets for the components used and find pretty much equivalent matches.
To test the repair on a SMPS you should always make sure it is connected to a load. I tend to use a lightbulb since it can take the full voltage of the primary side of the supply. Also a blown light bulb is easier to replace than components on a PCB that may be connected to a irreplaceable IC chip.
Fortunately, the light tested OK, and it stabilised under load for ten minutes, meaning that it was alive and well. I reassembled the light and tested it and it worked great. Absolute nightmare to get apart, alot of soldering but well worth it as these are designed to be disposable!