PSU Repair: A Case Study

Setup And Safety

In my LG repair, the power PCB was mounted a fair distance away from any surface, so I had no need to worry about possible insulation crushing. I was also only tapping into low voltage and relatively low-power signals, so I did not have to worry about insulation breakdown, electrocution or potentially very destructive shorts either. For this first and hopefully only set of test points I am going to need on the primary side, I will be tapping into circuits biased at somewhere in the neighborhood of -170V connected directly to mains through the input rectifiers, so a few extra precautions are warranted.

The first three points I want to look into are the 5VSB's auxiliary output, the 3844B's supply voltage and the bottom cap voltage, since I need to know what the other two I am interested in are referenced at. To tap into those signals, I used #24 wires and slipped plastic tubes on top of the sections that will end up under the PCB. This protects the wires' insulation from component pins and provides additional insulation in case the wire's insulation is dented or uneven.

Another precaution is that instead of attaching probes directly to loose wires and just spreading them out a little, I used a perforated prototype board as a spacer to prevent accidental shorts or wandering. To protect my wires from the metal housing, I removed the fan, slipped a PVC tube over them and made them exit through the fan opening. This way, my three wires have a direct exit path across the PCB, under the transformers where there are very few component pins.