Rebuilding a cheap bench power supply

A 30 V / 5 A linear supply bought second hand. It worked, but the fan ran at full speed all the time and the current limit knob had a dead zone in the first quarter of its travel.

The fan now runs from a small thermostat board glued to the heatsink: off below 40 °C, full speed above 55 °C. Most of the time the supply is silent.

The electrolytics on the output stage were bulging, so they were replaced with low-ESR parts of the same value. The current limit potentiometer was swapped for a ten-turn one, which makes small currents much easier to set.

Total cost was a little under twenty euros, and it is now the quietest thing on the bench.