DIY Fix for RXN305D Power Supply Voltage Fluctuations

If you're interested in DIY projects or repairing adjustable power supplies, you might want to take a look at this article.

The RXN305D adjustable power supply, after replacing two variable resistors, started showing erratic voltage readings. For example, when set to 10 volts, it would jump to 9.5 volts, then 10.2 volts, and after a restart, it would stabilize at 8.3 volts with no consistency. Additionally, when connected to a 12-volt light bulb, the voltage would drop to around 4 or 5 volts, even when the current was set to maximum. After some inspection, it was found that one of the large capacitors had a cold solder joint, which could be fixed by reinforcing the connection. However, if the voltage readings still fluctuate, the issue might be poor contact with the variable resistor. You can use a multimeter to check if the output voltage is stable.

If you're certain that the issue is with the display, you can open the adjustable power supply and look for the TL431 voltage regulator chip along the display wiring. Its control pin is connected directly to the cathode, with the anode grounded. Measure the voltage at the K pin—if it fluctuates between 2.5V, 2.7V, and 2.9V, the chip may be faulty. You can desolder the legs and measure again to see if the voltage returns to normal. If the issue persists after resoldering, any voltage over 2.5V is abnormal. If you have a surface-mount 431, you can replace it. If not, a through-hole replacement can be used instead. Once the display stabilizes, the issue is resolved, and the fault is cleared!