ATX Power Supply Repair and Troubleshooting Procedures

Essential Steps for ATX Power Supply Repair

1 .- If the fuse is blown, replace it before you start measuring other diodes or the bridge rectifier. The diodes conduct current in one direction only. If reversing the ohmmeter leads shows continuity in both directions, they are shorted and should be replaced. Never solder a wire instead of a fuse; this will cause the power supply to deteriorate further.

Testing Switching Line Transistors

2 .- Continue by desoldering and measuring the transistor switching line input. Most of them are NPN; remember that when measuring base-collector or base-emitter junctions, they should conduct in one direction only. If they show very low resistance, they should be replaced. In the majority of power supplies, including the ATX type, the BUT11 works well.

Checking Electrolytic Capacitors

3 .- Confirm that the "filters" or electrolytic capacitors are not defective. Visually, you can see if they have spilled oil or exploded. Use an ohmmeter to check if they are shorted.

Identifying Deteriorated Resistors

4 .- There are four resistors associated with the power transistors that often deteriorate, especially if the transistors are shorted. The values vary between brands, but you can identify them as two connected to the bases of these transistors (hovering at 330k Ohms) and two connected to the emitters (about 2.2 Ohms).

The Startup Circuit Components

5 .- The "start" of the power supply is obtained by a polyester-type capacitor in series with the input transformer and a resistance of about 10 Ohms. If any of these components are open, the power supply will not start.

Safety Precautions and Testing

6 .- CAUTION: Upon testing the power supply, since it works directly with line voltage, you should connect it to a line isolation transformer (220V-220V or 110V-110V). This will avoid unnecessary risks and the danger of electrocution. You can also connect a 100W lamp in series if there is any short circuit.

ATX Starting Pulse Requirements

7 .- ATX power supplies need a starting pulse to begin operation. You can connect the power supply to the motherboard without connecting other elements like floppy drives or hard drives. However, this should only be done after verifying that the power supply is not shorted, following the procedure in section 6.

Advanced Troubleshooting and Oscilloscope Use

8 .- If the unit still does not work after applying these procedures, you will need to check the oscillator; for this, there must be at least a 20 MHz oscilloscope. Consider the investment of time and the cost of the power supply before deciding whether to proceed. The integrated pulse modulators of most power supplies are listed in circuit manuals like the Philips ECG or similar. Start by checking the power of this integrated circuit and the voltages on the different pins. You can also check "cold" (without being connected to the source) for shorted diodes. These power supplies often use 1N4148 low-signal diodes that break down easily and Zener diodes that commonly short if you accidentally change the supply voltage. In most power supplies, there are integrated rectifiers that physically resemble transistors but internally consist of two diodes. These may need to be removed and measured outside the circuit; otherwise, the transformer they work with may make them appear shorted.