PC Power Supply Repair

Project overview

Document version 1.2 — This guide covers the standard repair workflow for ATX PC power supply units (PSUs). It is intended for technicians, repair engineers and electronics enthusiasts. The scope includes switch-mode power supplies (SMPS) commonly used in desktop computers, with power ratings from 300 W to 850 W.

This document is a living reference. It contains practical steps, component-level troubleshooting and safety guidelines. All procedures must be performed with appropriate ESD protection and under safe working conditions.

Safety first

Mandatory precautions

• Disconnect the PSU from mains and wait at least 5 minutes before opening.
• Measure the voltage across the large electrolytic capacitors — must be below 10 V DC before touching the board.
• Use a bleeder resistor (e.g., 10 kΩ / 5 W) with insulated probes to safely discharge capacitors.
• Wear safety glasses and use insulated tools.
• Work on a grounded ESD mat; use an antistatic wrist strap.
• Never bypass fuses or protection circuits — replace with identical ratings.

For a full safety checklist, refer to Section 3 of the internal standards document PSU-SAF-2025.

Tools & equipment

A properly equipped workbench is essential for efficient and safe repair. Below is the recommended tool list for PSU repair.

Calibrate your multimeter before each session

Diagnostics procedure

Follow this systematic approach to identify faulty sections. Start with visual inspection and then move to voltage checks.

Step 1 — Visual inspection

• Look for blown fuses, bulged capacitors, burnt resistors, cracked solder joints, or charred PCB areas.
• Check for missing or damaged components (e.g., MOV, NTC, bridge rectifier).

Step 2 — Standby voltage (+5VSB)

With the PSU connected to mains (via isolation transformer), measure the +5VSB line on the 24‑pin connector. Expected range: 4.75–5.25 V. If missing, the standby circuit (PWM controller, switching transistor, optocoupler, or feedback) is likely faulty.

Step 3 — Main rails activation

Short PS‑ON (green wire) to ground (black) using a jumper. Measure 12 V, 5 V and 3.3 V rails. If all are zero, the main PWM or protection circuit may be tripped. Check the primary side (switching transistors, driver) and secondary rectifiers.

Repair steps

Component replacement must be performed with the correct ratings and polarity. Use high‑quality replacement parts (105 °C capacitors, fast‑recovery diodes).

1. Replace fuse: Use only same type and rating (e.g., 5 A / 250 V slow‑blow). Never solder over a blown fuse.
2. Primary side: Check bridge rectifier, NTC thermistor, MOV, and main switching MOSFETs. Replace all damaged parts.
3. PWM controller: If the controller IC is faulty, replace with exact part number. Check surrounding resistors and capacitors.
4. Output rectifiers: Replace shorted Schottky diodes with equivalent or better (higher current, lower Vf).
5. Capacitors: Replace all bulged/leaking capacitors. Use low‑ESR types for output filters.
6. Solder joints: Reflow any cracked joints, especially around transformers and heavy components.

Testing & quality control

After repair, the PSU must pass a full load test and safety checks before returning to service.

Load test procedure

• Connect PSU to a suitable electronic load or a test motherboard with known good components.
• Apply 20% load, then 50%, and finally 80% load (if rated).
• Monitor +12 V, +5 V, +3.3 V and +5 VSB rails — all must stay within ±5% (±3% recommended).
• Check ripple with oscilloscope: should be below 120 mVpp for 12 V, 50 mVpp for 5 V/3.3 V.
• Verify PG (Power Good) signal and PS‑ON behavior.

Final QC checklist

Specifications & references

Common ATX PSU specifications and pinout reference for 24‑pin connector (ATX v2.4).

24-pin main connector (ATX)

References: ATX12V Design Guide v2.4, Intel PSU testing guidelines, IEC 62368‑1 (safety).