1. The ATX switching power supply circuit board features a high density of components and a "stereoscopic" layout. The lowest components are only 2mm tall, while the tallest can reach up to 50mm. In between, components are often arranged in 4-5 layers, especially with two large heat sinks that block visibility. This makes it difficult to inspect or test many parts directly. Some components may be visible, but their pins are too small or inaccessible for testing probes. If you attempt to test from the back, it's challenging to match the solder joints with their corresponding front-side components. Therefore, removing the two large heat sinks first is highly recommended, as it makes the board more transparent and simplifies the repair process, making it safer and more efficient.
2. Damage to the standby power supply is often severe and can recur during repairs. However, the PCB of an ATX power supply is usually narrow, and the pads are small, making them vulnerable to damage from repeated soldering. A common solution is to create a short jumper wire from a solder joint that might need multiple component replacements. By soldering the components onto this short line for testing, the number of solder joints can be reduced, minimizing the risk of pad lifting and improving repair reliability.
3. The fuses in ATX switching power supplies are typically rated at 4A, 5A, or 6A. While they provide some protection under normal output conditions, they may not function properly when the output power is low. If the power is turned on without caution, hidden faults may persist or even cause new issues. To prevent this, it's best to connect a 1A fuse in series during the initial power-up. If the 1A fuse blows, it indicates a short circuit in the standby power supply, and the issue should be addressed before proceeding. If the 1A fuse remains intact, replace it with a 2A fuse and continue testing. If the 2A fuse blows, the main switching power supply likely has a short circuit. If it doesn’t, the fault is elsewhere in the system, not a short circuit. The repair sequence should still focus on the main switching power supply after the standby unit, and the 2A fuse should be used as a safety measure during the process.
4. When there is no load, the +12V output may rise by 0.6V, while the 3.3V voltage, which is regulated via "reverse current feedback," may drop instead of rising, potentially falling to 1.86V. This can lead to misdiagnosis and improper repairs. To avoid such issues, it’s best to use a CD drive as a load. Once connected, the voltages across all channels should stabilize. The optical drive’s LED indicator can also serve as a visual cue for power output, and playing music through headphones can confirm that the drive is working. Since the optical drive consumes moderate power (5V/1A, 12V/1.5A), it meets maintenance needs effectively.
Finally, if the switch transistor and rectifier do not heat up, it’s safe to remove the large heat sinks. With a 2A fuse in place, this approach ensures accident prevention and makes the repair process much smoother.
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