DESKTOP MOTHERBOARD REPAIR NO POWER

desktop motherboard standby voltages, their function, and how to troubleshoot issues when they’re faulty:

⚡ What Are Standby Voltages?

Standby voltages are low-level power rails—most commonly +5VSB (5V Standby)—that remain active even when the computer is turned off but still plugged into a power source.

🔌 Purpose in the Circuit:

Power to system management controller (SMC) and BIOS/UEFI chips.
Enables wake-on-LAN, USB device charging, and soft power-on via the power button.
Keeps certain components in a low-power state ready to resume full operation.

🔍 Common Standby Voltages

Voltage Rail	Typical Value	Function
+5VSB	4.8V–5.3V	Powers motherboard logic when off
+3.3V	3.2V–3.4V	Used by RAM, chipset, and PCIe
+12V	11.7V–12.3V	Powers CPU, GPU, fans (active only when on)

The +5VSB is the key standby voltage. If it’s missing or too low, the system won’t power on properly.

🛠️ Troubleshooting Standby Voltage Issues

1. Check Power Supply

Use a multimeter to measure voltages at the 24-pin ATX connector.
Short the PS_ON (green wire) to GND to manually start the PSU.
Measure the purple wire for +5VSB. It should read between 4.8V and 5.3V.

2. Inspect Motherboard Behavior

If +5VSB is present but the system won’t power on:

Check for short circuits on the board.
Look for burnt components or bulging capacitors.
Try powering with minimal components (CPU + RAM only).

3. Swap Components

Try a different power supply to rule out PSU failure.
Remove all peripherals and test with just the motherboard and CPU.
If voltage drops when components are added, one may be drawing excessive current.

4. BIOS and CMOS