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I'm working on another Pavilion (the 750n this time) and when you plug it in, it automatically turns on without the power button, does one solid beep for probably 5 seconds, and turns off, won't let you press the power button to try again. The only way to get it to do the problem over again is to unplug it, and plug it right back in. I started with the power supply, tried it with my own power supply, same problem. I unseated each RAM module, and moved them from each slot to see if it was a slot. I swapped the video card, I unplugged the HDD, floppy, and both optical drives individually, removed the PCI cards, I even took apart an old Compaq I had here and tried a different CPU. All the fans are working. There is nothing displayed on the screen for the 7 or so seconds the computer stays running. Any other suggestions?

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Good, methodical troubleshooting by — that level of swap-and-check narrows the fault to something the board or firmware is doing rather than a single peripheral. was right to point toward beep-code references; the exact meaning of a long/continuous beep depends on the BIOS vendor, so matching the pattern to the board matters.

Try these focused checks before writing the board off:

  • Clear the CMOS: remove the coin cell or use the clear-jumper for a minute, then try a cold power-up. This rules out a corrupted BIOS setting that forces an immediate shutdown.
  • Do a bench test: move the board to a non-conductive surface, remove case standoffs and all non-essential connectors. Boot with only CPU+cooler, one known-good RAM stick (in the recommended slot), power, and the PC speaker attached to hear POST.
  • Isolate the front-panel: unplug the front-panel power/reset connectors and short the two power pins briefly with a screwdriver to rule out a stuck switch or wiring short.
  • Inspect visually and by smell: look for bulging or leaking capacitors, scorch marks around VRMs, or cracked components. Small damage around the CPU power circuitry frequently causes the exact symptoms you describe.
  • Identify the BIOS chip or markings on the board and compare the exact beep length/pattern to that vendor’s POST codes to narrow the fault to RAM/CPU/graphics/chipset.

If the machine is ultimately scrapped, salvage usable parts (RAM, PSU if tested good, optical drives, fans). Safety notes: always unplug mains, discharge capacitors by waiting a minute after unplugging, and use anti-static precautions. If none of the above gives a definitive result, the behavior you describe is consistent with a motherboard/firmware failure where replacement is usually the only economical fix for an older Pavilion.

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I contacted the owner and told him I thought it was the motherboard, and he told me to junk it. The computer is probably a good 8-10 years old. Rik: thanks for the information!

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