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I've recently got this error: 304-Keyboard or System Unit Error when booting my Compaq Prolinea. I've tested the keyboard on another computer and it's fine. I don't have another PS/2 keyboard to test, but the keyboard does briefly flash its lights when the computer first powers up, but then has no power.

Most people/sites online say that the original troubleshooting was to test the keyboard, then replace the system board. Obviously, I can no longer replace the system board, so I'm looking for a way to diagnose this problem and/or fix it. I've got a multi-meter and beginner-to-intermediate solder skills, but I don't know where to start, and I can't find any type of schematic for this old computer.

There was an old barrel battery (next to the PS/2 ports) that looked like it may have started leaking, but I removed it, and I couldn't find any obvious damage to the board.

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    You might want to look for non-obvious damage then ;-) Do you have pictures of the area around the battery? Also, how do the capacitors look? Any sign of bulging?
    – Michael Graf
    Commented Feb 11, 2021 at 6:55
  • @MichaelGraf I added a picture of the space where the battery was and the keyboard PS/2 connector. There are very few parts on the motherboard that are not surface mount, so I don't see any electrolytic capacitors with any problems. I'm not sure how to test the resistors or caps or whatever those things are next to the PS/2 ports, but they are not short (continuous from one side to the other).
    – Quasi_Stomach
    Commented Feb 11, 2021 at 7:24
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    If it's not the keyboard, it's likely the PS/2 keyboard controller. Originally that chip was called 8042, it may be integrated into a "super-controller" chip on your mainboard. So the first step is to look at the whole mainboard and identify at least the important chips. Datasheets for the chips can usually be found with googling.
    – dirkt
    Commented Feb 11, 2021 at 7:39
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    I think I see some greenish residue around the top 3 SMD components in the row of 5 above the label C14. If you can, I'd suggest you desolder the PS/2 connector and clean the entire region with vinegar, water and isopropanol (if it was an alkaline battery).
    – Michael Graf
    Commented Feb 11, 2021 at 8:29
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    There is definitely signs of obvious corrosion damage to the board from the leaked battery. The keyboard may still get supply voltage but data or clock wire may have been cut by the corrosion but it is not visible. Even a multimeter can be used to determine where the problem is.
    – Justme
    Commented Feb 11, 2021 at 10:04

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