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AST Advangage Pro! 486/50 model broken AT Keyboard Port

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First post, by JiveTurkey

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Hey, so I think I may have managed to cook my keyboard port. I've tested the keyboard on a different computer, and it works just fine there, so I think it's the port. I used an AT to PS/2 adapter to connect my keyboard to a KVM switch that carried two ps/2 ports and vga to four possible outputs; I then used a PS/2 back to AT to connect it to my computer, and from that moment on my computer won't recognize my keyboard; so I definitely think I cooked something in there.

Is there anything I can do to fix it, because it'd be such a waste if the whole thing was toast because of this; or am I just completely screwed?

Reply 1 of 6, by Shponglefan

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It can probably be repaired, but it depends what tools you have available for diagnosis and repair.

If you have a multimeter, first thing I would do is check the pins of the keyboard port on the motherboard to see what signals you are getting. As well as testing to see if anything is potentially shorted (other than the ground pun). I would also test continuity from the pins to the board to make sure there are no cracked solder joints.

Keyboard ports are often protected by fuses, so it's possible that if something was wired incorrectly (i.e. shorted) that it blew that fuse. Replacing these usually requires desoldering and resoldering a new one.

Of course, there could be some other issue at work, which is why testing and diagnosing things first is important.

Pentium 4 Multi-OS Build
486 DX4-100 with 6 sound cards
486 DX-33 with 5 sound cards

Reply 2 of 6, by JiveTurkey

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I'm really hoping it was a fuse, but I can't find schematics for the motherboard, to even know if it's fused, and which fuse it would be. I've never replaced one of those little face mounted fuses before.

Reply 3 of 6, by rasz_pl

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you wont find any diagrams
start with taking a good picture of pcb near keyboard connector
fuse will be marked Fsomething, there might also be fired ferrite beads (FBsomething)

https://github.com/raszpl/sigrok-disk FM/MFM/RLL decoder
https://github.com/raszpl/FIC-486-GAC-2-Cache-Module (AT&T Globalyst)
https://github.com/raszpl/386RC-16 ram board
https://github.com/raszpl/Zenith_ZBIOS Zenith Z-386 MFM-300 ZBIOS disassembly

Reply 4 of 6, by Shponglefan

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JiveTurkey wrote on 2026-04-09, 19:45:

I'm really hoping it was a fuse, but I can't find schematics for the motherboard, to even know if it's fused, and which fuse it would be. I've never replaced one of those little face mounted fuses before.

Before going down that road, you need to first test and diagnose things. It could be a fuse, it could be something else entirely.

Do you have a multimeter available?

Pentium 4 Multi-OS Build
486 DX4-100 with 6 sound cards
486 DX-33 with 5 sound cards

Reply 5 of 6, by JiveTurkey

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I have one, somewhere, but I haven't seen it in a little while; I haven't had the space to do any circuit building in quite a while. You're right though, first step should be continuity testing.

Reply 6 of 6, by wbahnassi

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It could also be mechanical stress that finally managed to crack the solder joints of the connector. Does it have any wiggle in it if you rock it gently back and forth? Cracked soldier joints are hard to see by naked eye. They look like faint rings on the back side of the solder holes.

Turbo XT 12MHz, EGA, MFM HDD
Intel 386 DX-33, Speedstar 24X, SB 1.5, 1x CD
Intel 486 DX2-66, CL5428 VLB, SBPro 2, 2x CD
IBM BlueLightning 100MHz, CL5428, SB16, 4x CD
Intel Pentium 90, Matrox Millenium 2, SB16, 4x CD
HP Z400, Xeon 3.46GHz, YMF-744, RTX2060

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