Can you test the fuse between the power connector and the keyboard port for continuity? If the fuse is blown, you can just replace it but first you need to determine why it's blowing up.

If the fuse tests OK but the keyboard still doesn't work it's probably the keyboard controller chip, which can also be replaced but that's more difficult since it needs desoldering. It can also be one of the components that are connected to it (logic chips, resistors, etc.).

Just to be sure: you're not hot-plugging the keyboard right?

HQ pictures! 😀
I can feel your pain. It's gonna get solved.

Have you tested continuity from keyb to keyb controller? Maybe lanes got burnt under the connector. I once had that.

Seems like there's a bridge between the keyboard and the mouse plugs from the KVM. So the 5v on the keyboard plug gets fed back to the ps2 mouse plug. And then the dumb adapter which I have now learned is the spawn of satan feeds those 5v to ground. Hence the fuse as in the attached picture. Looks blown to me, but I went into this not knowing there even was a fuse there - so what do I know until I measured? :)

Simply check with multimeter on both sides of the fuse with the contact buzzer.
If silent - it blew-up and needs replacement. You can simply shorten it for a quick test, but with that KVM of yours you risk burning something further down that is not easy to replace.

ltning wrote on 2023-03-27, 02:33:

And then the dumb adapter which I have now learned is the spawn of satan feeds those 5v to ground.

Indeed. Those passive adapters are to be used only to connect mice/keyboards that were specifically designed to work with this kind of adapter. This is true both for passive serial-to-PS/2 and passive PS/2-to-USB adapters. You need to be especially careful with serial-to-PS/2 adapters, because they provide pins with +/-10V (although at low current capability, so the chance to burn stuff is limited) on the PS/2 socket. Keep that in mind for further use: They are to be considered "part of the mouse/keyboard they came with" and must not be used on anything else.

The previous paragraph does not apply to active USB-to-PS/2 adapters (that allow to connect a standard PS/2 keyboard to a USB port), which are available for cheap. On the other hand, I have yet to see an active small PS/2-to-USB adapter (plugs into PS/2 port and has a USB receptacle) that is not passive, though, so I consider those adapters generally unfit for general use.

Also after testing the fuse and you find that is blown you can ad a 'low forward voltage drop diode' on the 5v line for those ports so if there is any reverse voltage 'injected' in to the motherboard from that kvm switch then it would be protected in the future and will not blow the fuse every time.

pshipkov wrote on 2023-03-27, 06:37:

Simply check with multimeter on both sides of the fuse with the contact buzzer.
If silent - it blew-up and needs replacement. You can simply shorten it for a quick test, but with that KVM of yours you risk burning something further down that is not easy to replace.

It's already fixed, I helped him troubleshoot it in discord last night. The KVM isn't the problem per se, all it does is share a common +5 line between the ps/2 keyboard and mouse ports on each output. The problem is he tried to use a ps/2 kvm through a ps/2 to serial adapter, which resulted in shorting that common +5 line from the keyboard into ground via the mouse connection.

maxtherabbit wrote on 2023-03-27, 15:36:

pshipkov wrote on 2023-03-27, 06:37:

Simply check with multimeter on both sides of the fuse with the contact buzzer.
If silent - it blew-up and needs replacement. You can simply shorten it for a quick test, but with that KVM of yours you risk burning something further down that is not easy to replace.

It's already fixed, I helped him troubleshoot it in discord last night. The KVM isn't the problem per se, all it does is share a common +5 line between the ps/2 keyboard and mouse ports on each output. The problem is he tried to use a ps/2 kvm through a ps/2 to serial adapter, which resulted in shorting that common +5 line from the keyboard into ground via the mouse connection.

Happy ending!
Note to self, check KVM and adapters before plugging in keyb/mouse.
New thing learnt.

maxtherabbit wrote on 2023-03-27, 15:36:

pshipkov wrote on 2023-03-27, 06:37:

Simply check with multimeter on both sides of the fuse with the contact buzzer.
If silent - it blew-up and needs replacement. You can simply shorten it for a quick test, but with that KVM of yours you risk burning something further down that is not easy to replace.

It's already fixed, I helped him troubleshoot it in discord last night. The KVM isn't the problem per se, all it does is share a common +5 line between the ps/2 keyboard and mouse ports on each output. The problem is he tried to use a ps/2 kvm through a ps/2 to serial adapter, which resulted in shorting that common +5 line from the keyboard into ground via the mouse connection.

Quite a journey .. and when reassembling today a cache chip turned out to have a broken pin. Just because :D

Anyway: thanks to everyone. Both for diagnosis help, troubleshooting tips, hand-holding and emotional support. :)