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Keyboards have fuses!
Just a couple days ago (Dec 4 2021), I was fiddling around with my DOS machine, trying to install one of those USB breakout brackets, and I suspect that in doing so, I somehow connected it wrong and shorted something, since the machine didn't really work right afterwards.
First of all, the memory test ran extremely slowly. It took around 40 seconds to check the 64MB of RAM, but worse, after the check was done, I got this message:
Keyboard error or no keyboard present
The keyboard didn't show any signs of life at all, not even a blink of the LEDs at startup. I pulled out the USB bracket, and while the memory now counted at the regular speed, the keyboard was still totally dead.
I tried 3 different keyboards, 2 PS/2 to AT adapters, cleaned all the contacts, checked the solder joints, looked for damage, but nothing. As it turns out, keyboard connectors have fuses. I didn't know this, but I guess it makes sense.
I identified the fuse as this small yellow barrel labelled "BUSS 3A" next to the keyboard connector. I didn't have anything to check if the fuse was good or not, so I made a simple device to check continuity.
You can make your own pretty easily, and it works pretty good in a pinch.
Just grab an LED, or another battery operated device. Grab some wire, cut it into 3 pieces and strip the ends. Wrap one piece around the positive leg of the LED, and another around the negative. Connect the negative wire to the negative side of a battery, and connect another wire to the positive side of the battery. Now when you touch the 2 ends together, the LED will light up.
So after stealing the CR2032 from the board for it, I used this device to check the fuse, and the LED didn't light up, indicating that the fuse was indeed blown.
For a quick test, I used a small piece of wire, and wrapped it around both legs of the fuse to bypass it. Of course it would be ideal to replace the fuse, or at least solder the wire, but this was just for testing.
And a quick test confirmed that the keyboard is fully functional now! I put the board back into the case, and it booted up just fine.
So if you're run out of options trying to figure out why your keyboard isn't being detected, give this a shot. This also applies for some newer motherboards as well, though it would be more difficult due to the size of the components.
Pretty sick computer, eh?
Updated Dec 5, 2021
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