View Full Version : 386SX does not recognize a keyboard

April 28th, 2009, 10:22 AM
So I got a AT to PS/2 keyboard adapter and tried to use it with my 386SX single board PC, it did not work. Now when I try to use my AT keyboard on it I get nothing.

The system boots just fine, but no keyboard input works at all. I tried to see if there was a fuse I could repair on the board, but as near as I can tell there is not.

When I plug the keyboard in and the system is on it beeps, but it looks like there is no power to the keyboard. None of the keyboard light go on when the system is powered up or the keyboard is plugged in after the system is on.

Any suggestions?

April 28th, 2009, 12:10 PM
1st off does the SBC have a PS/2 keyboard port and you are trying to use a AT keyboard, or does the SBC have an AT keyboard port and you are trying to use a PS/2 keyboard?

I have never seen a SBC that did not have a PS/2 type of port.

The AT ports had issues where people would break the ground connections when they wiggle the plug too much when inserting the keyboard connector. The round metal part of the plug is grounded, break that ground path and the keyboard will not work.

April 28th, 2009, 12:30 PM
The SBPC has an AT (5 pin) connector. This is connected to the board via ribbon cable that runs to the port on the case itself.

The case 2 such connectors with ribbon cables (one on the back and one on the front). I will try to connect the one on the front and see if it work. If it does, then it is most likely a ground issue as you suggested (and easily fixed).

April 28th, 2009, 05:12 PM
The connector on the front has a different kind of ribbon cable and cannot plug into the same place as the one on the back.

They Keyboard works in another machine, so its just this one, and it appears to be a problem with the board.

Any ideas? Could a component have blown?

I don't have the faintest idea on how to even begin here, so any ideas would be appreciated.

April 28th, 2009, 05:22 PM
I don't know about something of that age but did they use micro-fuses to protect the keyboard port in that era?

April 28th, 2009, 05:43 PM
That was my first thought, but there is nothing around the keyboard connector that even looks like a fuse, but then again I would not know what I am looking for.

This is very frustrating as this is the only machine I have that can read and write 360k floppies for my XT.

April 28th, 2009, 05:58 PM
Could this be a fuse???


The line in front of the F goes underneath it, so it does not appear to be an "I" or an "L" merely a symbol.

I tested it for continuity, and there is none.

April 28th, 2009, 06:44 PM
Looks like one but I can't say for sure.

April 28th, 2009, 07:11 PM
the silkscreen underneath it seems to say fuse...

April 28th, 2009, 07:17 PM
Looks like its marked L1 to me but meh. "FUS_" Looks like its directed at the IC above it. Hard to tell with a tightly cropped pic but I'd call the green item in question a fuse since I've seen others that look like it.

April 28th, 2009, 07:59 PM
Well I bit bullet and bridged the contacts on either side if this "fuse" and lo and behold the keyboard worked. So my conclusion is that it is a fuse.

I Googled LF 3A and it came up as "Little Fuse", though i could not find pictures of it to verify it.

So do I replace it? I think I should, but I am not sure with what, aside from the 3 amp. Does the voltage on a fuse (30v, 125v or 250v) matter? Is this a special kind of fuse or can a glass fuse work?

April 28th, 2009, 08:06 PM
Yeah, the poor fuse blown. Have a scrap board or something else you could get another off of? I wouldn't think something like that would be hard to find.

April 28th, 2009, 08:22 PM
Does OP have a multimeter ?
If so, check here for the pinout
and test if you've got the +5 volt line.
Also, if this was tried and I overlooked it in the posts, sorry, usually the lights on the keyboard will flash during boot, does this happen ?

April 28th, 2009, 08:35 PM
@Pat: Thanks for the tips. No lights come on, and there is no voltage at the keyboard.

It was infact that 3a green thing (which I assume is a fuse).

Here are some pics of my MacGuyver fix, and yes those are actually leads from my Voltmeter :D



April 28th, 2009, 11:01 PM
What you have there is known as a Pico Fuse. It looks like a resistor, but is, in fact, a fuse. ;)


April 28th, 2009, 11:50 PM
And you can safely bet your life savings on the fact that the "3A" on it means 3 amps, and the component designation is "FUS1".

April 29th, 2009, 06:30 AM
And you can safely bet your life savings on the fact that the "3A" on it means 3 amps, and the component designation is "FUS1".


April 29th, 2009, 10:04 AM
If you have room, you might want to consider replacing the picofuse (more common than you'd think) with a standard-looking pigtail fuse. It might eliminate confusion later.

One that threw me were the IC protectors/fuses (http://www.nedis.com/Articles/Rohm/ICP-N75.php) in what look to be ordinary epoxy transistor packages in the Amstrad Joyce. They're not common in the US and I replaced the blown ones with picofuses--there wasn't enough real-estate to use ordinary glass tubular fuses.

April 29th, 2009, 10:45 AM
If you have room, you might want to consider replacing the picofuse (more common than you'd think) with a standard-looking pigtail fuse. It might eliminate confusion later.

I saw some really small 3a regular looking fuses out there. I will most likely try to replace it with one of those.

Does the voltage matter on these things?

April 29th, 2009, 12:02 PM
Voltage and how fast they blow are critical in computer applications, try to get the same fuse.

April 29th, 2009, 12:48 PM
Low-voltage fuses rated at 32V will be okay, but a 250V fuse will do in a pinch.

Traditional fuses in electronics are slowly declining in popularity. More often in new equipment you're seeing PPTCs (http://en.wikipedia.org/wiki/Resettable_fuse). One of those may work in your application.