Well, swapping the 4 and 5 probably could kill the keyboard with the power applied in reverse.
But did you check the actual 4 and 5? Because when you look at a plug and the socket facing towards you - they will appear mirrored. Just imagine you plug that extender into itself - then check the lines knowing which one would actually go.

Do you have a multimeter?
If so, I would check the actual cable wiring - swapping those lines seems really odd...
It might be a documentation/translation error, so double-check the cable

(check continuity of cable pins and "holes" and see if they are connected as they would
connect if you plugged the cable into itself)

Reversing power could kill something in the power-device (keyboard).
Less likely on the mainboard (but possible if the reverse-powered keyboard
came through on another pin).

Has it been a "long time" since you had this keyboard unplugged from this system?
Its possible some corrosion or otherwise marginal connection in the connector got
disturbed. Especially possible If the miswired keyboard drew more current that usual.

Again, I'd probably start with a multimeter and follow the connections from the keyboard
through to the keyboard controller on the mainboard before I'd go about changing anything
else.

vstrakh wrote on 2025-05-09, 18:36:

Well, swapping the 4 and 5 probably could kill the keyboard with the power applied in reverse.
But did you check the actual 4 and 5? Because when you look at a plug and the socket facing towards you - they will appear mirrored. Just imagine you plug that extender into itself - then check the lines knowing which one would actually go.

Thanks for your reply. Yes I see now that the pins are actually mirrored and I understand why 1 goes to 3 etc.

DaveDDS wrote on 2025-05-09, 18:52:

Do you have a multimeter? If so, I would check the actual cable wiring - swapping those lines seems really odd... It might be a […]
Show full quote

Do you have a multimeter?
If so, I would check the actual cable wiring - swapping those lines seems really odd...
It might be a documentation/translation error, so double-check the cable

(check continuity of cable pins and "holes" and see if they are connected as they would
connect if you plugged the cable into itself)

Reversing power could kill something in the power-device (keyboard).
Less likely on the mainboard (but possible if the reverse-powered keyboard
came through on another pin).

Has it been a "long time" since you had this keyboard unplugged from this system?
Its possible some corrosion or otherwise marginal connection in the connector got
disturbed. Especially possible If the miswired keyboard drew more current that usual.

Again, I'd probably start with a multimeter and follow the connections from the keyboard
through to the keyboard controller on the mainboard before I'd go about changing anything
else.

Thanks for your reply. I've been trying a continuity check of the cable pins and holes, but the meter probes are a bit big for the hole side, and not very stable on the pin side. But I think I've managed to confirm continuity on all of them and they do match up correctly. I didn't know these pins were actually numbered out of order at first, which confused me. No it hasn't been a long time since I last used that keyboard with that PC. Just 2 days. And the keyboard works fine on my 286 with the same 5 pin keyboard connector (without using that extension). And now none of my known good keyboards work with the 486 since I used that extension. So I'm not sure what's going on. The 486 was working fine until I tried that extension. The extension seems to test ok for continuity. I don't know whether to scrap the extension or check it against the 286 to try to rule out the cable - but there is some risk there if the problem was in fact with the cable. I only have 2 retro systems. One more piece of info that might be important is that I had the keyboard going into a 'Y' splitter, with one side going to the 286 and the other the 486. Not sure if that would have caused damage anywhere. But the 286 seemed quite happy with it.

The Y splitter is a very bad idea and likely the cause of your trouble

maxtherabbit wrote on 2025-05-10, 01:34:

The Y splitter is a very bad idea and likely the cause of your trouble

Thank you for your reply. Do you think the motherboard is recoverable? Would it likely be just the keyboard controller that was damaged?

newbie1 wrote on 2025-05-10, 01:20:

I've been trying a continuity check of the cable pins and holes, but the meter probes are a bit big ...

Btw, a good addition to any "electric" toolbox is a few little flex wires ending in "alligator" clips with
insulated covers (I've found that many of the cheap sets of 4-6 premade such cables don't have reliable
connections between cable and clip - I take them apart and replace the "pressure fit" with a properly
soldered connection)

You can clip one end to the multimeter probe and the other to a dressmakers pin to get a much
smaller probe contact.

Anyway - using such an arrangement you should be able to verify the connection between the DIN jack pins
and the corresponding pin of the keyboard controller chip. I've had times when such connections have failed
due to something like poorly soldered connection on the board/corrosion or a thin PCB trace opening under
higher-than normal current.

Once you have verified that ALL connections to the DIN jack are properly connected ... then you could look into
replacing the keyboard controller itself.

newbie1 wrote on 2025-05-10, 03:12:

maxtherabbit wrote on 2025-05-10, 01:34:

The Y splitter is a very bad idea and likely the cause of your trouble

Thank you for your reply. Do you think the motherboard is recoverable? Would it likely be just the keyboard controller that was damaged?

Main candidates:
fuse (check for +5V coming from keyboard jack)
7405 inverter
keyboard controller itself

I would check the fuse near the keyboard connector.

The signal inverter could be a 7404/7405/7406/7407

Yes that's true

DaveDDS wrote on 2025-05-10, 11:18:

Btw, a good addition to any "electric" toolbox is a few little flex wires ending in "alligator" clips with insulated covers (I'v […]
Show full quote

newbie1 wrote on 2025-05-10, 01:20:

I've been trying a continuity check of the cable pins and holes, but the meter probes are a bit big ...

Btw, a good addition to any "electric" toolbox is a few little flex wires ending in "alligator" clips with
insulated covers (I've found that many of the cheap sets of 4-6 premade such cables don't have reliable
connections between cable and clip - I take them apart and replace the "pressure fit" with a properly
soldered connection)

You can clip one end to the multimeter probe and the other to a dressmakers pin to get a much
smaller probe contact.

Anyway - using such an arrangement you should be able to verify the connection between the DIN jack pins
and the corresponding pin of the keyboard controller chip. I've had times when such connections have failed
due to something like poorly soldered connection on the board/corrosion or a thin PCB trace opening under
higher-than normal current.

Once you have verified that ALL connections to the DIN jack are properly connected ... then you could look into
replacing the keyboard controller itself.

Thanks again for your help. Much appreciated.

maxtherabbit wrote on 2025-05-10, 11:43:

Main candidates: fuse (check for +5V coming from keyboard jack) 7405 inverter keyboard controller itself […]
Show full quote

newbie1 wrote on 2025-05-10, 03:12:

maxtherabbit wrote on 2025-05-10, 01:34:

The Y splitter is a very bad idea and likely the cause of your trouble

Thank you for your reply. Do you think the motherboard is recoverable? Would it likely be just the keyboard controller that was damaged?

Main candidates:
fuse (check for +5V coming from keyboard jack)
7405 inverter
keyboard controller itself

Thanks a lot for the help.