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I have actually two of these ModelM keyboards, they are a pleasure to use and make a fantastic sound. The other one is from the 90s, takes a small PS2 adapter and works without a problem.

This one however worked fine for a while then it started to sometimes wildly type random characters, which of course makes it mainly unusable. It does not always do it and tends to take some input between the randomness. This is the keyboard and its adapter:

Model M keyboard, cable and adaptor to USB

If anyone has any ideas of what else might cause the intermittent lapses into random characters please say, otherwise I'd like to get the cable replaced. Thing is, I'm not sure how to describe it and if such a cable can be ordered or made to order, as nowadays I don't have a soldering iron and am a bit out of the habit of that kind of DIY.

This is the damage: closup of damage to cable It was like that when I got it, but the problem only appeared many months after I got it which points to the cause being the likely degradation of the copper strands inside.

This is the end which plugs into the adaptor: maybe an AT male plug I think that might be called an 'AT' plug, but not sure.

This is the end which goes into the keyboard: plug inside the keyboard

...and a closup of the holes:

keyboard plug, closup of holes So again, before taking action, I wanted to ask those who know if I am right that it is probably the cable.

Secondly, how would one describe such a cable in words and is it possible to obtain one anywhere nowadays?

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  • You might be able to get a replacement cable from clickykeyboards.com Commented Jul 3, 2022 at 19:00
  • Cardamom, it looks (I am not sure) like there is a shield that is broken right at the place you show in the picture as 'damaged.' It's possible that the shield is an important conductor in your case. Is there any way you can temporarily reconnect it up (assuming my eyes are not deceiving me) to see if the problem goes away then? It would help to know.
    – jonk
    Commented Jul 3, 2022 at 21:05
  • The black connector in your third photo is 5-pin DIN. Not mini-DIN (aka PS/2), which is the connector at the other end of the gray cable. Commented Jul 4, 2022 at 23:11
  • 1
    I had a (non-IBM) AT keyboard at one point that would occasionally insert random characters due to a firmware bug. It could be fixed by replacing the firmware EPROM with a new one on which you had burned corrected firmware, possibly obtained by copying the EPROM from another keyboard that had a newer revision. But in your case the bad cable would be the first thing to fix. Commented Jul 4, 2022 at 23:16

2 Answers 2

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  1. I think what you have there is an IBM Model F keyboard.
  2. The pictured cable is certainly suspect, though it does not have to be the main/only culprit. If you can easily find a replacement cable, it is definitely worth a try to replace the cable and see if that fixes the problem.
  3. I think any cheap PS/2 keyboard could provide the necessary donor cable, which has the PS/2 mini DIN connector on one end, for connecting to your USB converter. A cheap PS/2 extension cable should also work, just by cutting off the unused end and re-wiring it.
  4. You should use a multimeter to determine which color-coded wires are attached to which pins in the black Molex cable end (blocky connector). The four connections needed are Data, Clock, +5V, and ground.
  5. Then wire the new cable into the Molex connector using the same scheme identified in #4.

enter image description here

Note that what I am calling the "Molex" connector is a brand name, and it's basically a female 7x2 header connector. It may be a challenge to reuse the existing one. Usually, you can coax the wires out of the housing. But then you need new crimp-on pins that match to connect the housing to the new cable. Alternatively, you might need to get a new 7x2 female header with the same dimensions and its own crimp-on pins. If you are good with soldering, you can also possibly splice the old connector onto the new cable. In the end, you need a reliable connection in which the wires are both securely connected and properly insulated.

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  • As you said - Molex is a brand name. I used to call the 3.5 inch HDD power sockets Molex connectors.
    – cup
    Commented Jul 3, 2022 at 15:29
  • I would call it a Berg connector - again, this is a brand name. Note the 'Berg' on the socket.
    – john_e
    Commented Jul 3, 2022 at 16:45
  • These days, I see a lot of people lazily using "Molex connector" to refer to the connectors for PATA hard drives, optical drives, and 5.25" floppies, "Berg connector" to refer to the connectors for 3.5" floppy drives, "Dupont connector" to refer to the unkeyed version of the black cable-side connector pictured in the photograph, and "IDC connector" to refer to the connectors on ribbon cables, even when referring to the PCB sockets for them which have nothing to do with insulation displacement.
    – ssokolow
    Commented Jul 3, 2022 at 20:59
  • That said, you can easily reuse the keyed connector shell as long as it's using the typical size of crimp pins. Just push each wire as far into the shell as possible, slide the end of a toothpick under the plastic finger to provide a ramp that'll cause the crimped end to lift the finger rather than catching on it, and pull on the wire to slide it out. Then crimp some of those female pins onto the new wires and click them into place.
    – ssokolow
    Commented Jul 3, 2022 at 21:02
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    A Model F XT, indeed.
    – Krackout
    Commented Jul 16, 2022 at 6:42
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Thanks for the answers and comments. The problem has been intermittent but really bothered me today so now that I have a soldering iron, I took a blade, cut out the damaged part of the cable, stripped everything, the outer black and 4 inner red, black, white and brown wires and joined the 5 of each side together (4 + the uninsulated Earth).

That did not fix the problem at all it was as wild as ever!

What did fix it was taking some fine silicon carbide black sandpaper and sanding the metal part of the backplate which touches the earth lug till it was totally shiny. Someone or the manufacturer had already removed the grey enamel around that screw hole but possibly there was metal oxide on the surface now.

The following two threads gave me the idea that it was an earthing / grounding problem :

https://deskthority.net/viewtopic.php?t=21157

https://deskthority.net/viewtopic.php?t=20113

No idea what the underlying explanation is of why the earthing needs to be so solid, can only assume that the encoding or modulation that this keyboard uses to send the information down the wires is some kind of high frequency that goes haywire without the backplate solidly part of the circuit.

As this problem is intermittent, I will not suddenly assume that it is solved, but after observing it for weeks and months may give this the green tick eventually if it never comes back.

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