IBM Model M - Need help troubleshooting ... (PC just keeps beeping)

[sofakng]

I've received a used IBM Model M (1391401) and I completely cleaned all of the keycaps and stems and made sure it was absolutely 100% dry.

When the PC boots, the NUMLOCK is correctly illuminated, but then the PC keeps beeping and displays Keyboard Error and nothing works on the keyboard.

I've cleaned the ribbon cables inside the keyboard (with isopropyl alcohol and Deoxit) but it didn't help.  I'm not sure if the membrane is broken and I was hoping not to remove the rivets but I'm not sure if the problem is the membrane or the PCB.

Can anybody help me diagnose this?

[Tactile]

Is it plugged directly into a real PS/2 port or are you using some type of a converter? Is this "keyboard error" message from the BIOS or the operating system?

[sofakng]

I'm using a real PS/2 port (on an older Pentium PC).  The keyboard error is coming from the BIOS.

If I unplug the 16-pin ribbon cable the beeping stops.  (ie. I can leave the 8 pin and 4 pin ribbon cables inserted)

[Tactile]

Here's an interesting bit of trivia for you: The num-lock LED is controlled by the computer, not the keyboard. When you press num-lock it sends a signal to the computer/OS.requesting different keyboard behavior. If the OS understands then the OS sends a signal to turn the num-lock LED on.

The point of all of this is that the keyboard isn't completely dead or the BIOS wouldn't be able to light that LED. In some BIOSs there is a setting to tell it to ignore keyboard errors - you might check to see if you have that option. The problem might be simply that this old keyboard draws more power than the BIOS expects, causing it to raise a fuss. Model M boards are pretty notorious for this power issue. Some computers don't care while others don't like the power draw. Also, there are so many different models of the Model M board that some folks never see this happen.

If you can easily lay your hands on a PS/2 to USB converter you might try that. Sometimes that will work 'cause the converter is more tolerant of the power draw and the motherboard simply sees a USB keyboard - so everybody's happy.

[fanpeople]

Have you tried it with a different computer?

Did you plug the keyboard in whilst the computer was turned off? If not, restart.

[Snowdog993]

I've received a used IBM Model M (1391401) and I completely cleaned all of the keycaps and stems and made sure it was absolutely 100% dry.

When the PC boots, the NUMLOCK is correctly illuminated, but then the PC keeps beeping and displays Keyboard Error and nothing works on the keyboard.

I've cleaned the ribbon cables inside the keyboard (with isopropyl alcohol and Deoxit) but it didn't help.  I'm not sure if the membrane is broken and I was hoping not to remove the rivets but I'm not sure if the problem is the membrane or the PCB.

Can anybody help me diagnose this?

Fun. What if I told you that your keyboard isn't working and your computer isn't working? Have you tried other keyboards with that computer? If so, were they PS/2 or USB keyboards?
If they were PS/2 keyboards and they worked fine with that computer, you can pretty well say that the keyboard may have issues.

If this is the case, you might want to be sure the membrane contacts to the controller are in direct contact in line with it. You may also want to check and see if your SDL cable is the culprit. If you don't have a spare SDL cable that could be difficult. A diagram might help (or confuse the issue).

There are the three membranes that connect to the controller.
First it is the 16-pin connector to the 16-pin membrane
Second is the 8-pin connector to the 8-pin membrane
Third is the 4-pin connector to the LED.
Second and Third usually go into the 12 pin second connector. Pins 1-8 go to the keyboard matrix and 9-12 to the LED's.
Be sure you have the membrane lined up with the pins on the controller, or you will not have the keyboard functioning properly (sending errors).

So what's this all mean? I'm utterly clueless about your keyboard, but can suggest you can be sure the membrane lines up properly to the controller and check it with a different computer if you can.

If it turns out to be bad, it could ultimately be the membranes need to be replaced. From there, you would need someone that can fix it for you.

Edit:
Another thing I forgot to mention is you may have a key stuck, such as a CTRL key or Enter key, Backspace key, Tab key etc.... Be sure all keys respond and click properly. Sometimes the hardest solutions are the most overlooked simple ones.

[Snowdog993]

I've cleaned the ribbon cables inside the keyboard (with isopropyl alcohol and Deoxit) but it didn't help.

This is a concern too. If the membranes are still wet, it may take several days for them to dry out because they are sandwiched between the backplate and the barrels. Liquids are not a friend to the Model M.
Here is an example:

[sofakng]

Thanks so much for the help everybody!

I've tested another PS/2 keyboard and it works so I know the PC is working.

I've also purchased some flat ribbon connectors to plug into a breadboard so I can manually check the contacts (and learn how keyboard matrices work!).

Which ribbon cable (16 or 8 pin) are the rows and which is the columns?

Can I send 5v to each column and then check each row contact for voltage?  If I understand correctly this is how keyboards matrices are polled and this could help me check the matrix?

(I also have PS/2 breakout coming which I'm going to use to check continuity on the SDL cable)

[sofakng]

Well, I think I found my problem.  (haha)

I took the rivets off and the membrane is absolutely destroyed.  It looks like something spilled in here so I ordered a replacement membrane:


Hopefully the logic board is still OK.