Odd problem with Model M

[crispy1989]

Hi,

I recently purchased a refurbished 1993 IBM Model M from an ebay seller with good reviews, but there are some odd problems with it, and I'd like to take a stab at fixing it, but I don't really know where to begin.

The first problem is, as seems to be common with older keyboards like this, some keys aren't working (g, h, both ctrl's, left alt, caps lock, possibly others).  However, the common causes that I've found while looking around don't seem to fit.  The keyboard is very clean, so I don't think dirt is a problem.  And from what I've read, broken rivets commonly affect keys around the outer edge - not central keys like g and h.  The broken keys are just as "clicky" as the working ones.

The second problem is odd, in that I haven't seen any reference to it online.  When numlock is turned on, the numlock LED doesn't work properly - instead, it sometimes flashes irregularly a few times before going out completely.  Also, the scroll lock key seems to work, but I don't see any response on the LED.

It's possible these problems are unrelated, but I don't have the experience necessary to know where to begin in figuring this out.  Thanks.

[rknize]

Are you plugging the keyboard into a PS/2 port or are you using a USB converter?  The Model M has unusually high current draw for a keyboard and the vast majority of adapters don't handle that properly.  What you describe sounds like controller misbehavior.

[crispy1989]

I am not using an adapter.  However, I've never used the PS/2 port on this motherboard before, so it's always possible that's the problem.  I'll pick up a USB converter today and see if the behavior is consistent, and try it on a different machine.

The affected keys feel the same as the working ones.  I popped off the keys and there's no sign of damage or dirt in the mechanism.

If using a converter and different machine don't work, I guess I'll need to get the right kind of driver, open it up, and test the traces.

[jkercado]

Make sure that you pick up a real PS/2 to USB converter, and not one of the cheapo plug adapters. The Model M draws more voltage than other modern keyboards, so the cheapo adapter won't work.

[urbanus]

I am not using an adapter.  However, I've never used the PS/2 port on this motherboard before, so it's always possible that's the problem.  I'll pick up a USB converter today and see if the behavior is consistent, and try it on a different machine.

Yes, that's a good first step.

The affected keys feel the same as the working ones.  I popped off the keys and there's no sign of damage or dirt in the mechanism.

If the springs are clicking, and the hammers are moving freely, then it's unlikely to be a mechanical problem.  If it were broken rivets, you'd have to see significant damage before keys stopped working (a few broken rivets is generally harmless).  An easy way to check for broken rivets is to pick up the keyboard and shake it: they will rattle around inside.

If using a converter and different machine don't work, I guess I'll need to get the right kind of driver, open it up, and test the traces.

Ah, well the membrane is sealed away between the barrel plate and the back plate, so that could be hard. 

While you're in there, take down the model/serial numbers on the controller board.  Contact Unicomp and see if they can provide a replacement.

[mich]

It seems that membranes are partially disconnected from controller PCB or damaged. If you are lucky, opening the keyboard and reconnecting membranes will fix it.