16 Sep 2016, 20:27
First, let's get a few generic troubleshooting questions out of the way:
Are you using an active PS/2-to-USB adapter? Passive adapters don't work.
Older keyboards draw more power and not all adapters are able to provide as much as some keyboards need. The adapter should also be connected directly to the computer and not through a hub.
If you connect the keyboard directly to the motherboard, you must do it when the computer is off. PS/2 and AT can not be hot-plugged or you would risk damaging the motherboard, the keyboard or both. Connecting through a USB adapter and hot-plugging that should work though.
Next:
One common fault in older hardware is that some electrolytic capacitor(s) has dried out. The IBM Model M2 is infamous for this but it is not alone. Open up and replace.