41CX halfnut keyboard not responding

[wojtek]

41c service manual would be helpful here, if you find one somewhere I would be interested in having a copy.
As regards your problem if the service manual is not available (and if the time you invest would be worth the result you'll get) perhaps the best approach to localize the problem would be to get hold of another, fully functional unit of 41cx, try to measure the voltages and see the data signals on oscilloscope while operating the keyboards on both units.
Of course you should double check that the main pcb you have is operative.