I am not sure, but I think C16 and Plus 4 apart from differences in Ram have the same chips in them
Also I blieve bigger chips are socketed in these.
Then you could try swapping chips from one to another. Best would be if you have another working unit, because then you can check each chip of the dead computers one by one in the working one and find out which chip is broken.
Unfortunately these machines often have a dead TED chip, which is almost impossible to find spare ones for....