From what I've read about both Cambridge players, they have different sonic profiles. The 840C often characterized as sounding highly resolved and warm whereas the 851C has been said to have a cooler, drier sound. So I would recommend repairing the 840C or perhaps looking into a Marantz player which will have a similar sound profile as the 840C.
To be very clear, all of what I'm saying here is based on what I have read not experienced.
All the best!
To be very clear, all of what I'm saying here is based on what I have read not experienced.
All the best!