A BM5000 is not repairable? Why? There is really no problem to fix it... Only if you drop it out of the window, then it is not repairable.
What's broken? Does it switch off?
Ähmmm, the preamp of BM5500 6500 and 7000 is identical. There was just the board and layout changed from BM5500 to the newer models 6500/7000. But the electronic parts are quite the same, there is no difference in the circuit. Only Powerlink was added... but that is a different expansion. It is possible to install a BM7000 preamp in a BM5500 and it would work.
Martin