I can not help you with a service manual but I can tell you that the MCBH45B's main board and power amp was the same as the MCH44B. Unless the microphone jack was rewired after leaving the factory, it was wired to the following:
Pin 1 = Open,or +13vdc, or RX audio - depending on the microphone ordered by the customer (standard microphone, Pre-amp base mic, or hand set microphone, respectively)
Pin 2 = ground
Pin 3 = PTT
Piin 4 = Microphone audio
Pin 5 = Hang up
Regards,
Bill
email:
bill@billscommsol.com
http://www.billscommsol.com