Which output port gives low output? In the service manual, there is a block diagram of the RXTX block. Check that TX signal goes into the TXRX block from the RF block.
Then from what port do you see low output? Its been a while since I have operated a CMU200, but it has 4 ports: One is a dedicated input, one dedicated output and two RX/TX ports. So check also following the RXTX block diagram to figure out if any of the RF switches inside it may be blown.
Also, I think the dedicated TX port has an RF amplifier in the RXTX block and that component is failure prone, just even by getting old. So if your issue is just that the dedicated RF out port gives low signal, the amplifier may be done. It is just a simple MMIC amplifier, I can upload some photos and the part number later, when I will get back to my PC.
Simply put: If yo error is in the RXTX block, you have a pretty good chance repairing it. If yo failure is in the RF block, you better start looking for a replacement module.