Trouble with a M7559 TK70 Controller

Just as I got my MicroVAX II booting, the TK70 controller has started to play up and is no longer working correctly. If I try to boot off the tape I get a CTRLERR, and if I boot VMS from the network and then try to mount a tape I get "%MOUNT-F-DEVOFFLINE, device is not in configuration or not available". It was booting from tape just fine a few days ago. I can see that the controller has two LEDs on it. When I power it up they both come on but then one of them switches off leaving the other one on. The light that stays on is the one on the right when viewing the board flat, looking at the ribbon cable connector, with the component side up. I think this means its self test has failed in some way, but I cannot find a manual anywhere online to tell me. Can anyone tell me what this might mean? Thanks Rob