R&S SMU200A (B) Synthesis PLL unlocked

[drhex]

Hi,
I have a two channel SMU200A here where the second channel doesn't lock to the reference signal from the first. I tried another synthesis board, same result, a 10 MHz signal is present at Ref out, 2V PP. Makes me think it isn't the ref output from the first synthesis board, the service manual sadly is very short on information bar "Switch the board". Has anybody seen this before? SSYN A and B are different part numbers, so probably not interchangeable?
Thanks,
Florian