[Yaesu] FT990 tuner problem. START makes tx, but does not start tuning.

[Sat Q]

Hello,
 
I have a problem with a 990. Somehow the tuner fails, when I press START only TX comes on, but there is no tuning action.
When I switch the backup switch on the tuner board off, an press MOX, for example in CW, the transceiver starts to tune on most bands (exept 160 and 80m). This makes me think the tuner still works, but why will it not work when pressing tune START. I browsed through the schematics, but I can’t find how the tuner control board gets the start tune command.
 
Can anyone tell me how this toner works, where to look for, or should I look for another tuner (control board)
 
Kind regards,
 
Edwin PE1PWF