Yamaha CS80

A question. 
If something is wrong in the PSU of CS-80, how to check why all of the CC voltages are faulty from the origin? Usually one separates the PSU unit from the machine and checks where it is bad. In the service manual (p. 59 if I don't get wrong)  it is alerted not to detach the connectors from the PSU, for the risk of damaging the whole. 
So, how to check the PSU unit? 
Moreover, if both two fuses are ok, is there any other  component protecting the system ?

Thanks
Fernando 


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