Roland D-550

PB=Patch Bank
PN=Patch Number
power-up while holding the following keys:

1> PB2+PN6+edit=identify rom version number
2> PB2+PN6+exit=enter test mode
3> PB2+PN6+enter=initialization (restore factory setting)
4> PB2+PN6+write=enter D/A adjustment mode 

Note on 4
Monitor system must be connected to output jack
press compare, d550 start generating a low level test sound,
now work on vr1 (main board) to get minimum distorsion.