If   I  append   a  specific   video  mode   I'll  get   "out  of   scan
range"   or  "no   signal"  depending   on  the   video  mode. This   is
"video=radeon:640x480-16@85" (no signal):

Ug. The default mode in the modeb must be not supported by your card. Can 
you try lowering the value for 85.



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