no video after install

[Jud Leonard <jud.leonard@gmail.com>]

I have a Power Mac G5 with NVIDIA GeForce FX 5200 Ultra with 64MB DDR SDRAM.  The G5 also has 3GB of DRAM, an 80 GB disk in slot A, and a 500 GB disk in slot B.  The big disk has MacOSX 10.5.8, which works fine.

I installed Debian 7.4 on the smaller disk, using a netinst CD.  The installation seemed to work fine, but when I booted Linux, my monitor (a Sony Multiscan 420GS) put up an error alert saying the input scan rate was out of range, on an otherwise blank screen.  I tried another monitor, which displayed nothing.  I repeated the installation, using “install video=ofonly”, with the same result.

I would appreciate advice on how to debug this problem.  TIA.
	Jud