The framebuffer driver in the kernel is not capable of determining the exact type of display that is attached. Thus, it uses a standard resolution that most monitors are capable of functioning at. The TFT display is not capable of using these settings, and switches itself off.
It is possible to indicate the correct resolution to use, with the following Kernel parameter
17" Display (Monitor)
video=aty128fb:vmode:20
<A HREF="http://sdb.suse.de/sdb/en/html/olh_ppc_apple_tft.html" TARGET=_blank>Using the Apple TFT Display under Linux
</A>
This might be helpful.
The next release of Mandrake-Linux for PPC should fix this problem.
you need to append this to the yaboot.conf file
video=aty128fb:vmode:20