I started playing with a PowerMac 4400 I recently got two days ago.

At first I've had monitor-sync problems, even with the bootvariables way.

I thought the machine was broken, and started reading about the
serial-console way, when I thought about something, which I couldn't
find in the FAQ, so I thought it was important to report it:

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I didn't have a mac-monitor at hand, and used a db-15 to VGA connector, with
a monitor (Philips 14c), which is 800x600.

When I tried boot variables the first time, the system would hang, until
I zapped the pram.

I suddenly remembed vague problems with 800x600 monitors with certain
videocards (had something to do with some video cards wanting to do
800x600 only at 56 Hz, while some want 60Hz, and likewise for monitors),
so I took a monitor that could do 1024x768 at 60Hz (instead of 800x600 at ?
Hz max) and voila, I'm at OpenFirmware 2.0.2

It is just a hunch, and I don't have a large array of 800x600 and other
monitors to verify it, but it seemed important enough to me to report.
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P.s. I'm still at that Open firmware prompt btw, wondering about what to do
:_)