After I replaced the hard drive in my old iBook G3 (with the help of iFixit guides), it will not boot connected to mains power, only with the battery. It also always returns its date/time settings to its "zero-hour", when it left the production line.
The G4 suffered a fall and no longer "finds" the HD to start. In addition it has a yellow vertical line on the monitor at the beginning of its rightmost third part to the left.
I suspect a logical board problem on the G3, and some physical damage on the G4's HDD.
Best Answer
Your G3 probably has a dead motherboard battery. Depending on the model, they are usually not that hard to replace and cost between $2 and $30.
As for the G4, I'd say physical damage from the fall is a pretty good assumption. Since it sounds like multiple components are damaged, it may not be worth the effort to fix unless you just like to tinker.