OK, I solved my own problem… sort of. The bottom line was that the logic board was NOT outputting any voltage for the “dim control” input to the Inverter board. According to some other info found on the Internet, the “dim control” pin should have a voltage ranging from 0 to +4 volts depending on the dimming desired (where 0V = no light, 4V = max light). Since the logic board wasn’t outputting any voltage on that pin, the backlight remained off. So, since I had no intention of buying a new logic board for such an old computer, I decided to just manually put +4V on the pin. Here’s how I did it…
First off, the CORRECT pin names and voltages to the input connector of the Inverter board are here (again, this is for Powerbook G4 17″ Aluminum). This is slightly different than what I wrote in my original message…
CHASSIS GROUND – Green wire (connector pin 1): Hardwired to chassis ground
+12V SUPPLY – Black wire (connector pin 2): +12V
CABLE GROUND – Gray wire (connector pin 3): 0V
+5V SUPPLY – Blue wire (connector pin 4): +5V
DIM CONTROL/ENABLE – Silver wire (connector pin 5): 0 to +4V depending on dimming
So, it was the DIM CONTROL that was dead from the logic board. I had both 12v and 5v available to me at the input connector to the Inverter board so, at first I figured I’d just connect the 5v to DIM CONTROL. At first it worked but, after about 1 minute, the backlight would shut off, presumably because the Inverter was being “overdriven” and it has some kind of overcurrent shutdown circuitry. So, I went to plan B and, though experimentation, found that a 3K ohm resistor placed between pin 4 and pin 5 brought the voltage on the DIM CONTROL to +3.8V which kept the backlight bright without it shutting down. See pictures below.
Now, for the bad news. Not only will the screen not dim (which I can live with), but it also won’t shutoff when going into sleep mode (the display goes away, but the backlight remains lit). For the record, it DOES shutoff when the computer is ShutDown. But, this poses yet another challenge. Since I generally only use my computer from home while it is plugged in, I may not worry about this one for now. But, for many people, this might be a showstopper. None the less, I did get the backlight working.