I bought a parts machine that was marked as "not working" mainly to get the plastic front cover. It turned out that the only thing not working on it was the power supply! These power supplies are notorious for dying capacitors. And well I could recap it, but its a pretty densely-packed PSU and I could easily mess it up. But I had another idea! Harvest the proprietary power cable, and turn it into a USB-C cable! Then I can use the same USB-C power bank I use for my modern MacBook Pro!
The 540c power supply outputs 16V, but the machine runs fine off of 15V (I guess the same power circuit handles the battery so it can deal with the whole battery voltage range). The way that USB PD (Power Delivery used by high-power USB devices such as laptops) is that the device can ask the power supply to increase the voltage to 9V, 12V, 15V or 20V. Generally any power supply that supports 30W or more will support 15V output. It turns out you can buy cables that take USB-C, negotiate 15V, and output that over a standard DC barrel jack!
The first step after buying the parts is to harvest the power cable. There was a post on 68kmla that suggested you just "Just throw it on the floor, comes right open!". So I did. And it did pop open! It also cracked the bottom case in half... Damn those cheap 90's Apple plastics! After taking any photos I would need to restore it, I desoldered the cable. Then I had to buzz out the pins to see which wire went where. This is the Apple Service Source pinout:
The cables inside the wire are colored:
Pin 4 didnt connect to anything, and my multimeter probe didn't go very far in, so I guess it's NC? Does "Sense" in the pinout mean it's there to see if GND is connected or not?
There's an Apple support document that says "The VBatt supply is used for charging the batteries while the VMain supply provides power for the PowerBook. Power from VBatt is automatically diverted to power the PowerBook if additional power is needed." so initally I tried to only wire in VBATT, but it wouldn't turn on. So I changed to VMAIN and it came alive! VBATT probably didn't work since I don't have a working battery and it has to go through the battery. Once I actually get a working battery I'm going to have to wire in both in parallel.
It might be cool to make this a completely internal solution - since I don't have a modem I could put a female USB-C socket there and just solder the 15V output directly to the power input. Then I could rehabitate the power cord and recap the original power supply.