Thanks again Doug, and Yair.
I searched the Peartree and Calumet websites, but it wasn't listed. I was about to email them, when Doug's words came back to me: "Looks like most AC adapters". So wouldn't any 9V AC adapter do?
I remembered I had bought a "universal" AC adapter from my local B&Q a while ago, around €20, with switchable voltages (3V to 12V) and a variety of changeable DC tips for different device sockets. I found the tip that fitted the Mamiya AFD, and set the adapter to 9V. I verified with a Fluke voltmeter that it was providing 9V (9.2V to be exact). Almost there!
All I needed now - before switching anything on! - was to ensure I had the right polarity. Can't take chances with electronics, and astronomical MF repair costs have been well publicised in this forum.
Those of you with a nervous disposition may want to look away now. The DC socket on the Mamiya body is surrounded by the baseplate, which is attached by 7 small screws. Off came the screws. The socket itself is attached to a vertical circuit board by 2 other screws. Off they came too. Quick look at how the socket is wired: the red wire goes to the central terminal, the black to the exterior. So yes, it is
centre positive. (Unless the Mamiya assembly line technicians are colour blind!
)
Reassemble everything; plug in the B&Q adapter; switch on the camera: well yes it's functioning normally, but is that because it's running off the batteries or the AC adapter?
Remove the battery cartridge from the grip: the LCD display stays on but it's frozen - it doesn't respond to changed settings and won't fire the shutter. Hmmm. Pull out the AC adapter and the LCD display goes off. Hmmm. Clearly the adapter is providing some power to the camera, but it's not functioning as it should. Could it be that the battery cartridge must be in place in the grip, for the camera to work properly?
Remove 1 battery from the cartridge - to open the circuit so that the batteries cannot provide power; reinsert the cartridge and the AC adapter; switch on.
Result! The camera is working perfectly off the B&Q adapter alone.
So now I know that
the battery cartridge must be in place for the camera to function, even if the batteries themselves are not present. I still don't know whether having a full set of batteries in the grip means that they will be run down even when the AC adapter is used. I could test this over time of course, but for now I'll just remove 1 battery, and keep the rest in the camera, to be sure that it's using pure AC power.