So, what was I doing in these iMacs? Well, neither of them were booting, and I tried to sort this out. It turns out that one of them just had a dodgy disk, so reinstalling OS 9 on that one was the way forward. In fact, I wanted to install Panther, but it kept panicking (something like 'cannot find drivers for platform imac'). So I tried Jaguar, and that kept failing with the message 'the installer has unexpectedly quit (return code 0)', so eventually gave up and just stuck with Oh Ess Nein.
The other iMac was slightly more interesting, and I still don't fully know what was wrong with it. Still, as I had the whole thing apart, I thought I may as well write about my experiences here in case it helps someone. At first, it appeared to be the battery, the terminal potential difference under load was 0. So I bought a new battery from watchbattery and set about installing it. Having taken the machine apart as described in my last post, the battery is quite clear to see, near the middle of the logic board. With the cable from the power filter board to the logic board disconnected (that's the one with the really wide connector, from the four you had to remove to extract the LB), press the power button on the front of the Mac. Now remove the battery, and wait at least ten minutes for everything to discharge. Insert the new battery. If you're lucky (I wasn't), the system should now work.
So I thought that maybe the CUDA was messed up. This can be reset using the, um, 'CUDA RESET' button which is labelled as such on the logic board. However, press it twice in quick succession, and you've lost it. Apparently this can cause the PMU chip to crash, which reduces the battery life from five years to two days. Not optimal, as I'm sure you'd agree.
That didn't help either; one time the computer would power up, go bong, then shut down as soon as the POST was completed and disk spun up, but other than that there was no joy. Actually, it did that twice. So I decided just to use it for spares, and removed the battery (trivial), CD-ROM (easyish; just unplug the cables and it will fall out) and hard disk (reasonably easy; there are two screws holding the mounting bracket to the logic board chassis - you need to unscrew these, then fiddle about a bit as the bracket has two tabs that fit into slots in the chassis). However, if anyone's got any bright ideas regarding fixing this Mac, I'd be glad to hear them