Have a TG-5 (very similar to TG-3) that has worked fine, but now automatically powers off after a few seconds, with...

This is the assembly for the battery latch. (I presume you had removed the bracket assembly that holds in the small speaker, which also holds in place the battery latch assembly.)

I inherited an old iMac (A1224) mid-2007 20" Core Duo 2 2.0GHz, with the rock-bottom specs for this model. I first bumped the RAM from 1GB to 4GB for $20. Next, I bought a X7900 (pulled from a A1225 iMac) on eBay for $59 and replaced my T7300 CPU, using the iFixit guide. (It took about 2 hours.) My Geekbench 3 (64-bit) score went from 2135 (T7300) to 2916 (X7900). While running Geekbench, the CPU temp went from 30 C up to 48 C (iStat menu). So I get about a 37% increase with the X7900 over the T7300, which is pretty much consistent with the Geekbench numbers reported on the Everymac website. I'm currently running OS X 10.8.5 on this iMac, with Office for Mac 2008 installed. Both Word and Excel run noticeably faster with the X7900. After a couple of days usage, I've experienced no problems with the upgraded iMac so far.

Thanks for a highly useful guide. Slowly and methodically, using a single-bladed, razor blade, I was able to open the case, following this procedure. Patience is, indeed, a virtue, in this case.

I was able to successfully replace the hard drive and battery.

The only snag I encountered was that the headphone jack ribbon was glued to circuit board adjacent to the connector under the hard drive. (I just left this ribbon be, not wanting to risk damaging the ribbon.) It was a bit awkward straightening the case edges, but I managed.

Overall, I'd give this guide a grade of "A"....it was nearly perfect and very clear. Bravo!