I had the same problem with a Core Solo machine. Turns out the people who installed some RAM (this was before I discovered ifixit) had not properly seated the ZIF cable in its socket. Steps 7 and 8 in the Audio Board install (Mac mini Model A1176 Audio Board Replacement) are where the solution lies. There is a painted line on the ZIF cable. Before you close the cable lock, make sure the cable is fully inserted - the painted line will be about 1mm above and parallel to the top of the socket. Push down the clip and all should be well.
I upgraded a Core Solo 1.5GHz with a T7600 Core 2 Duo 2.33 GHz. The guide here on ifixit is well done, and along with the notes on each step, it was all I needed to do the work. I would say having the proper tools helps a lot. Who knew a spudger was so important :) Gary O'Brien Photographer Charlotte, NC
I also just installed a T7600 2.33 GHz, and it was pretty much by the book. Read all the notes in each step before you do the step - they are a big help. Also upgraded RAM and HD. The little machine is stepping along quite nicely now. Gary O'Brien Photographer Charlotte, NC
This guide worked very well for me. Entire process took less than 90 minutes, and that was having to pull the upper frame a second time because I'd let the Airport antenna cable get snagged under the fan ductwork.
Adding to my note, the illustration for Step 21 is a bit misleading. You'll have to grab the barbs much closer to the end of the pin than is shown here.
The springs holding the nylon pins are stronger than you might think.
Use the finest tip needle-nose pliers you can find. The trick is compressing *and* pushing at the same time.
Around 2mm or so from the surface of the circuit board is the furthest point at which the barbs can be compressed sufficiently to pass through the hole in the board.
If you grasp the barbs closer to the circuit board, you won't easily be able to push the pin through the board.