Hi. I just fixed the overscan on my Sanyo DP42848 yesterday, so I know this works. This process worked best for me when I had a calibration disc with an overscan pattern onscreen so I could confirm (I used the Disney WOW! disc). Also, here's the standard disclaimer that you could really mess up your TV if you make a mistake, so you have been warned!
Here are the steps I used:
1. Unplug your tv. Hold down the Channel Down button on the side of the TV (NOT the Volume Down button; that brings up a different menu) and plug the TV back in. Keep holding down the Channel Down button until the service menu comes up.
2. Using either Channel Up/Down or Volume Up/Down on your TV remote, scroll through the indexes. You are looking for indexes 164 through 171.
3. Index 164 should say "SCALE_LT_DIFF_HDMIHD". Now, use the arrow button on the remote, move over to the "Value" box, and change the value from 0 to -69 (use the volume down button to change the value).
4. Do the same thing for Index 165 (SCALE_RT_DIFF_HDMIHD). Then proceed to do the same for 166 and 167.
5. While you're here, do the same for indexes 168-171 (these will say HDMISD at the end). This will ensure that both HD and SD programming use the same screen area.
6. If you are using an overscan test (and it's like the one on the WOW! disc that I used), the tips of the triangles should now be touching the edges of the screen. It should no longer be cropped.
7. Unplug your TV, then plug it back in normally. You should now have eliminated overscan.
Just so you know, you will begin seeing what I call "junk" at the top (and sometimes left or right edges) of the screen on some channels. That's because these channels are not broadcasting to fill the entire space (since they assume there will be some overscan). However, your Blu-rays should look fantastic, and you should be able to see the entire frame.
Hope this helps!