A15 BIOS Problem (Yes, I have read the other threads.)

Hi all, first post here and I hope someone can steer me in the right direction. History: My sister gave me her old A15-S129. She had put it in a box about three years ago because she lost the AC adapter. I bought a new one at a swap meet for $8.00 and plugged it in and got the proverbial "RTC battery is low or CMOS checksum is bad" error code. Hitting F1 does nothing. No problem, did a tear down with the help of this site and replaced the RTC battery, cleaned out the heatsink, replaced the thermal grease on the CPU, and generally cleaned it up inside. Still got the same error code. Next I swapped the RAM into the second bay. Still nothing. Then I removed the HDD and got an IDE error code before it loaded back to the RTC error code screen. Next I hooked up another keyboard, which powered on but I still could not use the F1 key. My question is what can I do/test to continue the process of elimination. Or, should I go ahead and just buy a tested motherboard off of ebay? Have I narrowed it down far enough to justify the $75.00-$100.00 cost of a new board or are there a few more things I can try first? Oh yeah, the DVD drive powers on at startup and the fan does operate and the CPU gets warm. Any help would be greatly appreciated. Please keep in mind that I'm in to this for under $20 bucks at this point. And I can be one stubborn mutha when I get wrapped up in something like this trying to find the cheap solution. Thanks.
Eric