Troubleshooting error 99

[Billyjaxin]

I was given a used 30D, by my brother who could not get it to work. The original owner didn't know that it wasn't working but it hadn't been used in some time.
- It took one photo with flash for me, and then nothing. I bought and charged a new battery.
- I get error 99 but the message doesn't always show. I now get no shutter action at all.
- All the display and setting functions appear to work.
- When I turn it on, the red light for the chip blinks once.
- Battery reads 7.61V
- I've cleaned the contacts on the battery and in the battery compartment, using alcohol on a wooden stick.
- I also tried a different card. Didn't seem particularly logical but didn't know what else to try!

Because it did take one photo at one time, I've been guessing that everything potentially works but that something isn't getting the voltage that it needs.
Any suggestions?

[Billyjaxin]

Great advice, thanks. I'm more comfortable with physical repair than I am with programming repair, and I enjoy it.  So it's tempting to start in the most interesting place instead of in the smartest place!

 Haven't tried a different lens yet. Will probably be able to do that within a few days.
It will be a week or two before I can get around to the software side.  Too many other things unfinished as it is, for now.

But I will certainly leave the screwdrivers where they are until the other avenues are exhausted. Thanks for possibly saving me from going down the wrong path!