Why my iPhone doesnt charge but it turns on!

hi ppl, i have an iPhone 3g witch doesn't charge but its turns on when i connect the charger (no usb) also the computer doesn't read it.

I have changed the battery, the lock flex, charging port, and back case of the phone practically everything... am tired of deal with it and i don't know what to do :( plzz help and thanxx.

and i almost forgot i cleaned the logic board with alcohol and flex connectors :D.