black screen but responds to being on charge

i have replaced the rear housing screen and digitizer on my 3g after replacing all the components and trying to power on nothing happend so thinking the battery may be flat i put it on my dock to charge and it responded with a charging tone but still no screen showing so i bought a new lcd and still nothing the phone seems to be on as it will ring when i call it im stumped ive checked all conections and pulled it apart numerous times now is there any hope?