direct tv.... each transponder is resposable for certain channels,say if transponder 30 is missing you won't have any of the channels that are related to it.
even or odd transponders are related to voltage sent to the LNB, 18/10 volts. measure voltage, check output at reciever<most swing from 10 to 18 volts while hunting signal>.
check for loose or corroded fittings, kinked or bad cable,possably a bad LNB,or dish thats damaged<did a basketball hit it, or limb fall on it> or out of alignment.
bad reciever can cause missing channels.
SQof 70 is threshold
DTV sets for a clear day most recievers will have some in the mid to high 90s and depending on the model reciever some as low as mid 70s. There will always be a spread .
multipul recievers some will always have a higher SQ that others in the system.