Keep in mind that some of this equipment may be approaching 30 or 40 years of age and could have been the first kit built by some smart high school student, so faulty wiring, cold solder joints, and wrong component location could all be a problem.  

Other things to consider (all items may not apply to your particular model):

    - bad or weak tubes

    - passive elements (resistors, capacitors, etc.) that have changed values

    - crystals that will not oscillate or are off frequency

    - dirty contacts on switches, controls, relays, and/or tube sockets and pins

    - poor alignment

    - broken or open coils

    - broken switch contacts

    - sub-par shielding (tube shields, coil cover, coil board separators, etc.)

    - final neutralization

    - wrong components, like tubes and panel lights

    - poor grounding or high SWR antennas

    - defective or wrong impedance microphone

    - sub-par speaker system

    - loose or open coax connecting cables and/or connectors

    - power supply problems (wrong voltages, excessive AC ripple, etc.)

    - faulty cable between PS and radio

    - improper setup or tuning