Explanation: A defective crystal in the Automatic Frequency Control (AFC) section compromises the receiver's ability to maintain a precise frequency lock. In radar systems, which often use a Plan Position Indicator (PPI) display, this frequency stability is critical for accurately processing target echoes. If the AFC crystal fails, the receiver's Intermediate Frequency (IF) will drift or become unstable. This results in the radar intermittently losing its "tune" to the incoming signals. On a PPI display, this manifests as "bright flashing pie sections," indicating sectors where the radar is temporarily unable to properly detect and display targets due to the frequency instability. The radar loses its coherent processing in these areas, rather than showing a stable, accurate picture. The other options are incorrect: A defective crystal in a critical control circuit like AFC would indeed cause serious operational problems, not "no serious problems." Spiking (C and D) typically relates to noise, interference, or specific display synchronization issues, not the broad frequency instability caused by AFC failure.