Explanation: The symptoms described—no target video at the center of the CRT and a blank spot that shrinks proportionally with increased range scale—are classic indicators of a defective TR (Transmit-Receive) Cell. The TR cell's primary function is to protect the sensitive receiver from the powerful outgoing transmit pulse and then rapidly switch to allow weak incoming echoes to reach the receiver. If the TR cell is faulty, particularly if it's slow to switch back to receive mode or remains partially conductive after the transmit pulse, it will effectively "blank" or desensitize the receiver for a short period immediately following transmission. This creates a "dead zone" at very close ranges, preventing the display of nearby targets. As the radar's range scale is increased, this fixed-duration dead zone represents a smaller proportion of the total displayed range, making the blank spot appear smaller in diameter on the CRT relative to the overall scan area. * **B) The local oscillator is misadjusted:** A misadjusted local oscillator would typically cause a general loss of sensitivity or no targets across *all* ranges, as the receiver would be tuned to the wrong frequency. It wouldn't specifically create a near-range blank spot. * **C) Video amplifier circuit:** A fault here would affect the display of all signals after detection, leading to weak, distorted, or no video across all ranges, not a specific near-range blanking. * **D) The IF amplifier circuit:** Problems in the IF amplifier would lead to reduced gain or no signals across *all* ranges, similar to a local oscillator issue, rather than a range-specific blank spot.