Explanation: The crystal mixer in a RADAR unit is a very sensitive semiconductor diode used for frequency conversion in the receiver. It is extremely vulnerable to damage from high RF power. The TR (Transmit-Receive) tube is a gas-filled switch designed to protect this delicate mixer by rapidly shorting out the receiver input during the powerful transmit pulse, preventing high-power energy from reaching it. If the crystal mixer fails, it's a strong indication that the TR tube may have also failed or degraded, allowing transmit power to leak through and damage the crystal. Replacing only the crystal without addressing a potentially faulty TR tube would likely lead to immediate failure of the new crystal. Therefore, to ensure proper protection and reliable operation, both the crystal mixer and the TR tube are typically replaced simultaneously.