Sony tuner LBT-D505 not working, display is very dim

Hi, it's about the LBT-D505 Sony tuner. The display is very very dim when pressing the [On/Standby] button. I hear the power relay switch on. If I measure the LA5667 voltage regulator on the Power Board on pin 5 it produces 5.6V as noted in the schematics. However if I switch on the tuner by pressing the [On/Standby] button the voltage drops down to 0.8V and the display shows FM 87.50 and stays on. Pressing the [On/Standby] button again then the display shows "OFF" and after a few seconds the power relay is switched off and the voltage on pin 5 returns to 5.6V. If I disconnect the 5.6V pin 5 and connect an external power supply at 5.6V to the circuitry then when pressing the [On/Standby] button the power relay switches on but after a few seconds it switches off. The current drawn in Standby mode is then 15mA and when switched ON it is 22mA. I think anyway that the power regulator chip LA5667 is defective because some circuitry on the tuner PCB has ruined it. What could be the cause of this?