The marker beacon antenna receives the 75 MHz signal and sends it to a 75 MHz filter in the VOR/marker beacon (VOR/MB) receiver 1. This filter tunes and rejects unwanted signals. The output goes to a radio frequency (RF) amplifier. A detector receives the amplifier output and sends the demodulated signal to three bandpass filters.
The bandpass filters are at 400 Hz, 1300 Hz, and 3000 Hz. The filters send the signals to an audio amplifier and to an input/ output (I/O) card. The audio amplifier sends audio tones to the REU. The I/O card sends data to the DEU and the flight data acquisition unit (FDAU).
The DEU shows marker beacon data on the PFDs.
Monitor and Test
The test switch starts a test of the VOR/MB receiver. The built-in test equipment (BITE) circuits do a test of the VOR/MB receiver. The test results show on the common display, light emitting diodes (LEDs), and through the flight interphone speakers and headsets.
During the test, a self-test frequency generator gives a 75 MHz RF signal that has 400 Hz, 1300 Hz, and 3000 Hz audio signals. The bandpass filters find the audio signals and send them to the audio amplifier. The audio amplifier sends the audio signals to the REU. The REU sends the audio signals to the flight interphone speakers and headsets in the flight deck.
The BITE circuits monitor the condition of the output data of the VOR/MB receiver.
The VOR/MB receiver gets an air/ground discrete signal from the proximity switch electronics unit (PSEU). The VOR/MB receiver uses this signal to prevent a test in the air.
An internal nonvolatile fault memory (NVM) in the BITE circuits use the air/ground discrete signal. The NVM uses this signal to calculate flight legs.
TEST – 1
When you do a test of the marker beacon system, you test the VOR receiver 1 at the same time.
When you push the test switch, the receiver does a check of the internal receiver operation and the interface with the NAV control panel. The test takes 48 seconds. This is the test sequence that shows on the LED status indicators during the test:
· 0 to 2 seconds - the LRU status and the control fail LEDs are red
· 2 to 4 seconds - the LRU status LED is green and the control fail LED is red
· 4 to 12 seconds - the LRU status and the control fail LEDs go off
· 12 to 42 seconds - test status shows.
The LRU status LED indicator shows green for an LRU test pass condition or red for an LRU test fail condition. The control fail LED shows red if there is no tune input from the NAV control panel or if the signal is invalid.
TEST – 2
You do a remote test of the navigation receivers with the navigation control panel. The navigation control panel does a test of these receivers:
· VOR/MB receiver
· ILS receiver.
When you do a test of the marker beacon system, you do a test of the VOR/MB receiver 1 at the same time.
To do a test of the VOR/MB receiver 1, select a VOR frequency and push the test switch on the front panel of the navigation control panel. FT shows for common display system indications. You must select marker beacon audio on at least one of the ACPs to hear the audio tones. All the marker beacon audio tones come on in the flight compartment when you do the test. The audio tones come on continuously and you hear the outer, middle, and inner at the same time.