(a)  
Advancing the function knob clockwise allows the pilot to select a low altitude trip setting throughout the full range.

(b)  
Depressing the knob actuates a test switch which allows the system self-test function to engage.

(c)  
In the event of loss of track, the altitude pointer moves to a point above 2500 feet and disappears behind the NO TRACK mask.

(13)
The ARINC output circuit is composed of four operational amplifiers that rescale the external range output signal to a linear-logarithmic signal as defined in ARINC characteristic number 552A. From zero to 500 feet, the output signal is linear. From 500 to 2,500 feet, the ARINC output signal is logarithmic. The ARINC signal varies from zero to +26.174 vdc. The high and low altitude trip circuits switch from 28 vdc to 0 vdc when their PRESET trip level is passed during aircraft descent.

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B.  Self-Test Operation
(1)  
The self-test mode of operation is the manually commanded mode. It provides end-to-end checkout of the receiver-transmitter, exercising all the major functional blocks. By interpretation of the results of self-test, faults can be generally isolated to the receiver-transmitter, the indicator, or the antenna system. The self-test altitude indication is set at 400 feet.

(2)  
The self-test sequence begins when the PUSH TO TEST knob on the indicator is pressed. This applies a ground to the input of the self-test gate, causing its output to become positive. This ground is also applied to the clamping circuits on the input to the T-zero amplifier, preventing the normal T-zero output of the transmitter circuit from being applied. The output of the prf generator, coupled through the PTT delay circuit and a differentiating circuit, is applied to the self-test gate as a high of approximately 5 volts with a width of 2,000 nanoseconds. This signal is then coupled through another gate circuit to the T-zero amplifier.

(3)  
In the self-test mode of operation, the input to the T-zero amplifier is the differentiated leading edge of the prf generator output signal. Because of the time difference between the transmitted pulse and the self-test input to the T-zero amplifier, the receiver-transmitter develops an output range signal equal to the preset self-test altitude. When the transmitter fires, a leakage pulse is generated and travels to the receiver input. While the system is in the push-to-test mode, the leakage signal from the transmitter cavity to the receiver input (mixer/local oscillator) is outside the range of src (zero to 200 feet). During normal operation, all leakage signals are controlled by the src.

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LOW RANGE RADIO ALTIMETER SYSTEM - TROUBLE SHOOTING
EFFECTIVITY
GJ ALL EXCEPT B-2509, B-2510
1.  General
A.  If a malfunction occurs in the radio altimeter system the defective component should be replaced with a component known to be in good operating condition. Excluding the malfunctions listed below in par. B., substitutions should always be in the following order.
(1)  
If system is malfunctioning, replace receiver-transmitter with one known to be in good operating condition.

(2)  
Perform operational check system. If the system is still malfunctioning, replace height indicator

(3)  
If system still malfunctions, inspect antennas for damage, airplane wiring and all connectors for improper connections or damaged items.

B.  The following malfunctions are traceable to the height indicator. If the system operation is normal and one or more of the following malfunctions occur, do not replace the receiver-transmitter first. Instead, replace the height indicator and recheck system operation. If not OK, replace receiver-transmitter.
(1)  
Dial lighting

(2)  
DH trip lights

(3)  
DH control knob or cursor/self-test

(4)  
Flag not visible when circuit breaker is pulled.

C.  Altitude trips supplied by the radio altimeter system but not used for altitude indication by the radio altitude system are covered in the chapter for the using system. See simplified schematic in 34-48-0, Description and Operation.
D.  DH light on ADI's can be tested for failed lamp by pressing light face.
　
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