(b)
The memory circuits receive the search/track information, control the action of the flag, and supply memory information to the servomotor drive control.
EFFECTIVITY
LH 001-010,031-050,071-081;
AF 101-120, 123-151; AZ 201-219; CONFIG 1 IB 405 01 Page 12
Apr 25/95
34-55-00
(c)
The servo drive control combines the information from the search/track switch and the memory circuits and controls the signal applied to the servo-amplifier. If the interrogator is in track mode, the servomotor drive control applies the signal from the chopper to the servo-amplifier. When the interrogator switches from track to search, the servomotor drive control applies a memory signal to the servo-amplifier and the servomotor is driven at the tracking rate it had prior to going into search. After the memory period is over, the servomotor drive control applies a search voltage to the servo-amplifier. The servomotor then drives the range potentiometer and he resolver until lock-on occurs.
(d)
The warning flag circuits hold the DME indicator warning flags out-of-view only when accurate distance information is being tracked and displayed. The readouts are covered during standby mode, search mode (when the DME completes the prescribed memory cycle without achieing lock-on), channeling mode, warmup or equipment failure.
(e)
When the test button is depressed on the VHF NAV control panel, the self-test circuits function to break lock-on, cancel memory and slew the search drive inbound to slightly below 0 mile, reverse and lock-on 0 mile. When ground potential is removed, the DME resumes normal operation. The self-test circuitry itself provides an accuracy of | 0.1 nautical mile.
(8)
Power Supply
(a)
The power supply provides a variety of voltage sources for the interrogator circuits. A built-in time delay maintains the DME in a warmup mode for up to 70 seconds after power is applied.
(b)
On airplanes LH 001-099; AZ 201-219, 115 volts ac and 28 volts dc from the airplane's radio bus are controlled through a power relay operated remotely from the VHF NAV control panel.
(c)
On airplanes AF 101-171, 115 volts and 28 volts dc are routed directly from the circuit breakers.
(9)
Mode Description
(a)
When the receiver circuits of the interrogator detect no signals above a usable level, the signal controlled search circuits place the interrogator in the standby mode. When received signals increase to a usable level, full power is applied to the interrogator for normal operation.
(b)
When the interrogator loses lock-on with a ground station or is channeled to a new station, signal controlled search circuits begin searching outward from 0 miles to the range limit. If this occrs while the indicator is reading less than 50 miles, the search cycle returns to slightly below zero before search begins. This assures that the correct reply is encountered first (for echo protection).
EFFECTIVITY LH 001-010,031-050,071-081;
AF 101-120, 123-151; AZ 201-219; CONFIG 1 IB 405 01 Page 13
Apr 25/95
34-55-00
(c)
If the interrogator is channeled to a station with no DME, the search cycle stops (if indicator reads less than 50 miles) or returns to zero and stops (if indicator reads more than 50 miles).
(d)
If the interrogator detects that it is locked on an echo reply pulse, a protection circuit causes the interrogator to break lock-on, turn off the transmitter and returns the search cycle to zero miles where normal outbound search is begun. Echo protection is disabled when the interrogator is in search mode or standby. The echo protection circuit is self-monitoring and will activate the indicator flag in case of most failures.
(e)
Should the interrogator receive two ground stations on the same channel, the stronger one is presented. If both stations have equal strength, the aural identification is garbled to give a positive indication of this condition.
B. Control
(1)
The DME system is turned on by closing DME 1 - AC, DME 1 - DC, DME 2 - AC and DME 2 - DC circuit breakers on electronic circuit breaker panel P7 and placing the mode switch on the VHF NAV control panel to the DME position. On the VHF NAV control panel operate the frequency selector to tune the desired VOR/DME channel.
(2)
The distance indicators will show flags and the system will remain in standby mode even after the warmup period since no interrogation can take place until the system is airborne.
EFFECTIVITY
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本文链接地址：747飞机维护手册AMM CHAPTER 34 - NAVIGATION 第34章导航4(88)
