(2)  
Access Panel
 621GB Fueling Station (RH side)

 E. Prepare for Test
 S 865-002
 (1)  Supply electrical power (AMM 24-22-00/201).
 F. Service Interphone System Test
 S 985-005
 (1)
Set the SERV INTPH switch on the right side panel P61 to ON.

 S 985-006

 (2)  
Use headsets to do a check for clear voice communications between D425 (service interphone jack on the nose landing gear control panel) and the jacks at these locations:

 (a)  D427 Right main wheel well service panel.
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 (b) (c) (d) (e) (f) (g)  D423 Main E/E Bay.D843 E5 Rack.D417 Right wing fueling panel (P28).D413 APU compartment.D429 Horizontal Stabilizer.D415 Aft equipment center rack.
 (3)  S 985-007Set SERV INTPH switch to OFF (P61).
 (4)  S 985-008Use headsets to make sure that communication between D425 and any other service interphone jack is not possible.
 (5)  S 865-009Remove electrical power if is not necessary (AMM 24-22-00/201).

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 CABIN INTERPHONE SYSTEM - DESCRIPTION AND OPERATION
___________________________________________________
 1.  General_______
 A.  The cabin interphone system provides facilities for interphone communication among cabin attendants, and between the flight compartment crewmembers and attendants. The cabin interphone system microphone circuits can be switched to the input of the passenger address (PA) system to permit announcements.
 B.  The cabin interphone system includes: cabin attendant handsets, attendant call lights, the pilots' call panel, and associated amplifiers and mixing circuits located in the audio accessory unit.
 C.  The system gets power from the 28v dc battery bus, through a circuit breaker on overhead panel P11.
 2.  Component Details (Fig. 1)_________________
 A.  Audio Accessory Unit
____________________
 (1)  
Two identical audio amplifiers with parallel inputs are located in the audio accessory unit on shelf E4-3 in the main equipment center. The amplifiers are used by both the service interphone and the cabin interphone systems. A placard on the audio accessory unit identifies and locates the two amplifiers. There are two outputs from each amplifier. One output goes to the cabin interphone audio on the audio selector panels. The other output goes to the cabin interphone handsets and to the service interphone jacks. The amplifiers have internal adjustments preset for compression, squelch, and volume.

 (2)  
The audio accessory unit contains: the interphone power supply card, the main logic input/output card, the call interface card, and three dual-handset interface cards. To adjust the audio level for the handsets, a potentiometer is located on the PA control card. All interphone control functions occur in the audio accessory unit automatically, when it receives tones from the tone encoder circuits in the handsets and the pilots' call panel.

 B.  Cabin Interphone Handsets
_________________________
 (1)  Each attendant station has a handset. Each handset has the following components: a noise-canceling microphone to decrease the input of airplane noise, a small speaker to listen, buttons to operate the handset, and a magnet-operated hook switch to disconnect the handset from the system when the attendant places the handset back on the hook.
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