C.  Test points on the front of the mach trim coupler may be useful in trouble shooting the system. Expected voltages for some of the test points are given in the adjustment/test procedures as part of system test. Most of the indications which are provided by the test points can also be determined by observing how the system operates. The following list indicates each test point and where it is connected in the circuit:
Test Points Description TP - 1 Flaps Down Logic TP - 2 MachPot No. 1 TP - 3 MachPot No. 2 TP - 4 Signal Ground TP - 5 26 Volts AC, In Phase TP - 6 Linear Synchro Feedback TP - 7 Comparator Output TP - 8 Mach Good Logic TP - 9 Brake Release Logic TP - 10 thru TP - 12 Spare
D.  Before trouble shooting, check that electrical power is applied to load control center P6, hydraulic systems A and B are operating and that elevator power control units operate satisfactorily. The mach trim, air data computer, indicator lights and master dimming and test circuit breakers must be closed and the autopilot system disengaged. Flaps must be up. The mach trim coupler self-test switch must be in the OFF position.
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2. Mach Trim System Trouble Shooting Chart MACH TRIM SYSTEM -ADJUSTMENT/TEST
TROUBLE  PROBABLE CAUSE  ISOLATION PROCEDURE  REMEDY 
Mach trim fail- Coupler, Air  Check coupler meter, if  Replace coupler. 
ure light illum- Data Computer,  not in red, and coupler  Replace air data 
inated with   actuator or  light illuminated, press  computer. Repair 
flaps up  connectors  reset button 10 seconds. If meter in red but light still illuminated, check self-test 5. If meter reads in red, coupler is faulty. If not in red, replace air data computer. With suspected components replaced, check coupler meter, if meter does not read in red with self-test switch off and coupler light is still illuminated conduct self-test series. If self-test 1 or more fails, check wiring and connectors. Wiring and connectors ok, actuator is faulty.  connectors and wiring. Replace actuator (Refer to 22-21-11,  Mach Trim Actuator.) 
System fails  Coupler or test  Press cockpit test switch  Replace coupler. 
cockpit test  switch  and hold for 15 seconds, if mach trim failure light does not illuminate during test or after switch is released, coupler is faulty. If light illumi-nates and remains on after switch is released, check per mach trim failure  light illuminated with  flaps up. If replacing coupler does not correct fault, check wiring and mach trim test switch.  Replace mach trim test switch. 
Mach trim fail- Flap switch or  Check for 28 volts dc at  Replace coupler. 
ure light illum- coupler  TP-1. If voltage, coupler  Adjust mach trim 
inates with   is probably faulty. If  flap switch. 
flaps down  no voltage, flap switch is faulty.  (Refer to 22-21-31, Mach Trim Flap Switch.) 

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1.  Mach Trim System Test
A.  General
(1)  This test is to check the mach trim system. It is assumed that associated systems are operating properly.
B.  Equipment and Materials
(1)  
VTVM - Hewlett Packard, Model 410C

(2)  
Portable pressure equipment to simulate 0 to 35,000 feet altitude and 0 to 450 knots airspeed, ERA 82305, Pitot Static Test Set; adapters and hoses, Nav Aids PSS22300-4-6-6 and F72823-40

(3)  
Scale (for measuring elevator travel) 0 to 2 feet, graduated in inches, tenths, and hundredths

(4)  
Dry air pressure source, 0 to 5 psi, with gage, fittings, hose, etc., for connecting to feel system pitot tubes, F72928-53(preferred), -52(alternate)

C.  Prepare to Test Mach Trim System
(1)  
Verify control surfaces are free to move and flaps are up.

(2)  
Provide ground power to the airplane and energize busses on load control centers P6 and P18.

(3)  
Close mach trim, master dimming, and dim test circuit breakers.

(4)  
Pressurize hydraulic Systems A and B (Ref 29-11-0 and 29-12-0, Maintenance Practices).

(5)  
Connect pitot static source (Ref Chapter 34, Pitot Static System).

NOTE:  Continuously energize air data computers for at least 30 minutes prior to start of test (Ref Chapter 34, Air Data Pressure Instruments).
　
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