TM 55-4920-402-13&P CHAPTER 4 AVIATION   INTERMEDIATE   MAINTENANCE   (CRC)   INSTRUCTIONS Section I. PREPARATION FOR MAINTENANCE, STORAGE, AND SHIPMENT 4-1. preparation for Maintenance. Ensure that a 28 vdc and a 120 vat, 60 Hz power source is available for the test equipment  listed  in  table  1-3. 4-2. Preparation for Storage. Use the following proce- dure  when  preparing  the  VIBREX  for  storage: a.  Place  the  Tester,  Accelerometers,  Magnetic  Pickups, Brackets, and Cables in the top compartment of the Carrying Case (fig. 1-l). b. Place the Balancer, Strobex, and Gram Scale in their Section II. THEORY 4-4. Functional Theory of Operation.  Before  perfoming the  checkout  and  analysis,  the  following  simplified  block diagrams  and  functional  descriptions  should  be  understood. CAUTION Extreme care should be exercised when using the VIBREX test set. This unit contains   delicate   electronic   com- ponents and assemblies which may be easily damaged if subjected to physical abuse due to improper handling. 4-5. Balancer section of Balancer. The electronics for the  Balancer  (fig.  4-1  and  FO-1)  are  contained  on  the Balancer  Circuit  Card  Assembly  C4146. a. Basically, the Balancer consists of two tuneable narrow band filters. With the VERIFY TUNE button re- leased, the filters are connected in such a manner that they  result  in  a  single  stagger-tuned  filter  centered  at  the frequency (RPM) determined by the RANGE switch and RPM TUNE dial. When the VERIFY TUNE button is pushed, it switches the filters from broad band (released) to narrow band (pressed). If the filter is not properly tuned, a large difference in phase shift through the filter is indicated as a sharp change in clock angle. When the filter is properly tuned, there is no phase shift and no clock angle change. b. Inputs A and B from the Accelerometers are con- netted to operational amplifiers U107A and U110A, res- pectively. The operational amplifiers are a constant- current source to the t wo-st age isolation amplifier in the Accelerometer. The output of U107A is applied to the A input of FUNCTION switch S1201 and the output of U110A is applied to the B input. The output of S1201 is applied to operational amplifier U107B that, in con- junction with PUSH FOR SCALE 2 pushbutton S1203 and VERIFY TUNE pushbutton S1202, changes the gain of the circuit to make the lPS meter read within scale. proper compartments in the Carrying Case (fig. l-l). c.  Close  Carrying  Case. 4-3. Preparation for Shipment. Perform all steps in para- graph 4-2 and the following: , a.  Place  Carrying  Case  in  shipping  container. b. Record missing or damaged components on the appropriate forms. c. Return VIBREX through proper channels. OF OPERATION AND ANALYSIS c. The output of U107B is applied to the first active filter  loop  consisting  of  operational  amplifiers  U105, U101A, U101B, U102A, and U102B. The filter is tuned to resonance by RPM TUNE dial R1201A, and by C106 through C111 selected by RPM RANGE switch S 1204. The output of the filter at U102B is applied to U103A in the second active filter loop. This loop consists of U106, U103A, U103B, and U104B. The filter is tuned to reso- nance by RPM TUNE dial R1001B, and by C112 through Cl17 selected by RPM RANGE switch S1204. The two filters  are  stagger-tuned  to  broaden  the  pass-band  to  make tuning less critical and less subject to small changes in the input frequency. Disconnection of the two filters by VERIFY  TUNE  pushbutton  S  1202  removes  the  stagger- tuned effect and narrows the pass-band to provide fine tuning. d. The output of the second filter is from U104A to operational amplifier U108B, whose output is rectified by a full wave bridge rectifier to drive IPS meter M1201. The output of U104A is also combined with the output of  U106,  via  U108A,  to  produce  a  negative-going  command at  the  output  of  U109B  for  each  positive-going  zero  cros- sing of the filter output signal at U104A. e. The output of U109B is differentiated and the nega- tive  spike  triggers  U109A,  a  monostable  countdown  cir- cuit. U109A provides the positive sample command to the Strobex and the Phazor circuits for phase indication. The monostable countdown circuit, for command rates higher than approximately 40 Hz, automatically divides that rate by integer numbers so the output does not exceed 40 Hz. Change 4 4 - 1