ARMY TM 5-6775-238-14 MARINE CORPS TM 08839A-14/1 a. Signal Processor Unit.  The  signal  processor unit (also referred to as SPU) chassis consists of an open type welded frame with a heat exchanger, hous- ed in an instrument case. All connectors and operating controls  are  located  on  the  front  panel.  Input  and  out- put  air  ducts  are  also  located  on  the  front  panel. b. Punched Tape Reader. The  punched  tape  reader (also referred to as tape reader) is housed in an instru- ment  case.  The  operating  control  (or  controls)  is located  on  the  front  panel  along  with  the  tape  spooler, time  totalizing  meter,  two  fuses,  and  a  connector. c. Buffer Unit. The buffer unit is contained in an aluminum  housing  with  a  sealed  cover.  Two  connec- tors  are  installed  in  the  housing.  Left  and  right  sup- ports  are  attached  to  the  housing. d. Transit Cases.  Three  transit  cases  are  provided for transporting the PADS test set or for storage during long periods of nonuse. These cases are water- tight and dusttight and contain a pressure relief valve. 1-9. Differences  Between  Models.  There is only one model  of  Position  and  Azimuth  Determining  System Test Set AN/USM-427. However, there are two models of the tape reader used in this test set. The earlier model tape reader part no. is 877406-1. The later  model  tape  reader  part  no.  is  877406-2. Whenever  the  tape  reader  is  discussed  and  there  are differences  between  the  two  tape  readers,  the  part numbers  will  be  included  in  the  discussion  as applicable. 1-10. System Application. The  PADS  test  set  may be utilized to check out the computer, inertial measurement unit, or control and display unit. When an inertial measurement unit (IMU) or control and display unit (CDU) is connected to the PADS test set for checkout, an operational computer must also be connected.  In  addition,  an  operational  power  supply must be connected to the computer. The PADS test set is also capable of loading the computer memory with the supplied IMU alignment tape. In addition, provisions are made for using a teletypewriter with the  PADS  test  set  during  testing  and  troubleshooting of the PADS equipment. 1-11. Tabulated Data.  PADS test set performance characteristics  are  given  in  table  1-1. 1-12.  Items  Comprising  an  Operable  Equipment. Items comprising the operable PADS test set are illustrated in figure 1-1 and listed in tables 1-2, 1-3, and 1-4. Legend for Figure 1-1 1. 2. 3. 3.1 4. 5. 6. 7. 8. 9. 10. Signal Processor Unit TS-3617/USM-427 Signal Processor Unit Case CY-7563/USM-427 Punched  Tape  Reader RP-239/USM-427  (Part  No. 877406-1) Punched  Tape  Reader RP-239/USM-427  (Part  No. 877406-2) Punched  Tape  Reader  Case CY-7562/USM-427 Buffer Unit CV-3404/USM-427 Test  Set  Accessories  Case CY-7609/USM-427 Cable  Assembly,  Power  Electrical, W201 Cable Assembly, Special Purpose, Electrical,   W203 Cable Assembly, Special Purpose, Electrical, W204 Cable Assembly, Special Purpose, Electrical,   W205 1-2 Change 4 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. Cable  Assembly,  Special  Purpose, Electrical,  W212 Cable  Assembly,  Special  Purpose, Electrical,  W209 Cable  Assembly,  Special  Purpose, Electrical,  W210 Card  Extractors Self-Test   Tape Take-Up Reels (six reels) Diagnostic Tape (two reels) Operational   Tape Card  Extender,  Tape  Reader  (used on  Part  No.  877406-1) Maintenance Manual: Army TM 5-6675-238-14  or  Marine  Corps  TM 08839A-14/1 RPSTL: Army TM 5-6775-238-24P or Marine Corps TM 08839A-24P/2 Shorting  Plug Solid  State  Program  Tape  (three  reels) Cable  Assembly,  Special  Purpose, Electrical,   W211