TM  55-4920-402-13&P at a steady rate and turning dial on rear panel smoothly  changes  flash  rate. (5)   Direct   lamp   at   ground   and   observe   a bright  3  to  4  inch  diameter  spot  of  light or at a nearby (10 to 20 feet) flat surface and   observe   a   bright   10-   to   15-   inch diameter   spot   of   light.   If   the   spot   of light   is   not   as   required.,   pm-form   the following   focus   instructions: (a)  Remove  six  4-40  screws  and  lockwashers from rear cover. Do not remove any other screws, (b) Open rear cover (fig. 3-8). (c) Energize the will and place MODE switch in position D and RPM dial to 500 to 600. (e)   If   a   dark   center   appears,   use   an   in- (d) Project beam on a dark surface, approxi- mately 3 feet in front of flash tube. sulated   screwdriver   to   turn   the three   nylon   screws   clockwise   until a  uniform  bright  spot  appears  (keep it  about  4  inches  in  diameter). (f)       If a small bright spot appears in a large light  pattern,  adjust  nylon  screws  counter- clockwise  until  a  uniform  4-inch  diameter spot is seen. (g)  Accomplish  final  focus  by  projecting  the beam on a wall approximately 15 to 20 feet away. Adjust for a spot approximately 1  foot  in  diameter. (h) Install rear cover with six 4-40 screws and lockwashers. d. Controls. Set Balancer and Strobex controls as re- quired for the test being performed. 3-11. Troubleshooting the VIBREX While Installed on the Aircraft. Most  malfunctions  may  be  readily  traced to a defective part of to an improper adjustment. Table 3-1  is  a  troubleshooting  guide  for  the  most  common  mal- functions.  After  reviewing  table  3-1,  and  the  VIBREX is  still  not  operative,  perform  the  following: Table 3-1. Troubleshooting Symtoms Probable cause Remedy No lights in Dc  polarity  wrong. Check polarity (pin B is hot +, pin A is ground). Phazor. Circult breaker, to circuit in use, Turn circuit breaker on. not tuned on. Unsteady  tail Oscillator not in Mode A. Strobex must be in Mode A when balancing tail rotor  image  when rotor. balancing. Vibration level is very low. When vibration level is low, clock angle is uncer- tain. Jittery image is indicator of good balance. Unable to ob- Strobex out of focus. Remove  rear  panel  and  adjust  focus. serve  targets. Reflective  Targets  worn  or  dirty. Replace,  and  avoid  handling  reflective  surface. Replace  as  necessary. Flash  tube  cracked  (weak  blue  flash). Replace  flash  tube. Not aiming Strobex correctly and/or Look directly over top of Strobex, and search in not  in  line  with  light  source. a W pattern along the tip path. Strobex and/or Balancer switched to Check  settings. wrong  position. Protective  varnish  or  coating  over Coating  kills  reflective  properties  of  exposed reflector   material. bead material. DO NOT COAT. Targets  appear Strobex  oscillator  ON. Check oscillator switch. MUST be in A or B. scattered   when tracking main rotor. Interrupter installed backwards or bent. Refer to installation instructions. No test pattern in Magnetic Pickup gap too large. Check  gap  between  Magnetic  Pickup  and  Inter- Phazor  when  TEST rupter. button is pushed. Faulty  Magnetic  Pickup  Cable  or Check  and  repair  or  replace  as  required.  Mag- Magnetic Pickup. netic Pickup should read about 1000 ohms. INTERRUPTER  LOGIC  switch Set  to  DOUBLE. set   incorrectly. 3-10 Change  2