TM 55-4920-402-13&P
Strobex RPM control, the blades may be spread for easy
resolution. When applied to the two blade rotor system
(Bell) where two interrupters are installed on the swash-
plate, position B is used for greater light output. The
oscillator is set to flash at a rate slightly slower than
bade rate (in accordance with the formula on the back
of the Strobex, RPM=blade rate X 0.4). Then, the interrup-
ter commands the Strobex before the oscillator, and it
operates as if there were no oscillator. In this case, the
targets cannot be spread. The oscillator may be set to
double the blade rate, in which case the Tip Targets will
be seen at twice as many positions around the rotor disc.
For instance, the targets of a two-blade rotor are typically
seen at 12:00 and 6:00 o’clock (as determined by location
of Interrupters). If the oscillator is set to double the blade
rate, the targets will also be seen at 3:00 and 9:00 o’clock.
(3) Mode C Mode C is a free-running oscillator
and is used as a tachometer for speed (RPM) measuring.
All external signals are disconnected and the Strobex
flashes only in response to its internal oscillator. Flash
rate is adjustable from 100 to 1,000 RPM (flashes-per-
minute).
(4) Mode D. Mode D is also a free-running oscil-
lator and is used for tail rotor tracking and speed measuring.
Since there is no Magnetic Pickup mounted on the tail ro-
tor, the flash rate of the free-running oscillator is adjusted
to double or four times the rotor rate (for a two-blade tail
rotor), so the single Tip Target appears as a stopped image
of two or four. Then, by viewing the rotor disc edge-on
from the cabin door, reflective Tip Targets can be seen
superimposed for a judgement of track. Flash rate is from
1,000 to 10,000 RPM.
(5) Mode E. Mode E is a locking oscillator like
Mode B, but operates at higher rates and is used for view-
ing the track of airplane propellers. It requires a Magnetic
Pickup input pulse.
4-9. Tester.
a. The circuitry in the Tester (fig. 4-4) includes a 2-
ampere fuse (F101) and a 36-volt zener diode (CR102).
These components protect motor drive circuit (B1) in the
event the unit is accidentally connected to an improper
power outlet.
b. CR101 is a polarity project diode which prevents
reverse dc voltage from being applied to B1. If reverse
polarity is applied, no damage will result, but the unit
will not operate until polarity is corrected.
c. Switch S101 selects either R102 or R104 to syn-
chronize B1 motor speed.
d. Switch S102 applies 28 vdc power to the B1 motor
drive circuits.
Section III.
TROUBLE ANALYSIS AND REPAIR
4-10. Trouble Analysis. This section contains instructions
trouble to the Balancer, Strobex, airframe mounted compo-
and procedures for troubleshooting and repair of the
nents, or interface cabling. Normally, this will have been
VIBREX. Use of the Tester is the basis for isolation of a
Figure 4-4. Tester Schematic Diagram
Change 4
4-7
