TM 55-4920-390-13&P
1-13. OSCILLATOR SECTION. A locking oscillator is on the circuit board in the electronics section. Its rate is
controlled by the controls located on the rear of the hand-held lamp (See Fig. 1-4). The control setting is determined by
use of the chart on the inner lid (Fig. 1-6). It is locked to a one-per-rev signal from the magnetic pickup previously fixed
to the rotor head. This locking signal is delivered via the 40' cable (P 102) provided for the purpose.
1-14. POSITION CIRCUIT. Means are provided to introduce a time delay in the locking signal from the magnetic pickup
(one-per-rev signal). This delay is controlled by R 101 found on the bulkhead (Fig. 1-2) and is set as indicated on graph
a) It is a means of positioning the tip target display to a convenient viewing point in azimuth.
b) It introduces a delay which makes possible the operation of an "inhibit" circuit, avoiding the
appearance of a blade out of position. The logic is as follows.
The internal oscillator rate is set near blade rate by setting the "Multi-turn Dial" by means of the
chart on the inner lid (See 4-7, b and c). The oscillator rate is then "fine-tuned" by means of the
vernier control on the rear of the hand-held lamp. If the flash rate is precisely the same as blade
rate the tip targets will be "stacked" (See 4-7, i). Assuming blade rotation right to left, the targets
will be spread to the left if the oscillator is adjusted slightly slower than blade rate, and to the
right if faster.
If the oscillator is set at a rate slower than blade rate, there is no problem because the last pulse
(fourth on a four blade helicopter) would occur after the sync pulse from the magnetic pickup.
Therefore, the sync pulse causes the first flash and starts the oscillator over again and the fourth
pulse never appears and a proper display is presented.
However, if oscillator rate is set faster than blade rate, the fourth pulse appears causing a flash
and immediately thereafter the sync pulse appears before the capacitors have recharged and the
light will not flash. This causes a visual horizontal displacement of one blade because the flash
should have occurred at the sync pulse.
This leads to the main reason for the "Position" control, for if we insert a time delay such that the
flashes occur roughly midway between blades (relative to the position of the sync pulse), the
sync pulse will appear before the fourth oscillator pulse, but the light will not flash because of the
delay. The delay allows operation of an "inhibit" circuit which prevents the fourth oscillator pulse
from causing a flash, and after the appropriate delay, allows the sync pulse to cause a flash.
This puts that blade back where it belongs and allows use of the oscillator at a rate either slower
or faster (or the same) than blade rate.
1-10