Atlas 210x, Installation, Operation And Maintenance Manual

Page: 50 / 58

SECTION 5 ACCESSORIES
5-1. NOISE BLANKER MODEL PC-120
The Noise Blanker is a plug-in accessory which replaces the standard PC-100C R.F. board in Atlas transceivers. If
Noise Blanker is owner or dealer installed, a threshold control which is attached to the PC-120B by a 3 wire cable,
must be installed on the front panel.
CAUTION: The Carrier Balance controls have been factory set. Do not alter the 500 ohm trim pot or mica
trimmer. They are located in the upper right hand corner of the PC-120B. Handle with care.
CIRCUIT DESIGN: The PC-120B noise blanker is designed specifically to blank out pulse type noise; the same
as all other noise blankers. Pulse type noises, such as ignition noise will be effectively silenced, while other more
continuous type noise signals will not be silenced or reduced as well. The PC-120B works on the same principle as
the famous Lamb noise silencer. Noise pluses are amplified separately by Q103, rectified into DC pulses which are
amplified by Q104, and then applied to Q105, the blanking switch. Q105 switches off Q102, the second I.F.
amplifier, whenever a sharp noise pulse comes through from the antennae circuit. The threshold control, R120. is
located on the front panel, and controls the gain of Q103. A second threshold control, R128, is a trimpot located on
the PC-120B circuit board, and is factory adjusted so that noise pulses will switch Q105 completely, but not
excessively.
OPERATION:
(a) When the threshold control is in full counterclockwise position the noise blanker is OFF.
(b) Advancing the control clockwise will increase the gain of Q103 until noise pulses will begin to control Q105.
At this point the audible noise level will drop sharply.
(c) If the threshold control is advanced beyond this point, no further reduction in noise
will be obtained, if the character of the noise is made up entirely of sharp pulses, it will be practically
eliminated. If parts of the noise are continuous, they will still be audible. In other words, the degree of
noise reduction will be directly related to what percentage of the noise is pulse type, and what percentage is
of a continuous nature.
(d) CROSS MODULATION may result from higher settings of the threshold control. This is likely particularly
on the lower frequency bands when a great many very strong signals are being received on a full size
antennae system. Turning the threshold control down to the threshold level will eliminate this condition,
while still retaining the pulse blanking action.
47