Westinghouse Type KR, Инструкция по установке и эксплуатации, Страница: 88 / 124

mately 250 millivolts ( to produce 10 rna relay current)
with input control R- 1 in maximum clockwise posi­
tion. About 10-db additional gain or 80 millivolt
sensitivity can be obtained by clipping out resistor
R-74, which is connected to the base of Q-4 and the
B+
supply for the IF. Approximately 10-db g ain or
25 millivolt sensitivity can be obtained by clipping
out resistor R-7�. which is connected to the base of
Q-5 and the B+ supply for the IF. Resistor R-7 1 has
been selected at the factory to provide an overall
gain of approximately 25 millivolts.
a. Sensitivity Adj ustment for Noise
If the maximum on-frequ ency noise level is known
or can be m easured, the receiver can be adjusted for
this level. Disconnect the coaxial cable and connect
a 60-ohm non-inductive resistor and a VTVM (HP
400C or equivalent) across terminals 1 1 and 12.
Energize the transmitter and adjust the transmitter
output control R-42 for the same output as the m ax­
imum noise. Then adjust the receiver input control
R- 1 to obtain 1-ma output current. If it is impossible
to obtain 1-ma output current with control R- 1 in the
maximum clockwise position, then clip out resistor
R-74, which is connected to the base of Q-4. If the
gain is still insuffi cient , clip out resistor R-7 3,
which is connected to the base of Q-5. After com­
pleting the adjustment of R- 1 , restore the transmitter
to full output.
b. Sensitivity Adjustment for Remote Signal
When the maximum on-frequency noise is unknown
and cannot be conveniently m easured, the receiver
may be adjusted for the remote signal. First deter­
mine the channel attenuation using a Sierra voltmeter
or some other convenient m ethod. Disconnect the
coaxial cable and connect a 60-ohm non-inductive
resistor and a VTVM (HP 400C or equivalent) across
terminals 1 1 and 12. Add 1 5-db to the channel atten­
uation. This compensates for average variations such
as channel and voltage variations. For example, if
the channel attenuation is 5-db, add this to the 15-db
making a total of 20-db . Taking this from 8 volts
output of the transmitter, the receiver should then be
set for a sensitivity of 0 . 8 volts. To do this, adjust
the transmitter output control R-42 for 0.8 volts and
then adjust the receiver input control so as to obtain
10-ma output current. If it is impossible to obtain
10-ma output current with control R- 1 in the maximum
clockwise position, clip out resistor R-74 , which is
connected to the base of Q-4. If the gain is still in­
sufficient, clip out resistor R-7 3, which is connected
8
to the base of Q-5. After completing the adjustment
of R- 1 , restore the transmitter to full output.
It must be kept in mind that the two preceding
adjustment procedures are to be used as a guide and
will cover the majority of cases; however, cases m ay
arise where conditions change. For example, if the
adjustment is made per part a. , the noise may in­
crease due to various reasons such as bad insulators ,
which would require adjusting the receiver to be less
sensitive. In the adjustment of part b. , the attenua­
tion may increase due to sleet or line switching,
which would n ecessitate increasing the sensitivity of
the receiver.
Sleet Detector
Due to different types of op eration and supply
voltages, it is not practical to have a calibrated dial
for this unit. However, calibration may be made at
installation by recording transmitter output ( either in
db or volts) at various knob settings of the sleet
detector unit. Then , by adjusting the sleet detector
so as to obtain 10 milliamperes relay current at a re­
mote receiver, any increase or decrease in line at­
tenuation may be noted as the difference between the
original s etting, and the setting required to obtain the
given 10 milliam peres.
258-Volt Resistor Unit
With the apparatus energized, adjust R-4 in the
258-volt resistor unit so as to obtain 1 29-volt at
terminals 17 (B-) and 19
(B+)
on the relaying set.
Frequency Change
If the frequency is changed in the field, it is
necessary to change the input filter FL- 1 to the de­
sired channel frequency. This filter is of the plug-in
type and may be removed by unsnapping the clamp
and pulling the filter out. The transmitter oscillator
crystal Y- 2 must be changed to the desired channel
frequency, The receiver oscillator crystal Y- 1 must
be changed to the desired channel frequency +20 kc.
Figure 7 shows the connections for the transmitter
output filter. It is necessary to unsolder the existing
connections and make the new connections per the
chart. After completing the preceding, the tuning pro­
cedure as described under Adjustments for trans­
mitter and receiver should then
be
followed.
Note also that capacitors C2 and C9 in the
transmitter and receiver crystal oscillator circuits,
respectively, may have to be changed. For fre-
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