Drake 4-NB Service manual

MODEL 4-NB NOISE BLANKER

The Model 4-NB is a solid-state noise blanker for

R-4C

use with the
or limiters commonly found in communications
equipment, the 4-NB is an advanced noise blanker
which actually mutes the receiver for the duration
of the noise pulse.
receiver gain is restored. Receiver AGC is affected
only by the desired signal and not by noise. The
4-NB is most effective on strong, periodic impulse
noise such as ignition noise. It is least effective on
random noise. Random noise is continuous in time
and the information it masks cannot be recovered

by either blanking or limiting techniques.

To install the 4-NB, perform the following: Unplug

the

R-4C

line cord. Remove the top row of three
screws on each side of the
the cabinet top. Remove the jumper plug from the
noise blanker socket; see the R-4C Manual figure
5-l for the location of the 4-NB noise blanker
socket. Retain the jumper plug for alignment pur­poses, and in case the blanker is ever removed from
the

R-4C.

The 4-NB mounts on the four plastic
printed circuit board stand-off insulators, which
are on the metal mounting bracket-shields on
either side of the VFO; refer to the
figure 5-1. On receivers with the metal dust cover,
remove this cover by removing the three mounting
screws. Two screws are located at the top of the
rear panel. The third cover mounting screw is lo-

cated at the top front corner of the inner 4-NB

mounting bracket-shield. Position the

the four holes in the corners of the 4-NB board are

aligned with the plastic standoff insulators. The

cable from the 4-NB should be toward the rear of
the

R-4C

Carefully push the 4-NB circuit board

onto the plastic stand-off insulators until it snaps
firmly into place. Dress the 4-NB cable toward rear
of the
then along the rear panel. Plug the 4-NB cable into
the socket on the R-4C. Re-install the dust cover.

Before replacing the top cover, turn on the
and follow the alignment procedure mentioned
below. After alignment, reinstall the cabinet top
and the six screws.

The 4-NB can be easily removed if it is necessary to
gain access to the crystal calibrator. Compress the
locking tab on each mounting insulator with a pair

of long-nose pliers, and pull upward on the corner

of the 4-NB board. Repeat at each corner until

board is free.

R-4C

Receiver. Unlike noise clippers

Between noise pulses, full

R-4C

Receiver. Remove

R-4C

Manual,

4-NB

so that

along side the power transformer and

R-4C

The 4-NB is controlled by the R-4C function
switch. Blanking action occurs with the switch in
the NB position. When strong signals are received,
some distortion may be noticeable with the
blanker switched on.

4-NB NOISE BLANKER ALIGNMENT.

WARNING
Always turn off the R-4C before
plugging in or unplugging the 4-NB.

The 4-NB blanker is pre-aligned; however, align­ment should be checked to assure optimum per­formance in your receiver. For initial setup, align­ment of C3, C6, and R 12 should be all that is
necessary.

28.5 MHz. Adjust C3 and C6 for maximum S
meter reading. Remove the NB cable from NB
socket and reinstall jumper plug. While receiving
calibrator signal, detune the preselector control
until the S meter reading is around S-5 to S-7, and
note the exact reading. Without disturbing any
control settings, remove jumper plug and plug in
the 4-NB. If S meter reading has changed, adjust
R12 (blue potentiometer in corner of 4-NB board)
for the same S meter reading as with jumper plug
installed. This completes setup alignment.

If for some reason complete alignment is necessary
the following procedure is used: Connect a VTVM
which will measure positive 15 volts DC full scale,

between the chassis and R45. Tune the

28.5 MHz while using the crystal calibrator as a
signal source. With the function switch in CAL
position, adjust C3 and C6 for maximum S meter
reading. With the function switch in the NB posi-

tion and the RF gain control fully CCW, adjust
R28 for maximum positive voltage on R45. Con­nect a jumper from pin 2 of the calibrator socket
to ground. This will allow operation of the crystal
calibrator and noise blanker simultaneously. Adjust
the RF gain control to maintain 10 volts on R45
while adjusting Cl 9 and C25 for minimum positive
voltage on R45. Remove the jumper from pin 2 of
the calibrator socket and return the RF gain to full
CW. With the calibrator turned on, adjust R12 so
that the S meter has the same reading with the
4-NB installed as it does with the jumper plug
installed.

Tune in a crystal calibrator signal at

R-4C

to

4-NB Voltage Chart

REF
DES

Q1
Q2

Q3
Q4

Q5
Q6

Q7
Q8
Q9
Q10
Q11
Q12
Q13
Q14
Q15

Q16 4.2 (0)

EMITTER/SOURCE

1.6

1.5

8.3

7.3
0

3.3 4.0

1.6

1.4

3.4 4.0

3.4

3.6
0

6.3

5.8

3.8

I

BASE/GATE

2.4

2.3

9.1

7.9
0

0
0

4.0 3.4

4.2

0.22

6.7 13.8

6.3

3.2

3.5(1.3

V)

1 COLLECTOR/DRAIN

--f====-

I

+_-

-.--_

6.3

9.1

12.6

13.2

7.9

13.5

13.7

13.7

10.5

9.9

6.7

13.8

3.2

9.2(2.9

_

V)

MEASUREMENT CONDITIONS:
Conditions are the same as the R-4C voltage chart except the R-4C FUNC­TION switch was in the NB position. Measurements were taken with an 11
Megohm

Measurements in parenthesis are RF voltages.

VTVM and a Boonton

0

Q

R45
(330

K)

91C

RF voltmeter.

c25

I

n

I

R28

0

0

Model 4- NB Noise Blanker Alignment Locations

2

0