DRAKE SPR-4 User manual

DRAKE
MODEL
SPR-4
COMMUNICATIONS
RECEIVER
TECHNICAL MANUAL

Table of Contents
1. GENERAL DESCRIPTION
1.1 Features
1.2
Specifications
1
1
2.
INSTALLATION
5
2.1 Unpacking
5
2.2 Location
5
2.3 Power Requirements
5
2.4 Antennas
5
2.5 Speakers
6
2.6 Headphone Operation
6
2.7 Viewing Angle Options
6
2.8 Rear Controls & Connectors
6
3.
3.1
3.2
3.3
OPERATION
11
Operating Controls
11
Simplified Operating Procedurel2
Dia1 Calibration
12
4. PRINCIPLES OF OPERATION
4.1 Loop Amplifier
4.2 R .F . Amplifier
4.3 First Mixer
4.4 Premixer
4.5 Second Mixer
4.6 1 .F. System
4.7
Detection Systems
4.8 Audio Amplifier
4.9 AVC System
4.10
S-Meter
14
14
14
14
14
15
15
15
15
16
16
5.
ALIGNM ENT
5.1 Removing Cover
5.2 Trouble Shooting
5.3 Alignment Instructions
19
19
19.
19
6.
ACCESSORIES 24
6.1 SCC-4 Crystal Calibrator
24
6.2 S-NB Noise Blanker 24
6.3 RY-4 Radio Teletype Adaptor
26
6.4 TA-4
Transceive Adaptor 28
6.5 AVC Alterations 31
6.6 Service Data on Accessories
34
Viewing Angle Options
Fig. Page
1
Front Pane1 Operating Controls 2
Rear Controls & Connections
3
Block Diagram 4
Top Alignment Adjustment Points
5
Bottom Alignment Adjustment Points
6
SCC-4 Circuit Board
7
RY-4 Calibration Connection to SCC4 8
SCC-4 Schematic Diagram
9
S-NB Circuit Board 10
S-NB Schematic Diagram
11
RY-4 Circuit Board Installation 12
RY-4 Pictorial Wiring Diagram
13
RY-4 Circuit Board
14
RY-4 Schematic Diagram 15
TA-4 Circuit Board Installation
16
TA-4 Pictorial Wiring Diagram 17
TA-4 Circuit Board 18
TA-4 Schematic Diagram
19
Bottom View - Circuit Board 20
Transistor
Lead Configuration 21
Loop Amplifier Circuit Board
22
RF Amplifier Circuit Board
23
Range Crystal Oscillator CircuitBoard24
Accessory Crystal Circuit Board 25
Premixer Circuit Board 26
Premixer Output Circuit Board
27
1st Mixer Circuit Board 28
2nd Mixer Circuit Board 29
1st IF Amplifier Circuit Board 30
2nd IF Amplifier Circuit Board
31
S -Meter Circuit Board 32
Notch Filter Circuit Board
33
50 kHz Filter Circuit Board
34
BF0 Circuit Board
35
AM Detector Circuit Board
36
Audio Amplifier Circuit Board 37
Power Supply Circuit Board 38
4
9
10
13
17
18
34
35
35
37
38
39
39
40
41
42
42
44
45
46
49
50
51
52
53
54
55
56
57
58
58
59
59
60
60
61
61
62
Accessory Crystal Chart
SCC-4
DC/RF Voltage Chart
S-NB
DC/RF Voltage Chart
RY-4
DC Voltage Chart
TA-4 DC Voltage
Chart
SPR-4DC Voltage Chart
SPR-4RF Voltage Chart
SPR-4Schematic Diagram
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33
--
34
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36
-- 40
--
43
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47
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48
Fold Out

1.1
1.2
1.2.1
1. GENERAL DESCRIPTION
FEATURES
The SPR-4 is an a11 solid-state communications receiver which can te
programmed with accessory crystals to
cover 150 kHz to 30 MHz. The
SPR-4 receives AM, CW, SSB
(Upper and lower ) and may be powered
from 120 VAC, 240 VAC and 12 VDC.
SPECIFICATIONS
GENERAL SPECIFICATIONS
Frequency Coverage : Can be programmed with accessory crystals for 23
ranges
(each tuning a 500 kHz band ) from
.5 to 30 MHz plus 150 to 500
kHz.Generous PTO
dial overtravel allows additional 50 kHz or more off
each end of range.Crystals supplied with the receiver allow coverage
on these ranges
;
150
-
500 kHz
.5 -
l.0MHz
1.0 -
1.5 MHz
6.0
-
6.5 MHz
7.0
-
7.5 MHz
Modes of Operation:
Selectivity: AM
SSB
CW
RTTY
9.5
- 10.0MHz
11.5
- 12.0MHz
15.0
- 15.5MHz
17.5
- 18.0MHz
21.5
- 22.0MHz
AM, CW, SSB (Upper and lower
) , and RTTY with
RTTY adapter.
4_8kHz@6dB, l0.0kHz@60dB
2.4kHz@6 dB, 7.2 kHz
@
60 dB
.44 kHz @ 6 dB,
2.7kHz@
60dB
With
RTTY adapter, RTTY can be received in SSB
( lower ) or CW .
Sensitivity: SSB , CW, and RTTY:
.2 5 microvolt for 10 dB S + N/N .
AM
:
.5 microvolt with 30% modulation for 10 dB S + N/N .
Intermediate Frequencies
:
1st IF 5645 kHz, 2nd IF 50 kHz.
Frequency Stability: At room temperature, drift for
a11 causes (including
+ 10% change in supply voltage) is les s than
+- 100 Hz.
-
Automatic Volume Control: AVC is used on a11 modes. Time constants
are selected for the optimum effectiveness on
each mode. Audio output
is held constant
to 6 dB over a 100 dB range of input signals.
Input Impedance: 50 ohms approximately , on ranges C - H. On ranges
A and B,
input is high impedance and plug in loop antenna is low imped-
ance.

1.2.2
1.2.3
1.2.4
Audio
Output Power: 3 watts into 4 ohm load (less into higher imped-
ance
loads ).
Hum and Noise: More than 60 dB below rated output.
Calibration: Dial is accurate to better than _
+ 1 kHz when calibrated to
nearest 100 kHz calibration point.
POWER CONSUMPTION: 18 watts on 120 V DC, or 240 V AC, and 6 watts
on 12VDC. 2.5 watts on 12 V DC with dia1 lamps turned off.
SIZE AND WEIGHT:
5-1/2” H x l0-3/4” W x 12-1/4” D,
18 pounds
.
SEMICONDUCTOR COMPLIMENT:
Symbol
Ql
Q2
Q3
Q4
Q5
Q6
Q8
Q10
Q11
Q12
Q13
Q14
Q15
Q16
Q17
Q18
Q19
Q20
Q21
Q22
Q23
Q24
Q25
Q26
Q27
Q28
IC-1
Type Function
2N3394
Loop Amplifier
2N3394
Range Crystal OscilIator
2N3563 PT0 Buffer
SFC4982, MFE3007 RF Amplifier
SFC4982, MFE3007 Premixer
2N5950. PT0 Oscillator
Q7 2N5245 1st Mixer
2N5245 1st Mixer
Q9 B5030 D . C . Regulator
SFC4982, MFE3007 2nd Mixer
2N5245 Crystal Oscillator
B5030
Audio Amplifier
B5030 Audio Amplifier
2N5949, MPF102
S-meter Amplifier
2N3394 Audio Amplifier
2N4125 Audio Amplifier
2N5950, MPF102 Crystal Oscillator Buffer
2N4 125
AVC Amplifier
2N3394 Audio Amplifier
2N3394 Audio Amplifier
2N3566
Negative Supply Driver
2N3394 Audio Amplifier
SFC4982, MFE3007 IF Amplifier
SFC4982, MFE3007 IF Amplifier
2N3.566
AVC Detector
2N4125 Mute Control
2N3394 BF0
2N3394 AM Detector
CA3053
S-meter Amplifier
- 2-

Symbol
CRl
CR2
CR3
CR4
CRS
CR6
CR7
CR8
CR9
CRl0
CRll
CR12
CR13
CR14
CR15
CR16
CR17
CR18
CR19
CR20
CR21
Type
lN270
lN4148
lN4 148
lN714
B5G5
B5G5
B5G5
Z13B
lN714
lN4
148
lN270
lN4 148
lN4 148
lN75 1
lN4 148
lN4148
lN4148
lN4148
lN270
lN270
lN270
Function
Premixer Switch
RF Amplifier Protection
RF Amplifier Protection
Zener Regulator
Power Supply Rectifier
Power Supply Rectifier
Power Supply Rectifier
Zener Regulator
Zener Regulator
AVC Switch
AVC Switch
Negative Supply Rectifier
Negative Supply Rectifier
Zener Regulator
AVC Switch
AVC Switch
AVC Switch
AVC Switch
Product Detector
Product Detector
AM Detector
- 3 - -

VIEWING ANGLE OPTIONS
Figure 1.
- 4 -

2. INSTALLATION
2.1
UNPACKING
Carefully remove the receiver from the shipping carton, and examine it
for evidence of damage. If
any damage is discovered, immediately noti-
fy the transportation company that delivered the receiver. Be sure to
keep the shipping carton and packing material, as the transportation
com-
pany will want to examine them if there is a damage claim. Keeping the
carton and packing material is recommended even when no shipping dam-
age occurs ,
as having the original carton available makes shipment of
the receiver
much easier should it ever be necessary to return it to the
factory for service.
2.2
LOCATION
The SPR-4 will work well in almost any location. Extremely hot areas
such as over a radiator should be avoided.No air circulation around the
receiver is required .
2.3
POWER REQUIREMENTS
The SPR-4 will operate on 120 or 240 VAC 50/60 Hz or from any 12 -
15 VDC source with negative ground .
The SPR-4 is shipped from the factory
ready for 120 V AC operation with
the power cord supplied.A
1/4 amp, slow-blow fuse, Buss type MDL
1/4, is installed in the fuse holder on the rear of the chassis .
For operating from 240 V AC , the same power cord is used and the 120/
240 volt slide switch at the
chassis rear is moved to the 240 position.
This requires moving the small metal keeper from the right to the left
mounting screw on the slide switch .
Also for 240 volt operation , the fuse should be changed to a 1/8 amp,
slow-blow, Buss type MDL
1/8.
WARNING: NEVER CONNECT THE SPR-4 TO 240 VOLTS
WITH THE SWITCH IN THE 120 VOLT POSITION.
For DC operation, a separate power cord is
a11 that is required. This cord
is available with a
cigar lighter plug installed.
2.4
ANTENNAS
For optimum performance the SPR-4 should be used with a good antenna.
For general broadcast and short wave listening, a long wire antenna from
- 5 -

50 to 100 feet long will give good results .An antenna kit (Mode1 AN-S)
is available.
For best single band performance,a half wave dipole or other resonant
antenna , fed with unbalanced 50 ohm coaxial table, may be used . How-
ever, Bands A and B require a long wire antenna.
For directional
reception on 150 kHz to 1600 kHz, the accessory AL-4
shielded loop antenna may be
used.
All antennas other than the AL-4
loop antenna,
connect to the Ant. jack on the rear of the chassis.
2.5 SPEAKERS
The SPR-4 has a built in 3” x 5” ceramic magnet speaker which makes op-
timum use of the available
space in the cabinet. It may be desirable to
use an
external speaker when the receiver is used at fixed locations .
The Drake MS-4 Speaker, housed in a cabinet that matches the SPR-4,
provides excellent audio reproduction and cornes equipped with a table
terminated in a phono plug matching the speaker socket on the rear of the
SPR-4.
2.6
HEADPHONE OPERATION
Best results will be obtained with a set of good quality headphones . The
headphone
connection is made through the jack at the bottom left of the
front pane1 of the SPR-4.A standard
1/4 inch phone plug will mate with
the
jack. Connecting the headphones to the receiver turns off the speak-
er.The SPR-4
will accommodate headphones of any impedance.
2.7 VIEWING ANGLE OPTIONS
As supplied, the SPR-4 sits with its base
parallel to the mounting surface
and its front pane1 vertical to the mounting surface. With the studs and
rubber
feet supplied, it is possible to position the SPR-4 for either of
two alternate viewing angles as shown in Figure
1. TO convert the SPR-4
to either option, remove the bottom row of three
screws on each side ‘of
the receiver, and remove the bottom
cover . For Option 1, remove the
rear
feet and snap the small rubber feet into the holes just inboard from
the original rear
feet and replace the bottom cover. For Option 2, remove
the front
feet, invert them , and remount them with the same screws .
Thread one of the accessory studs into each of the remounted front feet.
Remove the rear feet and screw them onto the studs on the front feet and
replace the bottom
cover.
2.8 REAR CONTROLS AND CONNECTORS
Refer to Figure 3 for the location of the
connectors and controls .
- 6 -

2.8.1 POWER
The power
connector mates with both the AC and DC power cords .
See
“2 . 3 ”
2.8.2 FUSE
The fuse holder must have the proper fuse installed for 120 or 240 V AC
operation . See “2
.3 ”
2.8.3 SPEAKER
The interna1 speaker is normally
connected to the speaker jack, when an
external speaker is used, disconnect the interna1 speaker by removing
the plug and
connect the external speaker to this jack.
2.8.4 ANTI VOX and INJECTION
The holes marked ANTI VOX and INJ are intended for installation of
connectors for the TA-4 accessory which permits the SPR-4 to transceive
with the T-4XB. The injection and anti-vox tables from the T-4XB are
plugged into these jacks .
2.8.5 MUTE
A shorted phono
connector is normally installed in the MUTE jack. When
the shorted
connector is removed, the SPR-4 will be muted . When the
SPR-4 is used as a companion to the
T-4XB, the mute table from the
T-4XB is plugged into this
jack.
2.8.6 GROUND POST
A binding post is provided to
allow connecting the SPR-4 chassis to
earth ground .
2.8.7 RTTY
The hole marked RY is intended for the installation of a
connector for the
RY-4 accessory which permits the SPR-4 to receive RTTY signals . An
ex-
ternal switch is plugged into this jack which enables RTTY reception
when the jack is shorted and the SPR-4 mode switch is in the CW or LSB
position
.
2.8.8 AUDIO IN/OUT
The AUDIO
IN/OUT jack provides approximately .08 volts of audio into
a high impedance load
such as a tape recorder. The output level is inde-
pendent of the setting of the AUDIO GAIN control.
- 7- -

The SPR-4 audio amplifier may be used with an external audio source
such as a tuner. The level of the external source must be approximately
.08 volts into a 3 K load . The received signals
may be eliminated by
turning the RF GAIN control counter-clockwise or by removing the mute
plug l
2.8.9 DIAL LAMPS
The DIAL LAMPS switch may be used to tum off the three dial lamps .
Since the power required to operate the dial lamps is somewhat greater
than the power required by the rest of the SPR-4, the operating time for a
battery power supply may be extended by turning the lamps off. The on
and off positions of the switch are interchanged when operating with a
DC power
cord .
2.8.10 ANTENNA JACK
The antenna jacks mate with a standard phono fitting. A special antenna
fitting (a pin with a spring clip
attached ) is supplied with the SPR-4 and
may be used by inserting it into the antenna
jack.
The antenna lead -in
is then
connected to the clip.
2.8.11 ACCESSORY CRYSTAL INSTALLATION
The top of the SPR-4 cabinet must be removed in order to install
addition-
al crystals .This is accomplished by removing the top row of screws on
both
sides of the set.Also, disconnect the
table from the speaker jack.
Be sure the power tord is disconnected.The top of the cabinet may now
be removed .
At the bottom of each segment of the crystal selector dial is a small num-.
ber (0
- 23 ) which corresponds to a particular crystal socket .The
crys-
tal location diagram which is located on the chassis behind the crystal
sockets indicates the number of each socket. No crystal is required for
reception of 15 0 - 500 kHz,therefore there is no
crystal socket numbered
zero .
Install the accessory crystal in one of the empty sockets . Locate the
appropriate
crystal dial decal from the sheet supplied with the crystal.
Remove the decal from the sheet by cutting along the appropriate lines
with a pair of scissors .Remove the
backing from the decal and install it
in the dial sector with the
same number as the crystal socket used. Re-
place the cabinet top and the speaker
table.
On certain ranges there are spurious interna1 signals which are unavoid-
able in this double conversion, super-heterodyne receiver. These are
due to outputs of interna1 conversion oscillators. There are five
frequen-
cies that should be avoided if possible. These are; 5.222 MHz, 5.595
MHz, 5.695 MHz, 9.963 MHz, and 15.334 MHz.
- 8 -

a,

3.
OPERATION
Refer to Figure 3 for the location of the operating controls.
3.1
OPERATING CONTROLS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
The
S-meter indicates the relative level of the received signals .
The
crystal selector switch selects the band determining crystals .
The
top line of the dial sector reads the lowest frequency of each
band in MHz, the middle line indicates the approximate position for
tuning the preselector and the bottom line indicates the proper
sett-
ing of the range switch.
The main tuning
dial reads 0 to 500 kHz with 1 kHz graduations.
When the tuning
dial reading is added to the crystal dial reading ,
the actual received frequency is indicated to 1 kHz.
The main tuning
dial consists of two concentric transparent discs
that
rotate at different speeds .Zero to 100 kHz is indicated on
one
disc and hundreds of kHz are indicated on the other disc. The dial
may
be calibrated by rotating the skirt with respect to the knob.
The preselector is set to the position indicated by the number on the
middle line of the crystal selector
dial and then fine tuned for peak
signal .
The range switch sets up the proper tuned circuits to
cover the de-
sired band of frequencies .It is set to the position indicated on the
bottom line of the crystal selector dial.
The headphone jack mates with a standard 1/4 inch phone
plug. Con-
necting the headphones to the receiver will turn off the speaker.
The audio gain control adjusts the audio output to the proper level.
The RF gain control controls the gain of the receiver for
all signals
which are below the AVC threshold level to which the control is set.
This control is normally set in the full clockwise position. Counter-
clockwise rotation reduces the gain of the receiver.
The mode selector switches in a
product detector for CW and SSB or
a diode detector for AM.
The proper I. F. bandwidth and AVC time
constant is also selected.
The notch control
may be used to eliminate or reduce an interfering
heterodyne or CW signal. The off position, which is shown in
Figure 2, has a positive detent. Counter-clockwise rotation of the
- ll-

knob from the off position moves the rejection notch across the re-
ceiver pass band .
11. The accessory switch turns on the S-NB Noise Blanker and the SCC-4
Crystal Calibrator if they are installed. The 5-NB and the SCC-4 are
off in the NORM position.
3.2
SIMPLIFIED OPERATING PROCEDURE
As an example , suppose that the SPR-4 is to be tuned to 7272 kHz. First
turn the crystal selector
dial to the 7.0 MHz position. Set the preselec-
tor to 5.5 as indicated on the second line of the crystal selector dial .
Next set the range switch to E as indicated on the bottom line of the
crystal selector dial.Turn the tuning knob until the 100 kHz
dial is be-
tween 2 00 and 300 and the one kHz dial is on 72. Select the desired
mode of reception with the mode switch.The RF gain control should be
full clockwise and the notch should be in the off position. Turn on the
receiver with the audio gain control and adjust it for normal audio output.
Fine
tune the preselector for maximum indication of the S-meter.
The frequency of the received signal is the sum of thè crystal
dial read-
ing and the main tuning dial reading as shown by the example:
Crys ta1 Selector Reading 7 .000 MHz
100 kHz Dial Reading .200
1 kHz
Dial Reading .072
Frequency of Received Signal 7.272 MHz
3.3
DIRLCALIBRATION
The main tuning dial calibration may be adjusted over a short frequency
range by using the known frequency of the SCC-4 crystal calibrator or a
station such as the National Bureau of Standards
, WWV, on 2.5, 5, 10,
15, 20, and 25 MHz. Usually 5 and 10 MHz WWV signals are the
easi-
est to receive .With the mode switch in the CW, LSB or USB position,
tune the signal from the SCC-4 or WWV to zero beat. Hold the main
tuning knob stationary and
rotate the skirt until the dial reads the correct
frequency .
- 12-

.;
-13-

4. PRINCIPLES OF OPERATION
4.1
LOOP AM PLIFIER
On Bands A and B, Q1 amplifies signals from the loop antenna and feeds
one gate of the R.F. amplifier Q4, through C52 and the tuned circuits
formed by
C9, Cl0, C13, C16, C18, C22, C23, C25, C27, C33, T2,
and T4.
On Bands A and B, signals from an external antenna connected to
J2 (AA)
bypass Q1 through Cl0 and Cl3 into the tuned circuits described above.
4.2 R.F. AMPLIFIER
On Bands C through H signals from an external antenna connected to
J2
(ANT )are coupled to Q4 through T3 and C52..T3 is resonated with C101
and the capacitors connected to S5R. Q4 is protected from large signals
by CR2 and CR3 which are reverse biased.CR2 and CR3 limit the signal
to a
safe level by conducting when a large signal overcomes their reverse
bias .
On Bands A and B the drain of Q4 is untuned and drivès the 1st mixer
through C90 and C96. On Bands C through H , the drain of Q4 is tuned by
T5, the capacitors connected to
S9F, and drives the 1st mixer through
C90.
4.3
FIRST MIXER
The 1st mixer is a balanced mixer comprised of Q7 and Q8 and driven by
the R .F . amplifier and the pre-mixer .On Band A, the pre-mixer is
by-
passed and the 1st mixer is driven from the P .T.O. through R25 and C99.
On Bands B through H, the pre-mixer drives the 1st mixer through T6 and
C99.
The 1st mixer is balanced by R40 and the output is tuned by Cl02 and T8
and drives the 2nd mixer through
C110, the 5645 crystal filter, and T9.
Heterodyning of the R.F. amplifier and pre-mixer (P.T.O. on Band A)
results in a
difference frequency output from Q7/Q8 of 5645 kHz.
4.4
PREMIXER
The premixer system consists of the P .T .O. , the crystal oscillator Q2,
the premixer transistor QS, and the tuned circuits associated with S1,
S4F, S4R, S8F, and S8R.
The
collector of the crystal oscillator Q2 is tuned by L5 and the compo-
nents connected to S1.The feedback winding on L5 is connected
through S2 to the proper range
ctystal and the output of Q2 is connected
-14-
-

to one gate of the premixer, Q5 through C26 and R15. The P.T.O. drives
the other gate of Q5 through R25 and C76. The P.T .O. is tuneable from
4955 kHz to 5455 kHz.
The output of Q5 is tuned by the components associated with
S4F, S4R,
S8F, S8R and drives the 1st mixer through T6, S6R, C99 and C96.
4.5
SECOND MIXER
The 2nd mixer, Q10, is driven by the 1st mixer and the crystal oscillator
buffer Q17. Q10is coupled to the 1st IF amplifier through the T notch
filter and the 50 kHz four pole Hi-Q ferrite LC filter. Heterodyning of
the 1st mixer and
crystal oscillator results in a difference frequency out-
put from Q10 of 50 kHz.
4.6 1. F. SYSTEM
The I .F . system is composed of the 1st I.F . amplifier, 423, which drives
through T 15, the 2nd
I. F. amplifier, Q24.C169 and C187
tune T15 and
T16 respectively to 50 kHz and T16 drives the
detection systems .
4.7
DETECTION SYSTEMS
4.7.1 AM DETECTION
The AM signal from
T16 is detected by CR2 1 and amplified by Q28 to a
level matching the output level of the product detector and is applied to
the audio gain control through C 167 and S 14F.
4.7.2 CW AND SSB DETECTION
CW and SSB signals from T16 are detected by the product detector made
up of CR19, CR20, R122, R123, and Q27, the beat frequency oscillator.
The output of the product detector is applied to the audio gain control
through S 14F.
4.8 AUDIO AM PLIFIER
A portion of the audio signal, determined by the setting of Rl0l , is ap-
plied to Q22 through a low-pass filter consisting of L10, L11, C164,
R97,
R95, C 163, and C159 which eliminates any residual 50 kHz signal
from driving the audio amplifier.Q22 amplifies this audio signal and
drives
Q20 through C 148 where further amplification occurs and Q2 0
drives
Ql6 directly and Q15 via Q19 which is unity gain common base
amplifier. Q12, Q 15 and
Q13, Q16 are darlington connected amplifiers
and the configuration of Q16 establishes a phase inversion permitting
Q12 and Q13
to operate in push-pull, Q12 and Q13 operate the speaker
or a headphone through coupling capacitor C
127.
- 15- -

4.9
AVC SYSTEM
The AVC threshold is established by the setting of R111, the R.F. gain
control.R111 is normally operated fully clockwise . This applies
approx-
imately +2 volts to the AVC line through R104, R100 and CR15 with no
received signal.Signals from T16 drive Q25 into base current on the pos-
itive R
.F . driving cycle which pulls the collector negative because the
emitter is returned to the - 5 volt line.
The negative potential on the col-
lector of Q25 is applied to the AVC line through CR18 and to C 171
through CR17.
The rate that C162 is charged determines the AVC attack
time.The AVC release time is determined by the rate that C 171
dis-
charges through R100 allowing Cl62 to discharge through CR15. The at-
tack and release times for Q4 are lengthened by R22 and C70 to eliminate
undesirable transients.
Large signals are kept from overloading Q4 by the application of a posi-
tive voltage on Pin 3, the signal gate.
This voltage is derived from the
S-meter and Q-18 and the delaying circuit of CR10, R47, R52, and C180.
On AM , S 14R shorts out CR10 to obtain a symmetrical attack and release
response.
4.9.1 MUTING
Muting is accomplished by placing an
open circuit at J9 which removes
base current from Q26, lowering its
collector voltage to -5 volts. The
cathode of CR16
will also be at -5 volts causing it to conduct, applying
cut-off voltage on the AVC line which turns off Q4, Q10, Q23, and Q24
and mutes the receiver.
4.10
S -METER
The S-meter is a voltmeter that measures the AVC voltage and is calibrat-
ed in ” S ” units . The source follower Q14, is connected to the AVC line
and drives IC- 1 which is a differential amplifier that operates the
S-
meter.
The zero adjustment is affected by R64 and the S-meter full scale
sensitivity is set by R73.
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