Kenwood TKR-820 User manual

SERVICE MANUAL
© 1990-3 PRINTED IN JAPAN
B51-8050-00 (O) 1403
REVISED
Panel
Case
(Upper)
Dressing
plate
Knob
(TAKE
OVER,
REPEAT,
MONI)
(A20-2666-11)}
(A01-1059-01)
(A21-1519-03)
(K29-3148-04)
x
3
Knob (POWER) Foot Knob (VOL, SQL) Dressing plate Rectangular receptacle (MIC)
(K29-31 46-04) (J02-0049-14) x 4 (K29-3147-03) x 2 (A21-1520-03) (E08-0673-05)
CONTENTS
GENERAL
.........ccccsceececeeesseesesscecescsencerceeeuserssseceeneestes
2
RX
VCO
:
Z2
(X58-3150-XX)
wesscseccecesseecnsenssrseesene
63
SYSTEM
SET-UP
.........:sscscessscstsrnssssccuseseserenseeseers
4
TX
VCO
:
Z5
(X58-B460-XX)
....eecreeceseersseessnsscereue
64
INSTALLATION
......0..cceccsssessesssensessensccescersssseneaeeeeeses
5
MIC
AMP
:
Z7
(X59-3210-10)
....scccssececesnsereseeen
-
65
CONVERSION
.........-cccsssesceeeetsessccevensssseceusvsseneesssssees
9
IF
:
ZB
(X59-3220-1O)
...
ee
eesecesssnssteteeeeseensneesneneetan
-
66
DISASSEMBLY
FOR
REPAIR
.............::sccsseessereees
11
BPF
/
VCA
:
Z9
(X59-3230-10)
...sccsesessrsrsesssereenn
-
67
BLOCK
DIAGRAM
............--:::scssccrsercessersetseeneecsenens
17
AVR
UNIT
(X43-3040-10)
oo...
ccseeesseseesssteesseeeeeene
-
68
CIRCUIT
DESCRIPTION
.............ceccssssesssersseeessereees
19
FINAL
UNIT
(X45-3250-XX)
...cssseccceeeeessecenstseeene
-
70
SEMICONDUCTOR
DATA
..........::cecccssscssesenteseseees
26
SIGNALING
UNIT
(X52-3140-10)
.....eceesstceesseren
-72
DESCRIPTION
OF
COMPONENTS
............scsceeees
31
DISPLAY
UNIT
(X54-3070-11)
.eecscccccccssreeneessenn
~
74
PARTS
LIST
........::cccescseeceescenseeresnnseneneceesscersscasenenses
33
TX-RX
UNIT
(X57-3270-XX)
oo
ecessessssseessceesesseneenn
-
78
DISASSEMBLY
.........ccscsssscccresenensessonnseusessconeresenses
53
SIGNALING
CIRCUIT
DIAGRAM
.................00008
-
83
PACKING
..........ccssssscssesseneescsesseccceeesssseosenancansrsseneses
55
SCHEMATIC
DIAGRAM
............ccssseesececssessenernennn
-
87
ADJUSTMENT
........:ccccccscsccorccessesusenscnscunsnrsesenezetsss
56
TERMINAL
FUNCTIONG.
.............:ccceeeeecesersencoeeens
~
95
PC
BOARD
VIEWS
/
CIRCUIT
DIAGRAMS
KMC-9
(BASE
MICROPHONE)
...........--cccsecsoeeetene
-
97
RX PLL: Z1, TX PLL : Z4 (X58-3120-10) ....sececees 62 SPECIFICATIONS. ...........csssscsseresessees BACK COVER

TKR-820
GENERAL
INTRODUCTION
SCOPE OF THIS MANUAL
This manual is intended for use by experienced techni-
cians familiar with similar types of commercial grade com-,
munications equipment. It contains all required service
information for the equipment and is current as of the pub-
licatton date. Changes which may occur after publication
are covered by either Service Bulletins or Manual Revi-
sions. These are issued as required.
ORDERING REPLACEMENT PARTS
When ordering replacement parts or equipment infor-
mation, the full part identification number should be
included. This applies to all parts: components, kits, or
chassis. If the part number is not known, include the chas-
sis or kit number of which it is a part, and a sufficient de-
scription of the required component for proper identifica-
tion.
PERSONNEL SAFETY
The following precautions are recommended for
personnel safety:
@ DO NOT transmit if someone is within two feet (0.6
meter) of the antenna.
@ DO NOT transmit until all RF connectors are verified
secure and any open connectors are properly ter-
minated.
@ SHUT OFF and DO NOT operate this equipment near
electrical blasting caps or in-an explosive atmosphere.
@ All equipment should be properly grounded before
power-up for safe operation
@ This equipment should be serviced by a qualified tech-
nician only.
® Be careful against electric hazard, for the commercial
power supply is being applied to the internal circuitry of
the radio even when the Power switch is OFF.
FCC COMPLIANCE AND TYPE NUMBERS
Type acceptance number |-Frequency range
Compliance
ALHSTKTKR-820-1
450 to 470MHz
Part 22, 74, 90 and 95A
ALHTKR-820-2
470 to 490MHz
Part 22, 74 and 90
ALHTKR-820-3
490 to 512MHz
Part 22, 74 and 90
ALHTKR-820-4
406 to 430MHz
Part 90
1. POWER-UP
To push on the radio. The POWER indicator will
illuminate to indicate power is ON.
2. TO RECEIVE
Operation Procedure
1. Disable OT Push on MONITOR switch.
If so programmed.
Turn SQUELCH control counter-
clockwise until noise is heard.
2. Unsquelch radio
3. Set VOLUME control Adjust VOLUME control for a
normal listening level.
4. Set SQUELCH control Advance SQUELCH control clock-
wise until noise just stops.
The radio will now receive all traffic on the selected channel.
5. Enable OT Press the MONITOR switch to OFF.
1f so programmed.
You will now hear messages for your system only.
3. TO TRANSMIT (in case a microphone is connectect}
Operation Procedure
1. Disable QT Depress MONITOR switchon front
panel or Microphone.
2. LISTEN DO NOT TRANSMIT if channel is
in use,
Press and hold the microphome PTT
switch. The Red LED on the front
panel will glow indicating he trans-
mitter is ON.
3. Key transmitter
Hold microphone at about2 inches
distance and speak at a nom al voice
level. Keep transmissions bief. -
4. Transmit message
5. Receive reply Release the microphone PTT switch.
6. Enable OT at end of
the conversation
Hf so programmed.
Depress MONITOR to theoust
position.

TKR-820
GENERAL
4. DURING OPERATION OF REPEATER
Operation Procedure
1. Enable repeat Press the REPEAT switch to ON.
When a signal including the abject
tone signal (provided that it has been
programmed) is input and if its level
is higher than the preset squelch level,
a signal modulated by the set tone
signal (provided that it has been pro-
grammed) and received audio signal
is transmitted.
2. Start of repeat
When the received signal disappears,
the transmission is stopped after the
set period of time. There indicator
lights while the signal is transmitted.
3. End of repeat
Press the REPEAT switch again to
OFF.
4. Disable repeat
PRE-INSTALLATION CONSIDERATIONS
1. UNPACKING
Unpack the radio from its shipping container and check
for accessory items. If any item is missing, please contact
KENWOOD immediately.
2. LICENSING REQUIREMENTS
Federal regulations require a station license for each
radio installation (mobile or base) be obtained by the
equipment owner. The licensee is responsible for ensuring
transmitter power, frequency, and deviation are within the
limits permitted by the station license.
Transmitter adjustments may be performed only by a
licensed technician holding an FCC first, second or general
class commercial radiotelephone operator's license. There
iS NO license required to install or operate the radio.
3. PRE-INSTALLATION CHECKOUT
3-1. Introduction
Each radio is adjusted and tested before shipment.
However, it is recommended that receiver and transmitter
operation be checked for proper operation before installa-
tion.
3-2. Testing
The radio should be tested complete with all cabling
and accessories as they will be connected in the final
installation. Transmitter frequency, deviation, and power
output should be checked, as should receiver sensitivity,
squelch operation, and audio output. QT equipment opera-
tion should be verified.
4. PLANNING THE INSTALLATION
4-1. General
Inspect the vehicle and determine how and where the
radio antenna and accessories will be mounted.
Plan cable runs for protection against pinching or
crushing wiring, and radio installation to prevent overheat-
ing.
4-2. Antenna
When an outdoor antenna is to be installed, select an
unobstructed position with good visibility.
The VSWR of the antenna should be no more than 2.
Select a coaxial cable with as small Joss as possible, and
its length should be as short as possible.
4-3. Radio
Install the radio on a rack using rack-mount brackets,
or on a flat surface that can withstand its weight. Do not
install the radio in a place subject to direct sunlight or near
heating equipment. Avoid wet place, and install it ina well-
ventilated place.
4-4. AC power supply
This unit has been designed for use exclusively with
commercial AC power supply. As the rated current for
transmission output attains about 2A, connect the unit to
a wall outlet that can handle this current with margin.
SERVICE
This radio is designed for easy servicing. Referto the
schematic diagrams, printed circuit board views, and
alignment procedures contained in this manual.

TKR-820
Merchandise
received
h
SYSTEM SET-UP
Preparation : Prepare an EEPROM writer, KPT-20.
License and frequency
allocated by FCC
Frequency range | RF Power Type
450 ~ 470MHz 25W TKR-820 K,M
Choose the type of _ 470 ~ 490MHz 25W TKR-820 K2,M2
transceiver 490 ~ 512MHz 25wW | TKR-820 K3
490 ~ 520MHz 25W TKR-820 M3
406 ~ 430MHz 25W TKR-820 K4
See KPT-20 400 ~ 430MHz 25W TKR-820 M4
Frequency, Instruction
EEPROM writer manual.
q
See KPT-20
Instruction
Signaling DATA manuat.
EEPROM writer
{for Signaling unit)
See page 5.
EEPROM
installation
Duplexer
used?
See page 6.
Duplexer *
installation
“Option
__|
DC Back up
modification
Parts used?
See page 9,
OC Back up
modification
J
Mounting
Bracket
used?
NO
See page 8.
Mounting Bracket
installation
|
Delivery

TKR-820
INSTALLATION
Installing the EEPROM of Signaling Unit
Note :EEPROM is inserted into the Signaling unit of the
TKR-820. Install it after writing the data using the
ROM writer (KPT-20).
1. Remove the 10 screws (‘®@) holding the upper case to
remove the upper case.
2. Remove the 11 screws ( @) retaining the shield cover to
remove the shield cover. 2)
3. Insert the EEPROM (®) on which the data has been Shield cover g
written into the IC socket (IC10)} on the Signaling unit.
iC(EEPROM)
93C46?1

TKR-820
INSTALLATION
Installing the Duplexer
Note :When installing the duplexer in the TKR-820, use
the either of the following products ;
1. CELWAVE (USA), 435 to 470MHz (PD633-6A)
2. ANTEN CORPORATION (JAPAN),
450 to 470MHz (L79-0691-05)
400 to 430MHz (L79-0896-05)
1. Remove the 10 screws (‘@) holding the upper case to
remove the upper case.
2. Remove the 4 screws (‘@) holding the lower case to
remove the lower case.
3. After removing the upper and lower cases, turn the set
upside down, as shown in the figure 3) ).
4. Remove the coaxial cables (@). @) connected to the
TX and RX antenna connectors.
TX Antenna connector
RX Antenna connector
a =

TKR-820
INSTALLATION
(‘@) and the 8 screws C7) ) provided with the set.
6, Connect the one end of the connector cabel (@) pro-
vided with the set to the duplexer antenna connector o
and the other end to the ANT (center) of the duplexer. { TF =)
7. Connect the connector cable (®) wired from the TX é lh i |
section and the connector cable (®) wired fram the O--~@ [> C]
|
5. Mount the duplexer ‘@) using the mounting hardware y—— Duplexer Antenna connector
u
RX section to the duplexer.
[o]
00%
ex.
©)
a
CELWAVE'S Duplexer °
_ eO@ 9
tre vo todh
*
=
|
on
a
=
Note :The input/output connectors of the duplexer manu-
facture by CELWAVE or ANTEN are located as REC. ANT TRANS. LOW ANT HIGH
shown in the figure. Pay attention when connecting eth it __.2, afk, Te
the connectors (as input/output location is reversed). ° o ° oO ° rs) ° °
ie) ie} ie) o
CELWAVE’S Duplexer ANTEN’S Duplexer
Note :When making adjustments after installing the
duplexer, remove the front panel and hold down the
sub-pane! (®), then perform adjustments from
the adjusting holes ().

KR-820
INSTALLATION
Attaching the Rack-mount Brackets (for EIA racks)
The brackets and screws are optional.
Brackets : J21-4250-04 (Common for left and right)
Screws : NO9-0704-05 (Use 2 screws per bracket)
1. Attach the 2 brackets (@) using 4 screws (‘®).
2. When the TKR-820 is to be mounted vertically :
Attach 4 brackets (@), using 8 screws (@}), to the
sides of the bottom panel.
7
SS SN
© 4
ia
6

TKR-820
CONVERSION
Modification of the DC Switching Circuit
(For
Backup
during
a
Power
Failure)
TKR820
A
L
The
following
parts
(optional)
are
required
for
the
Fe
circuit
modification.
WUT
[|
|]
_O©_
|
pine
|
Diode
($25VB10)
..........
000000.
1
pe.
Cit)
DC
cable
(E30-2076-10)
.............
1
pe.
Connector
with
lead
wires
(E31-3389-05)
..
1
pc.
COMO!
Connector
with
lead
wires
(E31-3455-05)
..
1
pe.
oS
Screw
(N87-4014-46)
.........00
0084
1
pe.
1. From
the
cover
(‘®)
attached
to
the
rear
panel,
remove
the
2
screws
‘@)
and
take
off
the
cover.
®
I
ia
:
a
_)
Jt
L}—}-—L_}
©
9K;
©
°
|
©
[le
2.
Insert
the
connector
with
lead
wires
(
@)
into
the
hole
m
on
the
rear
panel
‘@)
(so
that
the
red
lead
is
on
the
right).
3.
Place
the
diode
(
@)
so
that
its
notch
faces
the
direc-
a)
|
tion
shown
in
the
illustration,
and
attach
it
using
a
|
screw
(@).
C2
|
|
|
4, Disconnect the connector (@) from the QUT terminal
of
the
AVR
unit,
and
connect
its
lead
wire
to
pin
1
of
a
5
i
0
ou
the
diode
(
).
B
8B
°
our)
4
SS
ae
Red
lead
@|
i
=
iC

TKR-820
CONVERSION
5. Connect the optional connector with lead wires ( ©) to f
the OUT terminal of the AVR unit and to pin 3 of the
diode.
a)
6. Connect the red lead (©) of the optional connector
with lead wires to pin 2 of the diode, and its black lead
(®) to the negative (—} terminal of chemical capacitor
C101. Do not disconnect the lead wire which has been \
connected to the C101 negative terminal, but connect
the black lead to it.
7. Connect the optional DC cable to the connector on the
rear panel (®).
TKR-820
10

How to Remove the Case
1.
2. Remove the 4 screws (‘@) holding the lower case to
Disassembling the Transmitter/Receiver Section
1,
1.
2.
. While lifting up on the transmitter/receiver section
. Remove the 4 screws T7) holding the transmitter/
TKR-820
DISASSEMBLY FOR REPAIR
Remove the 10 screws (@) holding the upper case to
remove the upper case.
remove the lower case.
How to remove the transmitter/receiver section
Remove the 4 connectors (CN1 to CN4) connected to
the panel (Display unit) (@).
Remove the 3 screws (®) holding the transmitter/
receiver section.
(@), remove the power supply connector (@) and
the connector cables (@,@) connected to the frame,
and restore the transmitter/receiver section in place.
receiver section to the frame to remove the transmitter/
receiver section.
11

TKR-820
12
DISASSEMBLY FOR REPAIR
. How to remove the TX-RX unit
. Remove the 6 screws ‘d) retaining the shield cover to
remove the shield cover.
. Remove the 2 connectors (Q) of the Signaling unit and
the 6 screws ‘®) retaining the shield cover to remove
the shield cover.
. Remove the 6 screws ‘Q@) retaining the connectors
CN2 to CN4.
. Remove the 11 screws (@) attached on the bottom of
the TX-RX unit to remove the shield cover.
. Remove the 3 connectors CN1 (@), CN8 (@) and
_CN10 ( @) connected to the TX-RX unit.
. Remove the 4 screws (©) connected between the
TX-RX unit and Final unit.
. Remove the leaf spring fixing the two IC’s to the frame
with a screwdriver, etc. (®).
. Remove the 11 screws (®) mounting the TX-RX unit
to remove the TX-RX unit.
Driver
Frame
TX-RX unit

TKR-820
DISASSEMBLY FOR REPAIR
3.
Disassembling
the
Final
ass’y
unit
1)
1.
Remove
the
4
screws
(@)
retaining
the
shield
cover
to
1)
remove
the
shield
cover.
g
g
2.
Remove
the
2
screws
‘@)
retaining
the
power
module
=
IC
to
the
heat
sink.
or
~
3.
Remove
the
screw
(@)
retaining
the
transistor
to
the
ey
heat
sink.
c
NS
4.
Remove
the
2
screws
(Q)
retaining
the
power
supply
S4
cables.
‘
5.
Remove
the
8
screws
(@)
retaining
the
Final
PC
board
e
©
‘Sa
to
remove
the
Final
PC
board.
g
Note
:
When
replacing
only
the
power
module
IC,
remove
?
S
the
screws
(@)
and
then
remove
the
5
soldered
|
Py
.
parts of the power module !C’s terminals (©) to
remove the power module IC.
How to Remove the Panel
1.
Pull
off
the
VOLUME
and
SQUELCH
knobs
‘@
).
2.
Release
the
6
hooks
‘@)
located
on
the
upper
and
lower
section,
and
holding
the
sub-panel
by
pushing
them
up
with
your
finger.
Then
remove
the
sub-panel
3)
) by
pulling
it
out
toward
the
front.
13

TKR-820
14
DISASSEMBLY FOR REPAIR
Disassembling the Sub-panel
1. Remove the 2 nuts and 2 washers holding the VOLUME
and SQUELCH VRs (@). and remove the 2 screws
(@) holding the POWER switch.
2. Remove 2 of the screws c 3] ) holding the sub-panel, and
loosen the other 2 screws (@). then tilt the sub-pane!
toward the front (@).
3. Remove the 5 connectors ‘@) connected to the Dis-
play unit, and remove the connector (®) connected to
the POWER switch.
4. Remove the 2 remaining screws ‘@) holding the sub-
panel to remove the sub-panel.
5. Remove the 9 screws (©) holding Display unit (A/4}
and (B/4) to the sub-panel, and remove the Display unit
(A/4) and (B/4).
6. Remove the 4 screws (®) holding the speaker to the
sub-panel to remove the speaker.

TKR-820
DISASSEMBLY FOR REPAIR
Disassembling the AVR unit
1.
1.
2.
=
How to remove the AVR unit (X43-3040-10) (A/2)
Remove the 8 connectors (‘®) connected to the AVR
unit.
Remove the 4 screws ‘@) holding the AVR unit to the
rear panel, and remove the AVR unit from the rear
panel.
. Disassembling the AVR unit (X43-3040-10) (A/2)
. Remove the 4 screws ( @) hoiding the AVR unit.
. Desolder the 4 wires (‘@) connected to the bases and
emitters of transistor Q5 and Q6.
. Remove the 2 screws (@) on the radiation cover
attached to the heat sink to remove the radiation cover.
. Remove the 4 screws ‘@) of the transistors (Q5 and
Q6) attached to the heat sink, and remove the AVR unit
(A/2).
aad ony
2
[emma )
|_| pl
X43-3040-10 (A/2)
isla
Sm
—_,@
=H)
‘oo
15

16
KR-820
_
_
. How to remove the AVR unit (X43-3040-10) (B/2)
. Remove the 4 wires ( @) holding to the AVR unit.
. Remove the 4 screws (@) holding the AVR unit to
. Remove the 3 screws (©) holding the electrolytic
DISASSEMBLY FOR REPAIR
and the electrolytic capacitor (C101)
remove the AVR unit.
capacitor (C101).
. How to remove the power transformer
. Remove the 4 screws (() holding the power trans-
former to the bottom plate, and remove the power
transformer.
Transformer

MIC (OPTION)
TX=-RX UNIT
TKR-820 TKR-820
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TKR-820 TKR-820
CIRCUIT DESCRIPTION
Transmitter Circuit
The signal generated at the transmitter frequency syn-
thesizer is amplified by RF amplifier transistors Q1 (2SC
3357) and Q2 (2SC2954) and amplifier module (C1
(M57729) to a level of 25 watts in the power amplifier
unit. The signal is then routed to the antenna connector
after going through a harmonics filter.
The transmitter output is detected by D3, 4 (188101)
and is converted to a DC form. The DC signal thus detected
is level adjusted by APC control (VR1)} and is applied to
the base of O6 (2SC3326A). The base current of 03 (2SB
946Q) is varied according to the difference from the com-
parison voltage at Q5, 6 (2SC3326A) so that the collector
voltages of Q2 (2SC2954) and the IC1 first stage are con-
trolled to maintain the transmitter output level constant.
In the event an abnormal temperature rise occurs, the
temperature is sensed by a thermister (TH1) and reduces
the output power to a safe level.
The harmonics filter is of a fifth order butterworth type
lowpass filter having a minimum attenuation of more than
65dB at the second harmonics frequency with a passband
insertion loss of less than 0.5dB. With a characteristics of
the transmitter final power amplifier module, which has
more than 30dB of attenuation for the frequency at the
second harmonics or higher, total attenuation of harmonics
is more than 70dB.
TX-AX
UNIT
FINAL
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Fig. 1 Transmitter block diagram
19
20
CIRCUIT DESCRIPTION
Receiver Circuit
An incoming RF signal from the antenna is fed into a
bandpass filter (L7). Then eliminated RF signal pass a pro-
tecter D3 (1SS226).
The signal is then amplified by an RF amplifier (Q3,
2SC3358K) and filtered again by bandpass filters (LQ and
L10), After amplification and filtering, the signal is appli-
ed to a double balanced modulator (DMB, D4, ND487C1-
3R) for mixing with the first local signal generated at the
common frequency synthesizer.
The heterodyning action of the first mixer produces a
21.4MHz first intermediate frequency (first 1F), which is
applied to a Gpole monolithic crystal filter (MCF, XF1)
after being amplified by a buffer amplifier (04, 2SK125
and Q5, 2SK125 connected in paralle!). The signal out of
the MCF is further amplified by a first IF amplifier (Q6,
2SK302GR)} and sent to the IF unit (Z8).
The signal applied to Z8 is mixed with a 20.945MHz
signal at IC1 in 28, which produces a 455kHz second IF
signal. The signal obtained at the second mixer is filtered
by a 455kHz ceramic filter (CF 1) and amplified by limiting
amplifier stages in 1C1. The recovered audio signal from the
incoming signal is also obtained at IC1 by a quadrature type
FM detector. This recovered audio signal is then sent to the
audio amplifier circuit and to the noise actuated squelch
circuit.
In the receiver audio amplifier section, the recovered
audio signal from Z8 is first applied to a bandpass filter/
voltage controlled amplifier (BPF/VCA, Z9) unit. At this
BPF/VCA unit, the signal is amplified and sent to pin Q of
CN6 as the DET signal. The signal is returned to Z9 by way
of the signaling unit. 1C1 (NJM4558M) forms a lowpass
filter and a highpass filter, and IC2 forms a bandpass filter
and lowpass filter in Z9. The frequency components below
300Hz and above 3000Hz are attenuated in the above filter
Circuits.
The filtered audio signal is then applied to an elec-
tronic volume control (IC3, M522FP), where the audio
signa! level is controlled by a DC voltage sent from the
front panel volume control. The signal is then de-empha-
sized and sent to the audio power amplifier circuit (IC1,
pPC1242H) after going through squelch switches (Q9 and
010, 2SC3326A).
The alert signal is also applied to 1C1, when a specific
signaling board, which requires an audible alert through the
speaker, is installed. The signal, which is amplified by 1C1,
drives either the internal speaker or the optional external
speaker and this selection is done through the accessory
connector located on the final unit.
ANT.
TX-RX UNIT
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DISPLAY UNIT (A/4) Z9 : BPF/VCA AF PA
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Fig. 2 Receiver block diagram

CIRCUIT DESCRIPTION
Squelch Circuit
The high frequency component of the recovered audio
signal is fed to a noise amplifier within 1C1 (MC3361D) of
Z8 and it is further amplified by an external noise amplifier
(Q1, 2SC2712Y).. The signal is then detected by a noise
detector (D1, DA204K) and applied to the squelch switch
in 1C1. This detected noise is also routed to the squelch
control (VR3) through the display unit (C/4) for adjusting
the noise squelch sensitivity. :
The busy information is sent from the above IC1 (uPD
75104G) in the display unit in serial format to turn on or
off the busy LED. The squelch switch output and the audio
control (AC) signal from the signaling board are combined
at D8 (18$184) and applied to squelch switch transistors
Q9 (2SC3326A) and Q10 (2SC3326A)} along with the
inverted signal of transmit/receive control signal. The
squelch switch controls the input signal to the audio amp-
lifier to mute or unmute the receive audio.
While the busy LED is being controlled only by the
noise squelch circuit, the.actua! audio signal is controlled
by the following signals and in order to unmute the audio,
each signal must be in the condition as specified.
SQL signal = Low
R8 line = High
T/R signal = High
AC signal = Low
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Fig. 3 Squelch circuit
TKR-820
21
3
|

22
TKR-820
CIRCUIT DESCRIPTION
RX Frequency Synthesizer
The RX frequency synthesizer consists of three major
circuits. They are the temperature compensated crystal
oscillator (TCXO, Z3), RX voltage controlled oscillator
(RX VCO, Z2) and RX phase locked loop unit (RX PLL,
Z1).
The TCXO is operating at 12.8MHz and its frequency
stability is maintained within +/— 2.5ppm from —30 to +60
degrees Celsius. This output signal is fed to the frequency
synthesizer IC (1C2, JLC1O75DW) in 21. At 1C2, this signal
is divided by 1024 to become a 12.5kHz reference signal.
The output from the RX VCO operates at the frequency
of the receiver first local signal and a portion of the signal is
fed to a dual modulus counter formed by C1 (MB504F)
and |C2 in Z1. IC1 divides the incoming signal by 1/64 or
1/65 depending on the control tine status sent from 1C2.
The output of the dual modulus counter is also a 12.5kHz
and this signal is compared against the 12.5kHz reference
signal in a phase comparator at |C2. The output signal from
the phase comparator is then fed back to the RX VCO after
going through a charge pump and a lowpass filter to main-
tain the RX VCO frequency.
If this RX frequency synthesizer phase locked loop
becomes UNLOCK, the unlock condition is detected by
{C2 and it prevents the transmitter frequency synthesizer
from sending a transmitter signal to following amplifier
stages in order to prevent an unauthorized transmission.
TX Frequency Synthesizer
The TX frequency synthesizer consists of three major
circuits. They are the modulator/voltage controlled crystal
oscillator (VCXO, Z6), TX voltage controlled oscillator
(TX VCO, Z5) and TX phase locked joop unit (TX PLL,
Z4).
The audio signal from the microphone amplifier and
the signaling unit is applied to the TX VCO (Z5) and the
VCXO (Z6) operating at 12.8MHz to obtain an FM modu-
lated signal. And its frequency stability is maintained with-
in +/— 2.5ppm from —30 to +60 degrees Celsius. This out-
put signal is fed to the frequency synthesizer IC (IC2,
JLCIO75DW) in 24. At IC2, this signal is divided by 1024
to become a 12.5kHz reference signal.
The output from the TX VCO operates at the frequency
of the transmit signal and a portion of the signal is fed to
a dual modulus counter formed by 1C1 and IC2 in 24. 1C1
divides the incoming signal by 1/64 and 1/65 depending on
the control line status sent from C2. The output of the
dua! modulus counter is also a 12.5kHz and this signal is
compared against the 12.5kHz reference signal in a phase
comparator at {C2. The output signal from the phase com-
parator is then fed back to the TX VCO after going through
a charge pump and a lowpass filter to maintain the TX VCO
frequency.
lf this TX frequency synthesizer phase locked loop be-
comes UNLOCK, the unlock condition is detected by IC2
and it prevents the transmitter frequency synthesizer from
sending a transmitter.
Microphone Amplifier
The audio signal originating at the microphone is applied
to a microphone amplifier unit (Z7) after going through a
microphone sensitivity control (VR1).
The signal is amplified and voltage limited by 1C1 and
1C2 in Z7, then applied to an active lowpass filter/pre-
emphasis network (1C3).
The processed audio signal is sent to the modulator/
voltage controlled crystal oscillator (VCXO, Z6) and volt-
age controlled oscillator (TX VCO, Z5) in the transmitter
frequency synthesizer via [C3.