Kenwood TH 21 Service Manual

Page 1

1 KENWOOD

' SERVICE MANUAL

II-21A/AT/E 2121,11211111511

2m FM HAND-HELD TRANSCEIVER

0 pusw on
- -

51): YONi

CIRCUIT DESCRIPTION .................................. 2
PACKING .......................................... . 7
PARTS LIST ........................................... 8
Pc BOARD VIEWS
SWITCH UNIT (X41-1590/XX) ........................... 18
TONE UNIT (X52-12707xx) (TH-21 E ONLY) ................. 19
DTMF UNIT (TH»21AT ONLY) ........................... 19
RF UNIT(X44-1630-XX) ............................... 20
IF UNIT (x48-1410-XX) ................................ 22
) DISASSEMBLY ........................................ 23
LEVEL DIAGRAM ...................... V ................ 2 5
L'myooqflgsm ADJUSTMENT ........................................ 26
: BT-Z (AAA MANGANESE/ALKALINE BATTERY CASE) ........... 29
E32 (EXTERNAL C MANGANESE/ALKALINE BATTERY CASE) . . . _ 29
PB-21 (Ni-Cd BATTERY) ................................. 29
DC-21 (DC-DC CONVERTER) .............................. 30
SC-8/8T )SOFT CASE) ................................... 30
SMC-SO (SPEAKER MICROPHONE) .......................... 31
E a TU-6 (PROGRAMABLE TONE ENCODER) (TH21A/AT ONLY) ...... 31
g, ; WIRING ............................................. 32
.. ; REFERENCE DATA .................................... 32
f ,: SCHEMATIC DIAGRAM (TH-21A/AT) ........................ 33
an '2 SCHEMATIC DIAGRAM (TH-21E) ................. .......... 34
BLOCK DIAGRAM ..................................... 35
SPECIFICATIONS .............................. BACK COVER

Photograph shows TH-21AT type.

Page 2

TH-ZIA/AT/ E

CIRCUIT DESCRIPTION

Model Destination Frequency range (MHZ) FIPT-SHlFT Freq' (kHz) TONE Ref'
K1,M'I,M2 144.00-147.995
- $600 0 t'
TH 21A x 14400-145995 DUI:
DTMF
TH-2IAT K2,M3,M4 14400-147995
System used
T 1750HZ TRIO Brand
TH-21 E 144007145995 7600/REV TONE BURST
W 1750 HZ TONE

K : U.S.A. M : Gen. T : England W : Europe X : Australia/Newzealand

Table 1 Destination chart

RX Section

The TH-21A/AT/E uses a double super-heterodyne type
receiver with 3 IF frequency of 16.3MH2 and a second IF
frequency of 455kHz.

The received signal from the antenna is amplified by RF
amplifiers OI 25C2176IH) and 02 : 28C2668IY).
which are in connected cascade, and applied to BPF
LES-LB. The RF signal is then applied to the first mixer,
03 I ZSK192A, where it is mixed with the first local
oscillator signal from the PLL. The first mixer output
passes through a 16.3MHZ MCF (F1) and becomes the
first IF signal. This signal is amplified by IF amplifier
04 : 28C2714(Y) and is applied to IF unit 01 : MC3359P

Item Rating

Noninal center frequency (fol 16.3MHz

Pass bandwidth fo 1 7.5kHZ or more at 3dB

Attenuation bandwidth fo t 25kHz or more at ISdB

Guaranteed attenuation 30dB or more within fo :IMHz
Spurious : 15dB or more at

f0 ~ f0 +500kHZ.

Ripple 0.5dB or less
Insertion loss 1,0dB or less
Terminal impedance 1kn/1.5pF

Table 2 MCF IL71-0426-05) (RF unit F1)

Item Rating

Center frequency of GdB

bandwidth (f0) 455kHz t 1,5kH2

GdB bandwidth t 7.5kHZ or more

40dB bandwidth :2 15k Hz or less

Ripple 1.5dB or less (455 : SkHzl
Guaranteed attenuation 27dB or more within fo : IOOkHz
Insertion loss GdB or less at 455kHz

Terminal impedance 1,5kn

Table 3 Ceramic filter (L72-0335-05l (IF unit F1)

Rq=eoo

Fig. 1 TA7331F Block diagram (IF unit 02I

01 consists of the second mixer, second local oscillator.
second IF amp, FM demodulator. squelch noise amp and
control circuits.

The demodulated audio signal from 01 is amplified by
AF amplifier 02 : TA7331F, on the IF unit, via the AF
volume control (VRI on the switch unit) to drive the
speaker.

The squelch cirCUit, (an auxiliary circuit of the receiver
section) detects the high frequency noise component of the
demodulated audio output from 01 .

This signal is applied to pin 12 of 01 via the squelch
control, (VR2 on the switch unit). The noise component
applied to pin 12, is amplified and then output at pin 13.
The output at pin 13 is rectified by Di and D2 : 1N60As
and fed to pin 14. When this rectified voltage is applied to
pin 14, the squelch trigger circuit functions, pin 16 is
grounded, and O4 : 2SC2412K and 03 : 283698IE,F)
turn OFF. When 03 turns OFF, AF amp IC 02 : TA7331 F
is muted and no audio is output. When a signal is received,
the noise level contained in the demodulated output of 01
reduced, and the squelch trigger circuit does not function.
Therefore, 04 and 03 turn ON, the AF amp IC is powered,
and audio output is obtained.