Advertisement
CIRCUIT ANALYSIS
LBI-3999
A l l ICOM modules are i n d i v i d u a l l y
compensated a t t h e f a c t o r y , and
cannot be r e p a i r e d i n t h e f i e l d .
Any a t t e m p t t o remove t h e ICOM
MULTIPLIER BOARD (A413/A414)
Following t h e o s c i l l a t o r board a r e two
m u l t i p l i e r s t a g e s .
A413-Q1 o p e r a t e s as a
t r i p l e r , and Q2 o p e r a t e s as a d o u b l e r .
Q2
i s metered a t 5442-1 a c r o s s m e t e r i n g r e s i s -
t o r R6.
The o u t p u t o f Q2 i s coupled t h r o u g h
t w o h e l i c a l r e s o n a t o r c i r c u i t s t o t h e s o u r c e
t e r m i n a l o f t h e 1 s t m i x e r .
The h e l i c a l r e s -
o n a t o r s a r e tuned t o s i x t i m e s t h e 1 s t mul-
t i p l i e r o u t p u t f o r a t o t a l m u l t i p l i c a t i o n o f
24 t i m e s t h e c r y s t a l f r e q u e n c y
.
1ST MIXER (A4121
The 1 s t Mixer u s e s a F i e l d - E f f e c t Tran-
s i s t o r (FET) as t h e a c t i v e d e v i c e ( F i g . 2 ) .
The FET m i x e r has s e v e r a l a d v a n t a g e s o v e r a
c o n v e n t i o n a l t r a n s i s t o r m i x e r , i n c l u d i n g a
h i g h i n p u t impedance and an o u t p u t t h a t i s
r e l a t i v e l y f r e e o f harmonics ( l o w i n i n t e r -
m o d u l a t i o n p r o d u c t s )
.
RF f r o m t h e h e l i c a l r e s o n a t o r s i s ap-
p l i e d t o t h e g a t e o f Q1, and i n j e c t i o n v o l t -
age from t h e m u l t i p l i e r i s a p p l i e d t o t h e
s o u l - c e .
The m i x e r o u t p u t i s t a k e n from t h e
d r a i n w i t h t h e o u t p u t t u n e d t o t h e 1 2 . 4 M H z
h i g h I F f r e q u e n c y .
H I I F AMPLIFIER (A4211 A N D CRYSTAL FILTER
(A4227
A s e r i e s - r e s o n a n t c i r c u i t (A412-L2 and
-C3) c o u p l e s t h e m i x e r o u t p u t t o t h e e m i t t e r
o f t h e h i g h I F a m p l i f i e r A421.
The t r a n s i s -
t o r i s c o n n e c t e d a s a grounded-base
a m p l i -
f i e r which p r o v i d e s a low impedance f o r t h e
m i x e r i n p u t .
The a m p l i f i e r o u t p u t i s coupled
t h r o u g h t r a n s f o r m e r T1 t o t h e c r y s t a l f i l t e r .
H i g h l y - s e l e c t i v e c r y s t a l f i l t e r A422
p r o v i d e s t h e major s e l e c t i v i t y f o r t h e r e -
c e i v e r .
The o u t p u t o f t h e f i l t e r i s coupled
t h r o u g h impedance-matching
t r a n s f o r m e r
A424-T2
t o t h e b a s e o f t h e 2nd m i x e r .
2ND OSCILLATOR, 2ND MIXER AND 1ST L O I F
AMPLIFIER (A4241
A424-Q4 o p e r a t e s i n a C o l p i t t s o s c i l l a -
t o r c i r c u i t , w i t h f e e d b a c k s u p p l i e d t h r o u g h
C18.
The o s c i l l a t o r l o w - s i d e i n j e c t i o n
v o l t a g e ( 1 1 , 9 4 5 kHz) i s a p p l i e d t o t h e base
o f t h e 2nd m i x e r .
The High IF s i g n a l from t h e f i l t e r and
t h e i n j e c t i o n v o l t a g e from t h e 2nd o s c i l l a -
t o r i s a p p l i e d t o t h e b a s e o f 2nd m i x e r Q 2 .
The 445-kHz m i x e r o u t p u t i s a p p l i e d t o t h r e e
t u n e d low I F c i r c u i t s , L 1 , L2 and L3. T h e s e
t u n e d c i r c u i t s a r e r e q u i r e d f o r shaping t h e
nose o f t h e I F w a v e f o r m , and f o r r e j e c t i n g
any u n d e s i r e d o u t p u t f r e q u e n c i e s from t h e
2nd m i x e r .
The low I F s i g n a l i s a p p l i e d t o t h e
b a s e o f 1 s t low I F a m p l i f i e r Q 3 .
The o u t -
put o f Q3 i s R-C coupled t o t h e b a s e o f t h e
2nd low I F a m p l i f i e r .
2ND L O I F AMPLIFIER AND LIMITERS (A4261
A d d i t i o n a l a m p l i f i c a t i o n o f t h e low I F
s i g n a l going t o t h e l i m i t e r s t a g e s i s pro-
v i d e d by 2nd low I F a m p l i f i e r A426-Q1. T h i s
s t a g e i s metered a t 5442-2 t h r o u g h a m e t e r -
i n g network c o n s i s t i n g o f C19, CR3 and R25.
Following t h e 2nd low I F a m p l i f i e r a r e
t h r e e R-C
coupled l i m i t e r s t a g e s (A426-Q2,
-Q3 and - Q 4 ) .
The 1 s t l i m i t e r i s metered
a t 5442-3 t h r o u g h m e t e r i n g network C20, CR4
and R26.
DISCRIMINATOR (A4261
The l i m i t e r o u t p u t i s a p p l i e d t o
a
F o s t e r - S e e l y t y p e d i s c r i m i n a t o r , where
d i o d e s CR1 and CR2 r e c t i f y t h e 455-kHz
s i g n a l t o r e c o v e r t h e a u d i o .
The d i s c r i m i -
n a t o r i s metered a t 5442-10 t h r o u g h m e t e r i n g
network C16 and R23.
A U D I O
-
N O I S E
AMPLIFIER (A4261
The d i s c r i m i n a t o r o u t p u t i s coupled
t h r o u g h a low-pass f i l t e r ( C 1 6 , C18, R21
and R22) t o t h e base o f a u d i o - n o i s e ampli-
f i e r Q 5 .
The f i l t e r removes any 455-kHz
s i g n a l remaining i n t h e d i s c r i m i n a t o r o u t p u t .
Q5 o p e r a t e s a s
a
e m i t t e r - f o l l o w e r t o match
t h e d i s c r i m i n a t o r impedance t o t h e VOLUME
c o n t r o l , SQUELCH c o n t r o l , and Channel Guard
i n p u t .
The s t a g e a l s o p r o v i d e s power g a i n .
AUDIO AMPLIFIERS (A426)
Any a u d i o p r e s e n t
i n t h e incoming s i g -
n a l i s coupled from t h e e m i t t e r o f Q 5
t h r o u g h t h e VOLUME c o n t r o l and a de-emphasis
network t o t h e b a s e o f a u d i o a m p l i f i e r Q 6 .
The de-emphasis network c o n s i s t s o f C22, C23,
R30 and R31.
Audio d r i v e r Q 7 f o l l o w s t h e a u d i o a m p -
l i f i e r .
The a u d i o o u t p u t o f 07 i s coupled
i h r o u g h t r a n s f o r m e r T 1 t o p r o v i d e phase i n -
v e r s i o n f o r t h e p u s h - p u l l
a u d i o
PA
s t a g e .
Advertisement
