The Oscillator, Bias And Erase Circuit; The Power Supply - TANDBERG 12 Service Manual

5.1.10 The oscillator, bias and erase circuit
T h e s c h e m a t i c f o r t h e o s c i l l a t o r i s s h o w n i n f i g . 5 0 .
T h e o s c i l l a t o r i s o f t h e p u s h - p u l l t y p e , u s i n g t w o t r a n -
s i s t o r s Q l l 0 a n d Q l l l .
The DC supply is fed to the center-tap of the trans-
f o r m e r T 3 . V a r i a t i o n o f t h e o u t p u t v o l t a g e f r o m t h e
o s c i l l a t o r , c a u s e d b y t h e v a r i a t i o n o f t h e o s c i l l a t o r
l o a d b e t w e e n m o n a u r a l a n d s t e r e o r e c o r d i n g , i s c o m -
p e n s a t e d b y t h e d u m m y l o a d R i 4 8 , C l l 2 a n d 1 3 .
The oscillator frequency is adiusted fo 85,5 kc/s by
t h e t r i m m i n g c a p a c i t o r C l 2 0 . T h e b i a s c u r r e n t f r o m
t h e o s c i l l a t o r t o t h e r e c o r d h e a d s , i s a d i u s t e d b y t h e
t r i m m i n g c a p a c i t o r s C l 0 a n d C l l .
T h e s c h e m a t i c s h o w n i n f i g . 5 0 i s v a l i d f o r t h e f o u r -
t r a c k v e r s i o n , w h i l e t h e f i g u r e s m e n t i o n e d i n p a r a n -
t h e s i s a r e g i v e n f o r t h e t w o - l r a c k v e r s i o n .
R e f . m o d i f i c a t i o n l i s t p a r a 5 . 9 .
T O P L A Y 8 . P R E -
A M P L I F I E R
c H ' l
R E C O R D
P L A Y B , H E A D
T Y P E 4 0 H ( 3 9 H )
C H 1
c 1 0
l 0 - 6 0 p
R 1/.9
3 3 0
( 1 0 0 )
c l 1 2 {
u6onI
F i g . 5 0 , T h e o s c i l l o i o r . e r q s e o n d b i o s c i r c u i t , s c h e m o t i c .
C 1 4 B I A S
1 2 p A D J ,
4
E R A S E H E A D
T Y P E / . 1 H ( 3 4 H )
t 2 ' 1 \
T
117
300p
1000)
c
(
c 1 2 0
50- 250p
(1 s0 -750)
F o : 8 5 , 5 k c / s
R E C O R D A N D
P L A Y B . H E A D
T Y P E 4 O H
T O P L A Y B . P R E =
L 3
3 8 5 t
b
? : r t T
Q 1 1 0
u 28t gt2
R 1 4 9
( 1 0 0 )
R 1 5 0
130
( 100)
11't
c l 4 1
5000 p
1 1 ,
R 1 5 1
'
43K
L t q t
5 0 0 0 p
C 143
C1t t,
1 0 0 0 p 1 0 0 0 p
5 . 1 . 1 | T h e p o w e r s u p p l y
T h e s c h e m a t i c f o r t h e p o w e r s u p p l y i s s h o w n i n f i g . 5 l .
T h e s t a b i l i z i n g o f t h e l i n e v o l f a g e ( + 2 7 V ) i s o b t a i n e d
b y t h e t h r e e t r a n s i s t o r s Q l , Q 5 0 I a n d Q 5 0 2 a n d t h e
z e n e r d i o d e D 5 0 1 .
Q l w o r k s a s a v a r i a b l e r e s i s t o r . W h e n t h e l i n e v o l t a g e
l o a d i n c r e a s e s , t h e i m p e d a n c e o f t r a n s i s t o r Q 1 w i l l
d e c r e a s e t o s t a b i l i z e t h e l i n e v o l t a g e . Q 5 0 2 i s a D C
c o n n e c f e d a m p l i f i e r c o n t r o l l e d b y l h e l i n e v o l t a g e
v a r i a t i o n s u s i n g t h e z e n e r d i o d e D 5 0 l a s a v o l t a g e
reference.
T h e s i g n a l i s i n v e r t e d i n Q 5 0 2 a n d f e d t o t h e b a s e o f
Q 5 0 1 . Q l i s c o n t r o l l e d b y Q 5 0 1 . W h e n t h e l i n e v o l t -
a g e i s s h o r t c i r c u i t e d t h e e m i t t e r - b i a s c u r r e n t o f Q 5 0 I
w i l l d i s a p p e a r ( R 5 0 4 i s g r o u n d e d ) . T h i s w i l l c a u s e t h e
impedance of Ql to increase thus protecting the rec-
t i f i e r . W h e n t h e s h o r t c i r c u i t i s r e m o v e d , t h e t r a n s i s t o r
Q 5 0 l w i l l a g a i n c o n t r o l Q l d u e t o a s l i g h t " s t a r t i n g -
30
up" emitter-base current obtained bv R503 connected
directly to the rectifier. This emitter-bias current is
v e r y l o w a n d i n s i g n i f i c a n t f o r t h e c o n t r o l p r o c e s s . T h e
m a x i m u m c u r r e n t a v a i l a b l e f r o m t h e r e c t i f i e r i s l i m i t -
e d b y t h e r e s i s t o r R 5 0 1 . B y i n c r e a s i n g t h i s r e s i s t o r
m a x i m u m a v a i l a b l e c u r r e n t w i l l d e c r e a s e .
C 5 0 3 i s i n s t a l l e d t o p r e v e n t n o i s e f r o m t h e z e n e r
d i o d e . T h e c a p a c i t o r C 5 0 l i s i n t r o d u c e d t o p r e v e n t
t r a n s i e n t o s c i I l a t i o n s .
The N.T.C. (negative temperature coefficient) resistor
i s i n t r o d u c e d t o p r e v e n t t h e r m i c a l r u n - a w a y .
T h e
. l 6 5
v o l t s e c o n d a r y w i n d i n g i s c o n n e c t e d t o t h e
r e c t i f i e r 8 2 5 0 a n d f u r n i s h e s f h e h i g h v o l t a g e t o t h e
indicator tubes EAM86 and the motor. The 32 volts
secondary winding is connected to the rectifier B40
a n d f u r n i s h e s a l l t h e l i n e v o l t a g e s i o t h e f r a n s i s t o r
b o a r d s . T h e 6 , 3 v o l t s e c o n d a r y w i n d i n g f u r n i s h e s t h e
A C f i l a m e n l v o l t a g e t o t h e i n d i c a t o r t u b e s E A M 8 6 .
(