The Playback Pre-Amplifier; The Record Level Indicator Circuit - TANDBERG 12 Service Manual

5.1.8 The playback pre-amplifier
T h e s c h e m a t i c f o r t h e p l a y b a c k p r e - a m p l i f i e r i s s h o w n
i n f i g . 4 8 . T h e p l a y b a c k p r e - a m p l i f i e r i s a t w o s t a g e
t r a n s i s t o r a m p l i f i e r c o n s i s t i n g o f Q l 0 4 a n d Q 1 0 5 .
Both transisfors are connected in common emitter
connection.
T h e b i a s v o l t a g e o f t h e t r a n s i s t o r Q l 0 4 i s o b t a i n e d b y
t h e r e s i s t o r R l l 8 w h i c h i s c o n n e c t e d a c r o s s R i 2 2 .
C l I 3 w o r k s a s a f i l t e r f o r h i g h f r e q u e n c y s i g n a l s . T h e
s t a b i l i z i n g n e g a t i v e f e e d b a c k R l l 7 a n d C l 1 4 f r o m
f h e c o l l e c t o r o f Q l 0 5 t o t h e e m i t t e r o f Q l 0 4 i s f r e -
q u e n c y i n d e p e n d e n t . C l 1 4 i s a f i l t e r c a p a c i t o r f o r
h i g h f r e q u e n c y s i g n a l s . T h e s i g n a l f r o m Q l 0 5 i s f e d
v i a C I 1 8 , t h e c o n t a c t s G l 2 a n d C ] 1 9 , t o t h e b a s e o f
Q 1 0 6 .
T h e f r e q u e n c y r e s p o n s e o f t h e p l a y b a c k p r e - a m p l i f i e r
is f lat from 40 c/s to 20 000 c/s.
1lt
)L ' '
R f i 6
6 E K
c l ' t 8
R 1 2 1
3 , 9 K
TO PLAY B.
u202
EoUAL.AME
R 6 0 3
' t 5 0 K
1 W
=
R 5 0 2
1 0 0 K
, 0 1
c 6 0 5
8 o p
D 6 0 1
c 6 0 2
,02?|)
5.1.9 The record level indicator circuil
The schemaiic for ihe record level indicator circuit is
s h o w n i n t i g . 4 9 . T h e E A M 8 6 i s a c o m b i n e d r e c t i f i e r ,
t r i o d e a n d i n d i c a t o r t u b e . T h e i n p u t o f t h e i n d i c a t o r
circuit is connected to the center-tap of the poten-
t i o m e t e r R 6 0 . | .
The signal is rectified bye the double rectifier, con-
sisting of the diode section of EAMB6 together with
D 6 0 1 . T h e D C s i g n a l i s f i l t e r e d b y t h e d a m p i n g c i r -
cuit R604 and C602, and is then connected to the
g r i d o f t h e t r i o d e a m p l i f i e r s e c t i o n .
T h e c a p a c i t o r C 6 0 5 i s a f i l t e r c a p a c i t o r f o r H . F . s i g -
nals. The record level indicator is adiusted by R60 I
t o c l o s e a t a r e c o r d i n g l e v e l c o r r e s p o n d i n g t o 5 %
distortion from a recorded tape (400 c/s), when played
b a c k .
F R O M B A S E O F
Q 1 0 9
+ 250V
EAM 86
6 0 1
I
I
:
=
+
I
5 V
o 10s
BC 109 B
o 104
B C t 0 9 B
+
l
I
F i g . 4 9 . T h e i n d i c q t o r c i r c u i t , s c h e m o f i c .
* f ir-s
c l t 3 r * n i f e
,I
rooop= zzot
t
Ef'rnor.,, JravB. HEAD
=
J| ^^^ t
-''
s,t K
F i g , 4 8 . T h e p l o y b o c k p r e - o m p l i f i e r , s c h e m o l i c .
29