Technics ST 7200 Service Manual

Page 2

. SERVICE AID

9 FM sott muting
Concerning the muting of this unit, when the tuning point is gradually moved away from the point of
full output and the output gradually decreases, there will soon be no output. Thus, rather than out-
put suddenly changing from zero to maximum, and from maximum output suddenly to zero, the

sound gradually fades away and, for the listener, the listening sensation is more natural and more
pleasant.

.3 s a
a a a
u, 8 Mutlng paint LL Mutmg point
<1 LL 4

4 \

i /[

/
Noxse /
/
I
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FM input -- FM Input FM input
Without muting Conventional muting Soft muting

I Circuitry principles

This circuitry makes use of the VBE characteristics of TR106 2SA733. The detected voltage of the
M IF signal is applied to the base of 28A733. When the FM input signal is amplified, the voltage
is also amplified. Because the emitter current also increases when the base voltage is amplified,
the voltage of the collector approaches the value of B voltage,

Because this collector is connected to the 6th FM lF amplifier via the muting switch. a change in

the collector voltage limits the output of the 6th IF amplifier. When the FM input signal is amplified

and the base voltage of TRtOS reaches the operation point

(about 0.6V), the output reaches maximum without restriction.

Operation
lE paint

TRWOB
25/3033
FM IF Dtol.

input

82
6th FM IF V3 V
Amplifier bias

W
VRlOi

VEE curve at TRtOE

_ 0105
+8

To FM MPX IC

-~ Output
0
Q
m
o
S
U
E
n
C
2
m

- FM Elgllal input
FM input vs AF output cuwe

Adjustment oi the soft muting circuitry

(1) Apply SOdB FM signal to antenna terminal

(2) AF signal level at output terminal should then be OdB.

(3) Reduce FM signal applied to antenna terminal to 16 dB.

(4) Adjust VRiOt so that the output is then 3dB less than the previous output (when input was 60 dB).

I ST-7200