Circuit Description - Sony TFM-8100W Service Manual

1-2. CIRCUIT DESCRIPTION
7. Resonance-Indicating Circuit
The model TFM-8100W resonance-indicating
circuit uses an LED (Light Emitting Diode).
i-f
section
Fig. 1-2
The circuit is shown in Fig. 1-2. Resistors
R201, R202, R203, and diode D201 keep a constant
bias voltage (VB) on the base of Q201. The bias is
set at the cut-off point of Q201. Here, D201
stabilizes the bias against a variation in source volt-
age or temperature.
ZERO collector current flows at no input signal,
but the transistor has a high sensitivity to an input
signal. When an i-f signal (of 455 kHz or 10.7 MHz)
is added to the base, collector current flows in pro-
portion to the signal strength. Thus, the LED (D202)
is illuminated by the collector current.
Ic Q201 |C ~— VB curve
~ illumina ting
current
—-—at ZERO signal
VB
Fig. 1-3
2, VHF WEATHER Tuner
In the model TFM-8100W, the local oscillator for
the vhf weather band uses a crystal resonator XTAL
as shown in Fig. 1-4 to obtain stable reception of
vhf weather broadcasting. The resonant frequency
of the crystal oscillator is 57.75 MHz. Transistor
Q103 uses the 57.75-MHz signal on its base to
TFM-8100
produce harmonic components at its collector.
Resonant circuits which contain inductor L104 and
capacitor C114, or L103 and C115 are connected to
the collector of Q103 and are tuned to resonance at
the third harmonic (173.25MHz). Transistor Q102
mixes the vhf weather band signal (162.55 MHz) and
local oscillator signal (173.25MHz) at its base
and applies the resulting intermediate frequency
(10.7 MHz) to the i-f stage.
to the base
of Q102
(+) B
= VHF WB
osc coil
Fig. 1-4
—— Hardware Nomenclature
~ Pan Head Screw
— Pan Head Screw
with Spring Washer
Round Head Screw
Flat Fillister Head Screw
Set Screw
Retaining Ring (E Washer)
W -—- Washer
SW — Spring Washer
LW — Lock Washer
N — Nut
— Example —
— Type of Slot
@ P 3x10 = -
re i
Type of Head