Sony ICF-7800 - SECTION 1 CIRCUIT DESCRIPTION (Continued)

(3) Oscillator
Since a large part of the circuitry has been
replaced with ICs, it is no longer possible to check
the operation of the local oscillator directly with a
VOM. The oscillation can be checked, however, by
the following procedures.
FM Local Oscillator Check
Receive the local oscillation signal with another
FM receiver. The frequency should be received at
a position 10.7 MHz higher than the dial position.
SW Local Oscillator Check
Receive the focal oscillation signal with another
SW receiver. In this case, the reception is a little
more difficult, so it is suggested that the SW receiver
be placed quite close to the test receiver, and that
both telescopic antennas be fully extended. If the
oscillator is functioning properly, the frequency will
be received 468 kHz higher than the dial position of
the test receiver.
MW Local Oscillator Check
Connect the IC terminals directly to an oscillo-
scope and compare the waveforms with those shown
in the diagrams.
(4) AGC Circuit (Automatic Gain Control)
The AGC circuit in this unit is divided into two
stages, that is, a reverse AGC in IC4, and a shunt
AGC for Q2. The AGC circuit can be checked by
measuring the voltage at terminals 5 and 6 of IC4,
and terminal 1 of ICS.
The voltage of terminal 5 of IC4 starts to give
negative values when the tuning meter reads about 2.
A treading of 8 indicates a voltage of about -—0.2V.
The voltage at terminal 6 will range from 1.2V to
1.4V (tuning meter reading from about 8 and above)
for inputs of 74dB/m and above for MW, and 44dB
and above for SW. Voltages below this range are 0 V
shown. The voltages at terminal 1 of IC5 are about
half the value of those produced at terminal 6 of
IC4 above. z
| ICF-78(
(5) Power Amplifier
This small sized power amplifier produces a large
output due to its BTL circuit (Balanced Transformer-
less). The BTL circuit consists of two SEPP (Single
Ended Push-Pull) amplifiers combined together as
shown in Fig. 1-4. Out-of-phase signals are applied
to the inputs, while a load (speaker) is connected
across the outputs. By employing a speaker of equal
impedance and the equal B+ voltage, the output
obtained by the BTL is about 4 times as great as
obtained by an SEPP circuit. An OCL circuit (Output
Capacitorless) can also be formed with a single power
supply.
Fig. 1-4