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through the network of R721, C711 and R729. The outputs of H706 and H707 are led to the
H708 and H709, respectively. H001, H002, H003 and H004 are power transistors used in
complementary symmetry configuration and mounted on the heat sink.
To maintain overall amplifier stability and linearity, a degenerative feedback is utilized
throughout the amplifier. This feedback is also necessary to reduce distortion to within a
specified limit. The RC network of R728 and C710 conditions the feedback signal for the audio
signals. R729 and C711 also form a feed back loop provided to obtain a stable zero DC offset
voltage at the speaker output terminals. The R713 is a trimming resistor to adjust the DC offset
voltage.
A dynamic bias is applied to the bases of driver transistors H708 and H709. This dynamic
bias circuit comprises H714, H715 and R740. This provides a variable base bias for driver
transistors that automatically maintains the proper base voltage to temperature change. The
temperature sensitive biasing components of the dynamic circuit are thermally coupled through
the heat sink with the power amplifier transistors.
7. POWER PROTECTION CIRCUIT
A protection circuit for the amplifier is provided by sensing resistor networks and two
switching transistors. When the output transistors are over-driven, the current increase through
the power output transistor causes increase of the current flowing through R746. This increased
voltage potential is applied to the base of H710 through the resistor R732 and turns H712 on.
This means that the base of H708 is by-passed through the emitter-collector path of H712 to the
ground, since the emitter of H712 is connected through R730 to the base of H708. Thus, the
input signal to H708 is restricted to the value which maintains the operation of power transistor
within the safety area. A resistor network of R736, R738 and H720 also works as a sensing
network. If the center voltage (collector voltage of the power transistors) is excessively increased
to a positive value by some troubles, the voltage applied to the base of H710 turns the H712 on,
making bypass circuit to protect the power transistors. For the other half cycle of the drive
signal, the same operating principle is applied as described above.
8. SPEAKER PROTECTOR RELAY CIRCUIT
The speaker protection circuit consisting of H805, H806, H807, and associated parts protects
the speaker system against any loud "pop" sound developed. This circuit is so designed that no
sound is heard for the first three or five seconds after the power switch is turned on by the time
constant circuit consisting of C813 and R816. This circuit also protects the speaker system
against some troubles due to DC unbalance between the speaker system terminals by instantly
operating the relay and cutting off the speaker system from the circuit. If the DC unbalance
voltage (positive) is developed between speaker terminals by possible defects such as broken
power transistor, short-circuits, or broken potentiometer R713, as the base of H808 is connected
to the speaker terminal, the transistor H805 is turned on by the offset voltage developed. This
turns the transistors H806 and H807 off, thus cutting off the relay and disconnecting the speaker
from the output circuit. If a negative offset voltage is developed, this voltage directly turns the
H806 and H807 off, which disconnect the speaker from the circuit and protected.
The circuit also protects the speaker system from possible damage if the amplifier is
over-driven by very low frequency such as 7 or lower cycles per second.
9. DOLBY UNIT
The Dolby unit built in the Model 2325, which is a switchable processor, is inserted in each
of both R and L channels. The attached "DOLBY PROCESSING CHART" will facilitate you to
well understand the operation of the Dolby circuit.
An input signal coming to J601 is amplified by H601, and its output signal is led to the filter
which cuts off the tape bias. The signal passed through the filter is further amplified and comes
to the mixing circuit of the resistors R623 and R625 and to the phase inverting circuit of H607
and H609. The output signal is fed out of J607.
In the recording mode of operation, the signal is fed out of J605 preceding the mixing circuit
Digitized in Heiloo Netherland
5/30/2017
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