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Q,a to Q,, constitutes the power limiter stage.
This stage measures the current flowing through
the power transistors by means of the voltage
produced by the power transistor emitter resist-
ance. If the load impedance drops below 4 ohms,
or the output level exceeds 180 watts, Qrc - Qrz
are activated, thereby limiting the drive voltage.
Qre and Q15 operate for positive half-cycles, and
Qre and Qtt for negative half-cycles.
D,, Dr, D5, and D6 limit the drive voltage to
prevent drive voltages in excess of a fixed level
from being applied to the power stage, thereby
avoiding clipping in the power stage, and the
generation of oscillations.
NOTE:
If an excessiuely high driue voltage is applied to the power
stage, the Ds and D6 anode peah voltage will exceed the
power stage power supply uoltage, thereby activating D5
and D6, and limiting the driue uoltage.
Meter Amplifier (Figs 7,8)
In order to indicate the 0.01W- 400W range
with one meter without
range switching, a loga-
rithmic indication type meter must be employed
and the input signal must be logarithmically com-
pressed. The meter amplifier circuit is shown in
Fig. 7. This circuit consists of a logarithmic com-
pression circuit and a meter drive circuit. The
output signal of the power amplifier is applied to
the logarithmic compression circuit, and its dynam-
ic range is compressed. The principles of the
logarithmic compression circuit are given in Fig. 8.
The output voltage of this circuit is the value
divided by Rr
and Z. The attenuation at low
signal input is reduced and the attenuation at large
signal input is increased, by using the rise of the
diode current-voltage characteristic at Z.
The compressed signal is shaped by D5 and
applied to Q, of the meter drive circuit. Q, current
amplifies the DC voltage from D, to drive the
power meter.
c:=+
- I N P U T V 0 L T A G E
Fig. 8
Schematic diagram of logarithmic compressor
6 . 3 P O W E R S U P P L Y C I R C U I T
The L and R channels in the power stage are
both equipped with
independent coils, bridge
rectifiers, and a pair of 15000pF capacitors for
independent supply of 176V.
The class A amplifier circuits (equalizer amp,
flat amp, tone control circuit, filter amp, first
stage of the power amp, and the pre-driver stage)
are supplied with l68V and t32V via a coil, bridge
rectifier, and voltage regulator stage independent
of the porver stage.
The tuner section is supplied with +13.5V via.
lamp coils, voltage multiplier rectifier, and voltage
regulator stage.
Surge Current Suppressor Circuit (Fag. 9)
The power supply stage in this equipment em-
ploys toroidal transformers of low internal resist-
ance, and 4 high capacitance (150001F) electrolyt-
ic capacitors for smoothing purposes. Therefore
without
any effective countermeasuresi a sudden
Iarge surge of cunent would flow once the POWER
switch was turned on, resulting in possible damage
to fuses, the POWER switch itself. and other
components. In order to prevent this, a sur3e cur-
rent suppressor circuit has been incorporated.
When the POWER switch is tumed ON. the AC
cunent is applied to T (power transformer) via R,
and the micro temp (thermal fuse). The secondary
coil output in then rectified by diode D. resulting
in capacitor C1 being charged up, and relay Rr.
being activated. So until Rr is turned on, the volt-
age applied to the primary coil of T will be the
voltage divided by R, and the T primary coil im-
pedance, which means that the cunent flowing.in
when the POWER switch is first turned ON will be
very small. The time required for R; to be turned
on will depend on the D and C, time constant
Micro temp is a protection fuse designed to blow
U
J
F
F
I
OUT
z
I
+ S
-
I N P U T V O T T A G E
Fig. 7
Schematic diagram of meter amplifier
1 6
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