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HITACHI SR-2004
decrease, whereby the collector current of Q704 de-
creases and the current flowing through R707 decreases.
The sum of the emitter current of Q701 and Q702 is
kept constant in this way.
In addition, VA701 and
VA702 are the diodes used for temperature compen-
sation of the idle current; this is attached to the output
transistors to sense the temperature and compensate the
idle current.
5. Idle current compensation circuit against power supply
voltage fluctuations
The circuit shown in Fig. 3 compensates the idle
current by feeding the power supply voltage through
R7,17 back to R716, the common emitter resistor of
Q704 and Q705, negatively.
Assuming that the power voltage begins to rise, the
current which flows to R716 through R717 increases
and the voltage drop in R716 increases.
This increase is fed back to the base of Q706, decreases
the emitter current of Q701 and Q702, and decreases
the base potential of Q704 and Q705. Accordingly, the
collector current of Q704 and Q705 decreases, and then
Vidle decreases.
When the power supply voltage in-
creases, the heat generation of each semiconductor
element increases because of increasing power con-
sumption, and then the voltage between base and
emitter lowers and the idling current increases even if
Vidle is constant.
This circuit, however, lowers the Vidle as well as the VeE
as described above, so the idle current is kept constant.
When the power supply voltage drops, the circuit oper-
ates in the reverse of the above to keep the idle current
constant.
6. Protection circuits
(1)
Muting circuit
This set contains the muting circuit which cuts off
the relay approx. 3 - 6 seconds after the power
switch is turned on to eliminate the click sound
generated by power switch operation.
In Fig. 4, when the power switch is turned on, C552
is charged slowly through R554 and R558.
As a result, the voltage is generated between B - E of
Q555 to turn on Q555, and Q553 is turned off.
When C552 is charged up completely, Q553 turns off
and then Q553 turns on and the speaker circuit is
connected through the relay.
(2) ASO (Area of Safety Operation) detection circuit
(output transistor protection)
This circuit functions to protect the transistors Q714
- Q717 from damage.
Especially when the voltage (VCE) between C - E of
the output transistors is too high, the transistors are
likely to be damaged when excessive collector current
(Ie) flows. In this set, the protection circuit is design-
ed to operate when Ie and VCE exceeds a certain
value.
For protection of Q714 and Q715, Ie is detected by
R729 and VCE is devided by R721 and R722; and
both are impressed between B - E of Q709.
When this voltage exceeds 0.6V, Q709 is turned on
and the thyrister SCR551 becomes conductive. This
operation turns Q701 and Q702 off and main ampli-
fier circuit does not operate. For protection of Q716
and Q717, Ie is detected by R730, and VCE is divided
by R719 and R720; both are impressed between B - E
of Q708.
When this voltage exceeds 0.6V, Q708
turns on, and the current flowing to Q716 and Q717
is limited. (Fig. 4)
(3) DC voltage detection circuit (speaker protection)
When the DC voltage may appear at the speaker
terminals during a malfunction, it may make speaker
in trouble. To prevent this, the DC components in
the output terminals are detected by the filter circuit
of R561 and C551; when it is (+) voltage, Q551 is
turned on, Q553 is turned off, and the speaker circuit
is cut off by the relay; when it is (-) voltage, Q553
turns off by turning Q552, Q555 on and the relay is
also operated. Incidentally, this protection circuit is
reset automatically when the DC voltage disappears.
When the input terminals are touched by something
or when some ultra low-frequency noise enters,
speaker inputs are cut off temporarily but they reset
automatically. (Fig. 4)
e
•
Phenomena and remedy when the protection circuit operates
Type of protection circuit
Phenomenon when the
Cause
Remedy
protection circuit operates
No sound comes out about
1
Muting circuit
3 - 6 sec. after the power
Normal
switch is turned on.
Protection circuit
1. No sound comes out.
Short circuit of
Cut the power switch,
of the power transistor
2. Anode voltage of
speaker output
check whether the speaker
2
(ASO protection circuit)
SCR551 is OV.
terminal
terminal is short circuited
(+20V in normal
or not, and turn on again
conditions.)
after approximately 30 sec.
Speaker protection
1. Sound does not come
Trouble in the
Repair the fault.
3
circuit
out.
main amplifier,
(Be sure to check that
2. Neutral point voltage
etc.
neutral point voltage
is more than ±lV.
is within ±150mV.)
-10-
•
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