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HITACHI
SR-804
DESCRIPTION OF THE NEW CIRCUIT
IC PROTECTION CIRCUIT
(1) Muting circuit
A muting circuit is used to eliminate the clicking noise
which occurs at the start of circuit operation. This is
done by holding the relay OFF for 3.5 secs after the
power switch is turned ON.
In Fig. 16, when the power switch is ON, C502 is
charged via R504. As a result, the MUTING CIRCUIT
in IC501 operates via the TIME DELAY CIRCUIT.
When C502 completes charging, the MUTING
turns
ON,
and
current
flows
to the relay through
the
RELAY DRIVE CIRCUIT, turning the speaker circuit
ON and the protection indicator OFF.
INNER
VIEW (1C50})
INNENANSICHT
(IC 50!)
VUE
INTERNE
(IC5OI)
MUTING
(2) Area of safety operation detection
Reglage silencieux
RELAY
DRIVE
Relals —Treiber
Commande
de
telais
DC VOLTAGE
DETECTION
Gleichspannungs
- Detektor
Détection de
tanbion C.C.
AS.O.
DETECTION
ASO —Detektor
Detection ASO.
TIME
DELAY
Zeitrelais
Action différée
RSI3-R5i2
This prevents damage
to output transistors Q711—
Q714. Transistor damage may occur particulary when
excess
collector
current (Ic) flows while the C—E
voltage (VcE) of the output transistors is large. Hence,
the protection circuit of this unit is designed to operate
when the total voltage (Ic + VcE) exceeds a specified
value.
For the protection of Q711, Q713, Ic is detected by
R725, and VcE is divided by R717, R727 and R719,
and both are added between B and E of Q709. When
this voltage exceeds 0.6V, Q709 tums ON to control
PROTECTION
IND.
Schutzschaltung-Anzelgeleuchte.
Temoin
de protection
56K
5.6K
WN
Fig. 16
—10—
Abb, 16
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