Hitachi HMA-8300 Servicehandbuch Seite 8

5. Protection circuits
(1) Muting circuit
This set contains the muting circuit which cuts off
the relay approx. 7 seconds after the power switch is
turned on to eliminate the click sound generated by
power switch operation.
In Fig. 12, when the power switch is turned on, C502
is charged slowly through R508 and R505.
As a result, the voltage is generated between B - E of
Q503 to turn on Q503, and Q504 is turned off. When
C502 is charged up completely, Q503 turns off and
then 0504 turns on and the speaker circuit is con­
nected through the relay.
(2) ASO (Area of Safety Operation) detection circuit
(output transistor protection)
This circuit functions
Q714 - 717 from damage.
Especially when the voltage (V
the output transistors is too high, the transistors are
likely to be damaged when excessive collector current
(lc) flows. In this set, the protection circuit is de­
signed to operate when Ic and V
value.
For protection of 0714 and Q715, Ic is detected by
V
R729 and is devided by R721 and R722; and
ce
both are impressed between B - E of Q709.
When this voltage exceeds 0.6V, Q709 is turned on
and the thyrister SCR701 becomes conductive. This
operation turns Q701 and Q702 off and main am­
plifier circuit does not operate. For protection of
Q716 and Q717, lc is detected by R730, and
divided by R719 and R720; both are impressed be­
tween B - E of Q708. When this voltage exceeds 0.6V,
0708 turns on, and the current flowing to Q716 and
Q717 is limited. (Fig. 12)
(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
R503 and C501; when it is (+) voltage, 0501 is
turned on, Q504 is turned off, and the speaker
circuit is cut off by the relay; when it is (-) voltage,
Q504 turns off by turning Q502, Q503 on and the
relay is also operated. Incidentally, this protection
circuit is reset automatically when the DC voltage dis­
appears.
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. 12)
to protect the transistors
) between C - E of
ce
exceeds a certain
ce
V is
ce
6. Meter circuit
This meter circuit contains the logarithmic-compression
circuit. Fig. 13 shows the meter circuit in this set. R401,
CR401 and CR402 make the input signal logarithmic-
compressed. R402 corrects the logarithmic scale of the
meter. IC401 amplitudes the signal, which is logarithmic-
compressed, and its output signal is impressed to Q401
where both currents are rectified by CR403 and C405,
CR404 and C405, respectively ; and the peak value of the
logarithmic-compressed signal is impressed to the base of
Q402. Q402 is the transistor which drives the meter and
VR401 is the variable resistor that adjusts the sensitivity
of the meter.
High response speed meter, which is an external mag­
netic type is used to in order to respond to music signals,
which rise instantaneously.
The internal impedance of the meter is 1.5kl2 ± 10% and
the effective value of the peak detection is indicated.
Fig. 10
For temperature compensation of the idle current of output stage
Temperaturkompensation des Blindstromes der Ansgangsstufe
Pour compensation de température de courant déwatté
Fig. 11
Abb. 10
de l'étage de sortie.
Abb. 11