Pioneer SPEC-2 Manuale di servizio - Pagina 9
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Power Switch OFF Muting
Since +B3 normally passes from R7 to R6 and
flows to -B.2, Q28 base is maintained at cut off
potential. When the power switch is tumed OFF,
-B2 immediately ceases, since its time constant is
smaller than
+83.
Consequently, +83
passes
through Dl and is applied to Q28 base, tuming
Q28 ON. Point B potential then falls rapidly,
tuming Q27 and Q30 OFF.
lmpedance Selector Muting
When the impedance selector switch (S1) is
changed from 8Sl to 4Sl, uncharged capacitor C2
is introduced into the - 82 circuit, temporarily
reducing -B2 voltage. This causes Q28 to switch
ON, C1 to be discharged, and Q29 & Q30 to be
switched OFF.
SPEG.El
Although Q28 switches OFF again, Q29 and Q30
continue in the OFF condition for several seconds
while C1 is charging.
Temperature Detector
The temperature detector employs positive coeffi-
cient thermistors (posistors). These are temperature
sensitive elements possessing a positive temperature
response, rapidly increasing in resistance when a
certain temperature is exceeded (see Figs. 8 & 9).
The posistors (TH3 & TH4) are mounted on the
power transistor. If some abnormality increa.ses
the temperature to the posistor operating point,
their
resistance rapidly
increases, reducing the
potential at point C. This allows DB to conduct,
so that the potential at point D drops and Q27
switches ON.
q
4
o
, 2
o
4
A
p o i n t
-Tem0erature
t ("
Fig. 8 Posistor-temperature Response
Fig. 9 Temperature Detector Circuit