Pioneer SPEC-2 - Power Switch off Muting; Impedance Selector Muting; Temperature Detector; Relay and Lamp Drivers Circuit

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
point
-Tem0erature
t
("
Fig. 8
Posistor-temperature
Response
Fig.
9 Temperature Detector Circuit