JRC JRL-2000F Service Manual page 8

'.
/
OFF
OFF
0-
H
I
r-
KEY & PTT signal
L
ON
ON
@
No.14 pin of IC1 (3/4)
SV
L
ov
~--~-~-
SV
@
No.8pinofiC3(1/4)
(
\
(
L
ov
2.SV
@
Bias voltage of
( (
L
each PA
ov
Fig.2
2)APC (Automatie Power Control) Circuit
The APC circuit prevents excessive output by controlling the bias voltage of the power
amplifier when the output of the linear amplifier exceeds the rated value.
The outputpower signal Vf, detected by the CFG-111 matehing circuit, is fed to the
comparator IC1 (4/4) and compared with the reference voltage adjusted by the vari-
able resistor RV3.
When the Vf signal exceeds the rated value, the comparator is turned on and the
comparator output voltage controls IC3 (3/4) and IC3 (4/4) (DC amplifier to control the
bias voltage). As a result, the bias voltage drops and the outputpower is controlled to
be constant in cas-e of excessive output.
When the comparator is on, the OVER DRIVE signal is sent to the switch panel via
diode CD1, and lights up the DRIVE LED in red.
3)Temperature Detector Circuit.
The base-emitter voltage of the temperature detector transistor TR1 and TR2 attached
-to each of the two heat sinks is compared by the comparator IC6 (1/4) and IC6 (2/4),
and IC6 (3/4) and IC6 (4/4), respectively. -.
As TR1 and TR2 have a negative temperature coefficient of about -4.5 mV/
oc
(de-
gree centigrade) the base-emitter voltage drops when the temperature of the heat
sink rises.
Variable resistors RV1 and RV2 set the reference voltage of the comparators IC6 (2/4)
and IC6 (4/4) to the base-emitter voltage which corresponds to the temperature of 80
oc
of the heat sink.
At this time, the reference voltage of the comparators IC6 (1/4) and IC6 (3/4) is the
same as that which corresponds to the voltage when the temperature of the heat sink
is 50
oc.
Therefore, when the temperature of the heat sink exceeds 50
oc,
the com-
parator IC6 (1/4) or IC6 (3/4) is turned on, and "High" Ievei voltage is applied.
7
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