Pioneer KEX-70 Service Manual page 11

¢ DC-DC Converter
When the battery voltage has dropped, a voltage of approxi-
mately 11V is provided from the tuner HC power supply to
power the loop filter circuit used to generate the tuning
voltage. This is to prevent the tuner's tuning frequency from
changing. The digit signals fed out from the control IC
PD7004 are used as the switching signals, and a voltage
doubler circuit is configured combining the transistor
switches and electrolytic capacitors.
Fig. 15 shows a simplified view of the DC-DC converter and
also a description. D36, D37, Q33 and O34 are switching
diodes and transistors and D36 and D37 are driven by drive
transistor 035. D1, D3 and D5 digit signals are applied to the
base of this transistor and a virtually square wave signal is pro-
duced by the D24 ~ D26 diode OR circuit. As a result, 035 re-
4 —> Cy 49 Va 5c}
peatedly turns ON and OFF and so 033 and Q34 turns ON and
OFF alternately. Ths result is now detailed using the descrip-
tion figure. First, in the switch circuit indicated by the solid
line, the situation is the same in equivalent terms as when 034
turned ON, the current from power supply E passes through
D36 and C88 is charged.
Next, when 033 turns ON (in the case of the dotted Ine), the
voltage of power supply E and the charging voltage of C88 are
connected in series, they pass through D37 and exactly
doubie the E voltage is charged in C89. In the next period, D37
turns OFF so that the C89 charging voltage does not flow
back, C88 is charged (solid line) through D36, C88 and 034,
and the whole operation is repeated.
037 036
2 -
2.5 DISCRIPTION OF OTHER CIRCUITS
e PNS and MPX Decoder
1C2 LA2101 is an FM noise canceler IC in a 16-pin DIP
package. The FM detection output enters pin (1) while a low-
pass filter is configured by the RC elements across pins (2)
and (3). This filter circuit functions to allow signals with a fre-
quency of less than 100 kHz to pass through and also to delay
the signals.
The RC elements across pins (2) and (15) configure an active
high-pass filter which takes out noise components with a fre-
quency of over 100 kHz. The IC's signal path gate circuit is
switched ON and OFF by these noise components which cuts
out the signals for a short period of time only when there is
noise present. The above-mentioned low-pass filter is used
since it is necessary to delay the signal for the time until noise
Fig. 15
detection is made. Since the 19 kHz pilot signal is also cut out
by this process, the IC contains a circuit that generates a
19 kHz signal at pins (7) and (8) to get pilot signal and stereo
separation.
The IC3 LA3370 FM stereo demodulator functions so that
when the stereo composite signal accompanied by the 19 kHz
signal enters, the VCO inside the IC is locked onto the 19 kHz
frequency, a signal with double the frequency (38 kHz) is
created, the 38 kHz carrier is injected into the carrier-suppres-
sed double side band (CSDSB) stereo signal, this is detected
as AM, the stereo (L-R) sub channel is matrixed with the
(L + R) main channel and L and R signals are taken out by:
(L+R) + ({L-R) = 2L
(L+R)—(L-R) = 2R