9
Circuit Description
For transmission, the VCO
Q1046
(
2SC5555ZD
) oscillates
between 118 and 137 MHz. The remainder of the PLL cir-
cuitry is shared with the receiver. However, the dividing
data from the microprocessor is such that the VCO fre-
quency is at the actual transmit frequency (rather than
offset for IFs, as in the receiving case).
Receive and transmit buses select which VCO is made
active by
Q1038
(
DTC144EE
).
Q1077
(
2SA1774
) and
Q1078
(
UMW1
) amplify the Tune
voltage for application to the tracking band-pass filters in
the receiver front end.
When the power saving feature is active, the micropro-
cessor periodically signals to the PLL IC
Q1041
(
MB15A01PFV1
) to conserve power, and to shorten lock-
up time.
Push-To-Talk Transmit Activation
The PTT switch on the microphone is control to pin 22 of
microprocessor
Q1024
(
LC87F74C8A
), so that when the
PTT switch is closed, pin 31 of
Q1024
(
LC87F74C8A
) goes
high. This signals cut off the receiver by disabling the 5 V
supply bus at
Q1034
(
DTA143EE
) which feeds the front-
end, AM/FM IF subsystem IC
Q1058
(
TK10931VT1
), and
receiver VCO circuitry. At the same time,
Q1055
(
DTA143EE
) and
Q1053
(
DTC144EE
) activates the trans-
mit 5 V supply line to enable the transmitter.
Summary of Contents for VXA-300 Pilot III
Page 4: ...4 Exploded View Miscellaneous Parts ...
Page 5: ...5 Block Diagram ...
Page 6: ...6 Block Diagram Note ...
Page 10: ...10 Circuit Description Note ...
Page 16: ...16 MAIN Unit Note ...
Page 31: ...31 ...
