Theory Of Operation.5 - Radio Shack PRO-60 Service Manual

THEORY OF OPERATION
The PRO-60 is a PLL (Phase-Locked Loop) synthesized VHF/UHF, AM/FM receiver, controlled by a CPU (Central
Processing Unit) via a keyboard.
Receiving mode and search step are initially set to correspond with the frequencies entered. When a frequency
within the FM broadcast band is keyed in, the receiving mode is set to wideband FM (WFM). When a frequency in
the active radio band, such as police, fire, and ham radio, is keyed in, the mode is set to narrowband FM (NFM),
and when a frequency in the aircraft band is keyed in, it sets to AM mode. The mode can also be changed by the
AM/FM/WFM key.
All functions, such as the receiving frequency range, frequency determination, scanning an 1 delay time, etc. are
controlled by the CPU (IC1). The CPU is able to do only the assigned functions and no me' .cation of the CPU is
feasible.
The following paragraphs explain the operation of the circuit in terms of the functional blocks:
Varactor (variable capacitance diode) tuning (automatic tuning system) is employed on all bands.
The RF input circuit consists of the bandpass filters (BPF). A signal generated by VCOI or VC02 is applied to the
1st mixer and mixed with the RF signal. The 1st mixer is employed to facilitate 30 MHz to 999.9875 MHz mixing.
The 1st IF (Q106) is 608.005 - 611.2 MHz, and the signal is mixed with VC03 frequency at the 2nd mixer (Q107)
to produce a 45 MHz signal. Corresponding with the input from the keyboard, the CPU selects VCOI or VC02
and determines the AM/NFM/WFM data of the PLL circuit that will function; then it outputs the necessary data.
A signal entered to NFM/AM IF is mixed with crystal oscillation frequency 44.545 MHz at the 3rd mixer (IC301)
and converted to a 455 kHz signal. A signal entered to WFM IF is mixed with crystal oscillation frequency 34.3
MHz at the 3rd mixer (IC302) and converted to a 10.7 MHz signal. The signal is further amplified to be detected as
an AF signal.
The AF signal of AM, NFM, and WFM is CPU-controlled and applied to the AF power amplifier (IC306) via a
switching circuit. Squelch signals are comprised of noise products from detector output, and amplified by IC301
through a switching signal, which controls the AF mute and CPU.
CXI (4.19 MHz) is a clock which controls the CPU. Figure A shows the clock waveform at IC1 pin 58.
The unit has a battery-saving control in manual mode. When the unit is in operation, IC1 pin 61 is MH" and IC309
is on. If there has been no signal input or no key input for more than 5 seconds in manual mode with squelch on,
pin 61 goes "L" (Figure B), turning off IC309 to shut off the power supply for most of the circuits (except the CPU
and audio circuit). In this battery-saving mode, the unit uses only 40% of the power required for normal operation.
Any unstable supply voltage to the CPU can produce a CPU malfunction, such as wrong data processing, wrong
data transfer, etc. To overcome this, the CPU can be initialized when necessary. To initialize the CPU, press and
hold CLEAR and 0, and then turn on the power.
Figure A
Figure B