PRESIDENT JOHNNY III ASC 12/24, Service Manual

Page: 22 / 66

UNIDEN CORP.
CIRCUIT DESCRIPTION
Model: HARRY III 12/24 V
1. FREQUENCY SYNTHESIZER
Both of TX and RX local frequencies are generated by means of a phase locked loop frequency synthesizer.
These frequencies are directly obtained from VCO (Voltage Controlled Oscillator). A 10.240MHz Crystal
oscillator is provided as a reference frequency that is divided by 2048 in order to produce 5kHz of basic value for
the 10kHz channel spacing. This frequency is produced as Q301. And fed to 1/N programmable counter in the
PLL IC of IC301.
Therefore, the final frequency is determined by calculated as follows:
(Example: 26.965MHz in channel No.1 TX mode)
F TX = F TX VCO
Since last digit of TX and RX frequency is 5kHz, therefore; N = 26965 / 5 = 5393
In case of RX mode;
Since a 1st IF is set to 10.695MHz, the following formula can be expressed.
FL = F RX VCO = FR + 10.695 (MHz)
Where FR is receiving frequency and FL is local frequency. When the FL is specified for incoming signal, each
dividing ratio can be obtained the same step as taken in TX mode.
IC403 is CPU that recognizes the channel selector switch and also read out the data from the read only memory
for conversion into serial data. The serial data is then fed into PLL IC(IC301).
2. FREQUENCY STABILITY
Overall frequency stability of the unit is determined depending on crystal oscillator X301. Stability rank of
X301 is +/- 25ppm maximum in the temperature range from -10 degree C to +55 degree C.
3. TRANSMITTER and TX INHIBIT CIRCUIT
The carrier frequency produced the VCO (Q301) is amplified by TX PA circuit and then fed into antenna
terminal. When the PLL output frequency incorrect (PLL unlock state), the un-lock detect function activate and
then the IC403 of the CPU which shut down the TX state.
4. FOR ATTENUATION OF SPURIOUS EMISSION
The Low Pass Filter circuit which consists of the capacitors (C201, C202, C216-C219, C222,C225, C227-
C229) and the inductors (L201-L204) is installed between the TX Final Amplifier (Q502) and the RF output
terminal (J501) to reduce the spurious harmonic emissions.
5. LIMITING MODULATION
This transmitter is equipped with a Automatic Modulation Control (AMC) circuit which automatically and
instantaneously prevents modulation in excess of 95%.
The AMC circuit is constructed by Q407, Q409, Q415.
6. LIMITING RF OUTPUT POWER
The RF Output stage (Q502, Q202, Q203, Q204) of this transmitter is designed to generate the maximum
transmitter power within 4 Watts. Further, the regulated variable DC power supply circuit Q420, Q423 are
controlled the TX final power amplifier’s supply voltage for setting the transmitter power to the rated value.