5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS
RT2048
RT2048
capacitors C29, C30, C31 and R49. The output from the loop amplifier is further filtered in a low pass filter formed
by C36, L2, C34, and C35, to reduce the level of the reference frequency sidebands on the VCO output.
5.3.7. RX Buffer Amplifier
When receiving, the bandswitch diode D8 is turned on, and the output from the VCO buffer amplifier is further
amplified in a single tuned amplifier formed around Q16 with L8, C70, and C73 forming the tuned circuit. The output
is attenuated and impedance matched by means of resistors R93 and R94. The typical output level at connector
J3 will be 5 mW.
5.3.8. TX Buffer Amplifier
When the set changes to transmit mode, the band switch diode D7 will be turned on and the output from the VCO
buffer amplifier will be fed to the two stage amplifier circuit.
The level is amplified to about 30 mW in Q14 and the final stage with Q13 boosts the level to about 200 mW
measured at connector J4.
The first stage is working as a class A amplifier with the base on Q14 biased through R80, D7, R79, and R86. The
output stage is working as a class C amplifier with a small base bias through resistors R72 and R73.
5.3.9. X-Tal Oscillator
The oscillator is of a Colpitt type and the oscillator transistor Q1 is oscillating by means of a 14.850 MHz crystal X1.
The trimmer capacitor C77 is for fine adjustment of the oscillator frequency.
RF signal for the second mixer in the receiver is taken from the collector of transistor Q1, connector J1. The oscillator
signal is buffered and amplified in transistors Q2 and Q3 before it is fed to the reference divider.
5.3.10. Reference Divider
The fixed divide by four reference divider U2 divides the X-tal oscillator frequency down to 3.7125 MHz before it
is fed to the microcomputer via connector J2 and the phase locked loop reference divider included in U3.
5.3.11. RX Filter Control Amplifier
This variable slope DC amplifier made by means of an operational amplifier U1 converts the VCO control voltage
to an appropriate control voltage for the variocap. diode tuned filters in the receiver front-end.
The maximum output voltage is adjusted by means of potentiometer R95.
5.3.12. Preempbasis Network
Capacitor C18 in conjunction with R35 and R96 introduces the needed 6 dB/oct. response of the AF input signal.
The input sensitivity is adjusted by means of R96.
5.3.13. AF Amplifier/Compressor
This circuit, formed around two operational amplifiers in U1, comprises both an amplifying and voltage limiting
function. Amplifier No. 3 is a fixed voltage amplifier, the output of which is sensed by means of the comparator made
by amplifier No. 2. When the peak output voltage exceeds a certain level, adjusted by means of potentiometer R97,
the comparator will turn on transistor Q6, which in turn will charge capacitor C14. So the gate-source voltage on
transistor Q17, acting as a variable resistor, will decrease and also the AC resistance between pin 10 on U1 and
ground, which in turn will reduce the input level to the amplifier.
The rise-time of the compressor is mainly determined by resistors R38, R31, and capacitor C14. The decay-time
by C14 and R30.
5.3.14. AF Post Filter
This second order active filter, built around operational amplifier No. 4 in U1, secures an appropriate high frequency
roll-off of the AF signal. Resistors R64 and R65 give an appropriate attenuation of the AF signal before it is fed to
the modulating diode D6 in the VCO.
Important Note:
Due to a change of the RF Power Module (U04) from the Mitsubishi S-AV6 to the Mitsubishi RA35H1516M, a
hardware redesign of the original RX/TX module 4 (625448) and Synthesizer module 3 (625451) has been
necessary.
The revised Synthesizer Module 3 is identified as 60-127230 and comprises the change of transistor Q13 (2N4427)
to obtain a reduction of TX Buffer output from app. 200mW to app. 60mW. This is accomplished through a shift from
using the Motorola manufacture of this transistor 2N4427 to the Central Semiconductor manufacture. The change
of part number identification of this module is to distinguish between the two different manufacturers of this
transistor which may be found in this board.
Pls. refer to schematic diagram 93-127230 rev. A (or later).
The Synthesizer module 60-127230 can be used only with the RX/TX module 93-127059.
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Summary of Contents for RT2048 VHF
Page 1: ...INSTALLATION MANUAL SAILOR RT2048 VHF ...
Page 4: ......
Page 11: ...1 INTRODUCTION RT2048 Page 1 7 9403 BLOCK DIAGRAM ...
Page 12: ......
Page 22: ...2 INSTALLATION RT2048 Page 2 10 9346 626945 625473 ...
Page 24: ...2 INSTALLATION RT2048 2 8 REAR VIEW OF VHF RT2048 25567 9350 Page 2 11 ...
Page 26: ......
Page 31: ...3 Service RT2048 3 10 PIN CONFIGURATIONS Page 3 5 ...
Page 32: ...3 SERVICE RT2048 Page 3 6 ...
Page 33: ...3 Service RT2048 Page 3 7 ...
Page 34: ...3 SERVICE RT2048 Page 3 8 ...
Page 38: ...5 CIRCUIT DESCRIPTION AND SCHEMATIC DIAGRAMS RT2048 Page 5 2 ...
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Page 90: ...Thrane Thrane A S info thrane com www thrane com M2048GB Issue E 0820 ...