FX 146
•
124
?
M18. Install Q11, NPN transistor type 2N3904. Be sure to orient flat side
as shown.
?
M19. Identify and install R46, the yellow color modulation control
trimmer potentiometer. It differs from trimmer capacitors by having three
terminals. Simply place it in position and solder all three points.
NOTE: The remaining three transistors used in this section are PNP types,
2N3906 or similar, marked 228256. It is essential to use the correct
transistor type for Q12, Q13, Q14.
?
M20. Install Q12, PNP transistor type 228256. Orient flat side as shown.
?
M21. Similarly, install Q13, PNP type 228256.
?
M22. Install Q14, PNP type 228256.
Install the following additional parts:
?
M23: R60, near Q12, 10K (brown-black-orange).
?
M24: R62, also 10K (brown-black-orange).
?
M25: R111, near Q13, 4.7K (yellow-violet-red).
?
M26: R40, 100K (brown-black-yellow).
?
M27: R63, 470 ohms (yellow-violet-brown).
?
M28: R64, also 470 ohms (yellow-violet-brown).
?
M29: R112, 4.7K (yellow-violet-red).
?
M30: R39, near Q11, 47K (yellow-violet-orange).
?
M31: Install diode D11, 1N914/1N4148.Orient the banded end as
shown.
?
M32: Similarly, install diode D12, another 1N914/1N4148.
IMPORTANT: The following parts are located in areas of the PC board that
already have been assembled. If the ham in you pursued an assembly
sequence different from these step-by-step directions, you may already have
soldered in these parts. If so, no harm is done. Be certain now, though, that
the following are installed:
?
M33: R21, 470 ohms (yellow-violet-brown).
?
M34a: PIN diode D2, type BA482, near R21. BA482 PIN diodes have
an orange body, with a red cathode band. (D2 grounds the receiver
input during transmit.)
?
M34b: PIN diode D7, also type BA482, which passes RF to the antenna
FX-146
•
23
thereby increasing available modulation voltage.
The VCO requires a very pure source of well-filtered DC, free of AC hum,
alternator whine or other disturbance. R19 and the 47 uf C40 form a basic
low pass filter. Transistor Q4 serves as an electronic capacitance multiplier.
The actual effect of the filter is that the beta of Q4 multiplies the 47 uf for a
virtual capacitance effect of a much larger device.
Stage G: The FX- Transceiver Synthesizer PLL
The MC145152 IC incorporates the equivalent of 8000 individual transistors
and contains the following circuits:
?
A crystal reference oscillator governed by Y2, 10.24 MHz.
?
A counter or "frequency divider" circuit set externally to divide the
crystal oscillator output by 2048, for a Reference Frequency output
of 5 KHz.
?
A second counter or frequency divider that divides the frequency
from the Prescaler (U3) by the externally programmed number that
we call "N".
?
A third frequency divider ("A") also used for programming
?
Control logic circuitry which permit the "N" and "A" counters to
work together for channel programming.
?
The Phase Detector (or "phase corrector") which compares the 5
KHz Reference Frequency with the "intended" 5 KHz output of the
N-divider and sends correcting pulses to the VCO to keep the
output of the N-divider right at 5 KHz.
?
A "lock detect signal" circuit. The reference oscillator is internal to
U6, governed by Y2.
The precision of the 10.240 MHz reference oscillator can be adjusted by
trimmer C81. The R divider feeds 5 KHz to the phase detector section of U6
(10240 KHz divided by 2048).
The output of the TD6128 Ã64/65 prescaler U3 is AC coupled via C57 to pin
1. U3 is a dual modulus prescaler, controlled by pin 9 of U6. The prescaled
output of the VCO is fed to the A and N counters. The "N" number
programmed on the diode matrix is predetermined to divide this frequency
down to 5 KHz for phase comparison with the 5 KHz output of the crystal
controlled reference divider. Maximum "N" is 65,535, achieved by switching
on all 16 parallel inputs.
Unlike simpler PLL IC's, U6's phase detector has TWO outputs at pins 7
and 8. These outputs go through very simple low pass filters (R44-C68, R53-
C91) to cut back the 5 KHz whine sound of U6 at work. Op amp U5:A sums
together the phase detector outputs and the output of U5:A is passed
through a network of 2.2 uf electrolytic capacitors (C67,70,90,92) to smooth
out the phase detector pulses to clean DC for controlling the VCO.
