©2008 Hamtronics, Inc.; Hilton NY; USA. All rights reserved. Hamtronics is a registered trademark. Revised: 6/3/10
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swings up to 5 kHz on modulation peaks.
This will provide the optimum setting,
with sufficient audio gain to achieve full
modulation but with the limiter occasionally
clipping voice peaks to prevent over-modula-
tion. Avoid setting the audio gain higher than
necessary. Although the deviation limiter will
prevent over-modulation, background noise is
increased and some distortion from excessive
clipping may result.
Frequency Readjustment.
All crystals age a little over a long period
of time; so it is customary to tweak any
transmitter back onto the precise channel fre-
quency once a year during routine mainte-
nance.
The adjustment should be done using an
accurate service monitor or frequency
counter. Of course, make sure the test
equipment is exactly on frequency first by
checking it against WWV or another frequency
standard. No modulation should be applied to
the transmitter during the adjustment period.
The channel frequency is trimmed pre-
cisely on frequency with a small variable ca-
pacitor, which is accessible through a hole in
the top of the TCXO shield can. The proper
tool is a plastic wand with a small metal bit in
the end. (See A2 Alignment Tool on our web-
site.)
Setting Channel Frequency.
The channel frequency is determined by
frequency synthesizer circuits, which use a dip
switch in conjunction with programming in a
microcontroller to set the channel. The mi-
crocontroller reads the dip switch information
and does mathematics, applying serial data to
the synthesizer ic whenever power is applied.
Following is a discussion of how to set the dip
switch to the desired channel frequency.
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NOTE:
If the frequency is changed
more than about 500 kHz, a complete align-
ment of the Exciter should be performed, as
described in later text. Optimum operation
only occurs if the synthesizer is adjusted to
match the frequency switch setting and all the
tuned amplifier circuits are peaked for the de-
sired frequency.
To determine what channel frequency to
use, the microcontroller adds the frequency
information from the dip switch to a “base”
frequency stored in eprom used for microcon-
troller programming. Each model of the T301
Exciter has a particular base frequency. For
example, the T301-2 has a base frequency of
144.000 MHz, as shown in Table 1.
Dip switch settings are binary, which
means each switch section has a different
weighting, twice as great as the next lower
section. Sections have weights such as 5 kHz,
10 kHz, etc., all the way up to 2.56 MHz
. (See
Table 2 or the schematic diagram for switch
values.)
For very large increments, there is
even a jumper which can be added to the
board between E6 and E7 for a 5.12 MHz in-
crement, although this is rarely used, except
on commercial bands.
Begin by subtracting the base frequency,
e.g., 144.000, from the desired frequency to
determine the total value of all the switch sec-
tions required to be turned on.
For starters, if the difference is less than
5.120 MHz, you don’t need to jumper E6 to
E7.
If the difference is greater than 2.560
MHz, turn on switch #1, and subtract 2.560
from the difference frequency to determine
the remainder. Otherwise, skip switch #1.
Do the same for each of the other sec-
tions, from highest to lowest weighting, in se-
quence. Each time you consider the
remainder, turn on the switch section with the
highest weighting which will fit within the re-
mainder without exceeding it. Each time it is
found necessary to turn on a switch section,
subtract the value of that section from the
remainder to get the new remainder.
As an example
, let us consider how to set
the Exciter for output on 146.94 MHz. The fol-
lowing discussion is broken down into steps so
you can visualize the process easier.
a.
146.940 - 144.000 base freq. = 2.940
MHz remainder. Turn on switch #1, which
represents the largest increment to fit re-
mainder.
b.
2.940 - 2.560 value of switch #1 =
0.380 MHz. Turn on #4, which is 0.320 MHz,
the largest increment to fit the remainder.
c.
0.380 - 0.320 = .060 MHz remainder.
Turn on switch #7 and switch #8, which have
values of .040 and .020, respectively, which
adds up to the remainder of .060 MHz.
d.
When we finished, we had turned on
switch sections 1, 4, 7, and 8.
Note:
Dip switch information is read by
the synthesizer only when power is first ap-
plied. If switch settings are changed, turn the
power off and on again.
Shortcut ---
If you have access to the internet, our
website has a long table of numbers which
gives the equivalent binary number settings
for every possible frequency. We couldn’t
print it here because it takes many printed
pages of space. Surf to our website at
www.hamtronics.com
and look for Dip Switch
Freq Programming for T301 under Reference
Info near the bottom of the Table of Contents.
Look up the frequency, and it will give you all
the binary switch settings.
ALIGNMENT.
General Procedure.
A complete alignment is needed whenever
the frequency is changed by more than about
500 kHz. Alignment ensures that the fre-
quency synthesizer is optimized at the center
of the vco range and that all
RF
amplifier
stages are tuned to resonance.
Equipment needed for alignment is a dc
voltmeter, a good vhf 50 ohm
RF
dummy load,
an
RF
wattmeter accurate at vhf, and a reg-
ulated 13.6Vdc power supply with a 0-1000
mA meter internally or externally connected in
the supply line.
The slug tuned coils in the exciter should
be adjusted with the proper tuning tool to
avoid cracking the powdered iron slugs. The
models covering bands below 216MHz use the
A1 hex tool shown on our website. Models
above 216MHz use the A28 square tip tool.
Variable capacitors should be adjusted with a
plastic tool having a small metal or ceramic
bit, such as the A2 tool.
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NOTE:
Following are some ground
rules to help avoid trouble. Always adhere to
these guidelines.
1.
Do not operate without a 50 ohm
load. The output transistor could be damaged
from overheating.
2.
Class C amplifiers can become spuri-
ous if under driven. Therefore, do not attempt
to reduce power output by detuning the drive.
Better ways to reduce output substantially are
to reduce the B+ to as low as 10Vdc by adjust-
ing the power supply or to remove the output
transistor and replace it with a blocking ca-
pacitor if you really want low output.
3.
RF power transistors Q5 and Q6 run
quite warm at full drive, but not so hot that
you can't touch them without being burned.
a.
Connect 50 ohm dummy load to
phono jack J1 through some form of vhf
wattmeter suitable to measure up to 5W.
b.
Check output voltage of power sup-
ply, adjust it to 13.6 Vdc, and connect it to B+
terminal E1 and ground terminal E3 on the pc
board. It is permissible to use the braid of the
coax cable or the mounting hardware to the
chassis as a ground if the power supply has a
good low-resistance connection through this
path to the ground on the board.
CAUTION: Be sure to observe po-
larity to avoid damage to transistors!
A 1000 mA meter or suitable equivalent
should be connected in the B+ line to monitor
current drawn by the exciter. This is important
to indicate potential trouble before it can
overheat transistors. Better yet, if using a lab
supply for testing, set the current limiter on
Table 2. Frequency Settings
Device
Frequency Weight
Jumper E6-E7
5.120 MHz
Switch #1
2.560 MHz
Switch #2
1.280 MHz
Switch #3
640 kHz
Switch #4
320 kHz
Switch #5
160 kHz
Switch #6
80 kHz
Switch #7
40 kHz
Switch #8
20 kHz
Switch #9
10 kHz
Switch #10
5 kHz
