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should be less than 1.5:1. Less than 2.0:1 is acceptable but some reduction in power may be
seen and the amplifier will work less efficiently and generate more heat. At about 2.0:1 the am-
plifier will signal an alarm by flashing the Alarm LED (
reaches about 2.5:1 will enter protection, by removing the bias to the transistors and switching
the RTX relays to bypass, operation of the amplifier will be blocked. A message on the display
will indicate the current alarm condition. A power reset should be performed and the amplifier
should not be used again until the VSWR has been reduced.
MODE:
The LA250 may be used for all of the common narrow band transmission modes such as SSB,
CW,AM,FM, SSTV and data modes etc.
Warning:
Transmit Time.
Hi Duty cycle modes such as FM, Data modes and AM etc operate the amplifier at full power
all of the time unlike modes like SSB and CW that are either intermittent or only reach peak
output for very short times, these high duty cycle modes will run the amplifier much harder and
generate more heat in the same amount of time. It should be noted that the amplifier although
capable of being used with these modes should not be operated continuously. A transmission
time of more than a few minutes should be avoided to avoid excessive transistor junction tem-
perature. The transistors are mounted on a copper heat spreader (200x60x3mm) which greatly
improves the transfer of heat to the whole heat sink. The exact time for transmission in these
modes will depend on numerous factors such as if the amplifier fitted with the cooling fans,
how good the ventilation around the amplifier is if there is sufficient space for freely flowing
air to circulate, etc. If the ambient temperature is high this will reduce the total time in transmis-
sion. Common sense should be exercised if the heat sink is becoming too hot to touch then suf-
ficient time should be allowed to let it cool down before reuse. The amplifier constantly meas-
ures the heatsink temperature and if it reaches 56°C it will block the operation of the amplifier
automatically. The alarm LED will illuminate (
audible tone. The display will show the nature of the error as excessive temperature.
Fan Model:
The LA250V model is fitted with two large diameter fans to improve heatsink cooling. These
will allow longer transmission times or higher duty cycle operation such as contest type opera-
tion and also help to improve cooling if the amplifier is to be used in high ambient tempera-
tures.
The fans partly due to the large diameter are very low noise. They will be constantly active as
soon as the amplifier is switched on and rotate at low speed. As the heatsink temperature in-
creases the fans will increase in speed. There are 4 levels. The default low speed, at 38°C the
speed will increase to level 2, at 51°C the speed will increase to level 3 and at 55°C the speed
will be maximum. At 56°C the amplifier will shutdown from excessive temperature. The fans
continue to run until the temperature falls to an acceptable level 50°C at which point the ampli-
fier will automatically reset, and normal use may continue.
a
front panel) and when the VSWR
a
front panel) and the amplifier will emit an
Protection:
Excessive Heat sink temperature:
a
5 short beeps LED (
front panel) ON
LCD "Over Temp"
Automatic reset when temperature falls to an acceptable level
Excessive Input Power:
a
5 short beeps Alarm LED (
front panel) ON
LCD "Error Power TRX"
Manual power reset required
Excessive VSWR:
a
5 short beeps Alarm LED (
front panel) ON
LCD "Error SWR >2.5:1"
Manual power reset required
Input drive and power output:
The amplifier should give full output with approximately 10-12W input. Excessive input power
should be avoided and the amplifier should always be operated in a responsible manner.
Maximum output power considerations:
All amplifiers have a maximum output and this occurs shortly after gain compression where by
Pin no longer produces a proportionate increase in power output . The amplifier should always
be operated at a point below its saturated output. Trying to extract every last watt by overdriv-
ing the amplifier will not actually help your signal to be stronger, you will in fact cause higher
levels of distortion which will make your signal less intelligible at the distant receiver station.
Running the amplifier a little under max output will also allow the amplifier to run cooler and
make it more reliable for many years of use.
As an example consider the following situation.
1 „S‟ point on a receiver is usually approximately calibrated at 6dB so for example the differ-
ence between S5 and S7 2 „S‟ points is 12dB.
The difference between 10W and 230W is about 13.6dB a healthy increase to your signal
strength, more than 2 „S‟ points, with the same antenna. Now lets say for example you run the
amplifier at a moderate 180W output by slightly reducing the input power, the difference be-
tween 230W and 180W is only 1dB which when you compare this to 6 dB per S point is actu-
ally very little and as the amplifier is not running at its absolute maximum will give a cleaner
output with less distortion that will actually make a difference at the distant receiver for the
better!!
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