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 (
a
front panel) and when the VSWR
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 heatsink. 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 (
a
front panel) and the amplifier will emit an
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.
