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The ALC circuit can be used to reduce power by any amount on constant amplitude
carrier modes such as RTTY or FM. The ALC cannot cause modulation distortion or
pumping on these modes.
Never use the ALC to control the level of AM transmissions. Using the amplifier's ALC
circuit to control AM output power will cause pumping of carrier amplitude on AM
transmissions during modulation.
The ALC indicator on the front panel of the ALS-600 is driven by comparitor IC3B.
Resistors R24 and R25 set the turn-on level of the front panel "ALC" indicator. Factory
selected resistor values cause the "ALC" LED to illuminate as soon as negative ALC
voltage begins to appear at the ALC output jack.
If the exciter used with this amplifier does not "fold back" until appreciable ALC voltage
is developed, the "ALC" LED may become fully lit even under operating conditions that
are producing very light ALC action.
Caution:
There are no industry standards for transceiver or exciter ALC input voltage
levels, input resistance, or attack and decay times. While every attempt has
been made to make the ALC circuit in this amplifier compatible with various
exciters, the exact operation of the ALC circuit will vary with the exciters
response to external ALC control voltages. This ALC circuit will function
with negative voltage ALC control systems requiring less than 10 volts.
Thermal Overload
The ALS-600 has a built in thermal cut-out that by-passes the amplifier if excessive heat
sink temperature is produced. This circuit minimizes the risk of excessive temperature
damage to the FET's and other components.
If the amplifier "XMT" LED remains lit while the amplifier suddenly stops transmitting
the thermal overload has probably tripped. If the thermal overload has tripped full exciter
power will appear on the RF output meter.
The thermal overload resets automatically and all amplifier functions are restored when
heat sink temperature returns to a safe value. The usual cause of thermal overload is
excessive heat buildup during a transmission period longer than a few minutes. To
correct this condition be sure that the power level and duty cycle limits are being
followed, that the SWR is as low as possible, and that the cooling is not being hampered
by air flow restriction or excessive inlet air temperatures.
Load Fault Indicator
The "LOAD FAULT" LED indicator will light and the amplifier will not go into a
transmit mode if the band switch is set on the wrong frequency range, if the load SWR is
excessive, or if the output power level is excessive.
