AMERITRON ALS-500MXCE Instruction Manual - Page 3

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ALS-500M

Instruction Manual

GENERAL INFORMATION

The ALS-500M features a load fault trip circuit that bypasses the amplifier if the amplifier frequency

switch is set to a lower frequency range than the exciter or if the load reflected power exceeds 70-100

watts. This circuit virtually eliminates the danger of component failures due to operating errors.

The Combiner circuit board near the PA contains the components that sense the reflected power at the

input of the low-pass filters used in the output circuit. Selecting a filter that has a cut off frequency

below the operating frequency or operating into loads with reflected power levels that exceed 70-100

watts will trip the overload and bypass the amplifier. SCR (Q5) on the Bias board keeps the amplifier

"locked out" until the POWER (ON/OFF) switch is cycled off and on.

The Bias board contains the temperature and bias control circuitry. The output transistor temperature is

monitored with diodes D1 and D2 on the PA module. The voltage drop across the diodes decreases as

the PA transistor temperature rises. The reduction in diode voltage with increasing temperature causes

IC 1c/d to reduce the PA base bias current supplied by Q2/3. Diodes D1/2 on the bias board limit the

available bias voltage if a failure in the bias circuit occurs.

IC 1a compares the voltage from the Power Amplifier temperature diodes to the reference voltage from

the temperature sensitivity control R28. If the temperature becomes too high (voltage too low) IC 1a

switches high and Q4 turns the cooling fan on. If the temperature continues to increase IC 1b will

eventually pull low and the voltage for the transmit relay and the bias voltage will be removed. The

THERMAL OVERLOAD LED is illuminated when the output of IC 1b is low. This circuit

automatically resets when the temperature drops to a safe operating range.

If the band switch is in the wrong position, the antenna SWR is high, or the amplifier is being driven

into non-linear operation a "load fault" will occur. When a load fault occurs gate voltage is applied to

SCR Q5. This causes Q5 to latch in a conducting state. Q5 will remain in this condition and the

amplifier's relay line will be disabled until the OFF/ON switch is cycled off and on. The LOAD

FAULT LED on the front panel is illuminated whenever SCR Q5 is in conduction and the amplifier is

bypassed from a load fault.

The ALS-500M normally requires much less than 100 watts of drive to produce full output power.

This drive power varies considerably with the gain of the transistors used in the amplifier and the

power supply voltage. As a general rule, exceeding 100 watts of drive for long periods of time may

cause component failures. Exceeding 70 watts of peak drive power may cause distortion and increase

the bandwidth of the transmitted signal.