Ameritron als 500m schematic

Ameritron ALS-500M

500 Watt PEP Output Solid State Linear Amplifier Instruction Manual

The Ameritron ALS-500M is a 500 watt PEP output solid state linear amplifier using
rugged, conservatively rated bipolar RF devices in the power output section. The ALS­500M operates at full power with continuous frequency coverage from 1.5 through 21.6
MHz. This amplifier requires no tuning adjustments. Broad band 5 pole filters provide
output harmonic suppression in excess of 60 dB for all frequencies above 8 MHz, and
more than 70 dB on all TV channels. Export modifications are available to extend
operation to 30 MHz with a copy of a valid amateur license.
The ALS-500M uses two pairs of 2SC2879 linear transistors operating at 13.8 volts
nominal in a push-pull output configuration. High quality RF components combine with
load fault sensing, temperature protection, and one switch frequency selection to make
this one of the simplest and easiest to operate amplifiers available today. The ALS-500M
operates on 13.8 Vdc electrical systems and draws a peak current of 80 amperes at full
output.
Warning: THIS AMPLIFIER IS DESIGNED FOR USE IN VEHICLES WITH
NEGATIVE GROUND 12 VOLT ELECTRICAL SYSTEMS ONLY. The battery should
be maintained between 13.8 and 16.5 volts dc.

ALS-500M Features

Rugged devices. The ALS-500M uses four rugged linear RF power devices.
Fast warm-up time. The ALS-500M has no vacuum tubes to warm-up.
No tune operation. Tuning adjustments are not necessary. A simple one knob

selector switch permits operation on frequencies between 1.5 and 21.6 MHz.

Load Fault Protection. The PA load impedance is monitored and the amplifier is

bypassed if the bandswitch setting or the external load is incorrect.

Current meter. The ALS-500M has a DC current meter to monitor collector current.
Off/On switch. This switch allows the amplifier to be by-passed for "bare-foot"

operation without disconnecting the high current leads.

Remote Off/On control. A separate connection on the power connector allows low

current control of the amplifier Off/On function for remote mounting.

Compact size. The ALS-500M weighs only 7.5 pounds and measures 3 3/4"H x 9"W

x 15 1/2"D .

General Information

Load Fault Circuit

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.

Bias Control Circuits

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.

Drive Power

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.

Supply Voltage

Low voltage solid state amplifiers are very sensitive to power supply voltage changes.
For maximum performance, the voltage should be maintained as high as possible
consistent with component life. The devices used in this amplifier have demonstrated
excellent life with supply voltages up to 16 volts, and will produce the rated output power
at 14 volts dc. The output power will decrease by approximately 85 watts per volt as the
supply voltage is lowered from 14 to 12 volts.

SWR Considerations

SWR (Standing Wave Ratio) causes higher voltages and/or currents to appear at the
output connector of the amplifier. This problem occurs with all amplifiers regardless of
whether tubes or semiconductors are used in the output stage.
The devices used in the ALS-500M have very good overload tolerance. The low pass
output network in the ALS-500M not extremely sensitive to load impedance changes.
This makes the ALS-500M less sensitive to frequency and SWR changes than many
other similar amplifiers. The multiple section output network and push-pull output
configuration used in the ALS-500M also offers excellent harmonic suppression.
The only danger presented by a high SWR is that the current and power dissipation in the
output devices may exceed safe limits. This can result in heat damage and failure of the
amplifying devices. If the SWR exceeds 2:1 the power level should be reduced until the

reflected power is 60 watts or less. At approximately 70 watts of reflected power the
internal safety circuitry will disable the amplifier.

Periodic Maintenance

The lack of high voltage in this unit eliminates the chance of dirt and dust causing high
voltage arcing in this unit. The only deleterious effect of dust and dirt is a reduction in
cooling efficiency. The primary maintenance requirement is to monitor the amount of dirt
and dust in the amplifier, and remove the dust and dirt before cooling problems begin.
Low pressure compressed air can be used to blow the dust out of this amplifier whenever
a noticeable accumulation of dust builds up.
A soft bristled brush and a small amount of alcohol can be used to clean stubborn dust
from the fan or the other components. Be careful not to get any cleaning compounds on
relay contacts or in switches. If it becomes necessary to perform maintenance on any
switch or relay contact Ameritron. Ameritron has free technical support for customers
that wish to service their own equipment.

Technical Assistance

Ameritron has free technical information bulletins that provide assistance with the most
common operational problems. Technical assistance is also available by calling (601)
323-8211 or FAX [phone] during our normal business hours (8:00 to 4:30 Central
Time on weekdays) for questions that are not properly addressed by the technical
bulletins. Ameritron's policy is to provide complete technical support to our customers.
Before calling, please have the following information available:

The model and serial number the unit.
The date of purchase and the dealer.
A brief description of the station equipment and antenna system.

A complete description of the problem along with all meter readings.
Written assistance is also available. Due to time delays in processing mail, please allow a
minimum of three weeks for a written reply. Send questions to:

Ameritron,

116 Willow Road,

Starkville, MS 39759

Export Modifications

A simple modification will allow operation on frequencies above 22 MHz. A parts kit to
make this modification is available by sending a written request for an 19-10600 export
modification kit along with a copy of your valid amateur license to the Ameritron. There
is a charge for this kit. Export models are shipped with this modification installed and
have an "X" following the serial number.

Installation

Location

Do not locate this amplifier in excessively warm locations, near heater vents, or in
unventilated areas with ambient temperatures exceeding 120 degrees Fahrenheit. Be sure
that air can circulate freely around and through the amplifier cabinet. Do not obstruct the
air inlet at the right front side of the amplifier or the outlet air holes on the lower left side
of the amplifier.
The best location for this amplifier is one that allows the meter to be easily read and the
controls accessed during operation. The length of each 13.8 volt high current cable is