Lafayette HA-250 50-Watt Mobile Linear Amplifier

General Description

The Lafayette HA-250 is a completely self-contained 50 watt (100 watts PEP) mobile linear amplifier covering the 10 and 15 meter amateur bands, and the 11 meter Citizens Band (illegal on Citizens Band in U.S.A.). The unit is designed to operate with any signal input: AM, FM, SSB, DSB and CW. The 'PI' network will effectively match loads from 40 to 600 ohms. The linear employs an RF sensing relay which automatically activates the B+ and antenna circuits upon transmission. The tuning meter measures relative RF current and provides an indication of when the antenna is properly loaded and the final is tuned. The meter also reads relative RF antenna current when the linear's circuits are not energized, thus permitting the transmitter to be matched to the antenna "through the linear". The unit was designed to operate from 11.5 to 14.5 volts DC, negative ground only.

Mobile Installation

Before installing the linear in a vehicle, be sure to choose a location which is convenient. Some typical locations are as follows: under the dash, the underside of a transmitter or transceiver, in the trunk. The linear has holes in both the top and bottom covers on the left and right sides to facilitate mounting with the brackets and hardware supplied.

Figure 1 illustrates top mounting; for installations utilizing the bottom mounting (trunk, etc.), simply reverse the mounting brackets and attach them to the bottom of the linear's chassis as illustrated in Figure 2.

Figure 1: Shows the Lafayette HA-250 linear amplifier chassis with mounting brackets. The diagram illustrates how the brackets can be attached to the top or bottom of the chassis for mobile installation, allowing it to be secured under a dashboard or in a vehicle trunk.

DC Power Connections

CAUTION: This unit is designed for use only in vehicles employing a negative ground system. DO NOT USE IN POSITIVE GROUND VEHICLES.

The fused lead of the linear (positive) should be connected to a 12 volt positive battery source. Since the linear will draw up to 8 amps, it is recommended that the lead be connected directly to the positive battery terminal or the battery terminal on the voltage regulator.

The other lead (negative) should be connected to the metal fire-wall or any other point that is connected to the vehicle chassis.

IMPORTANT: The solid-state DC power supply circuit is designed to operate within an input voltage range of 11.5 to 14.5 volts DC. To avoid possible failure of the transistors, you should make sure that the vehicle supply voltage does not exceed 14.5 volts at any time. If necessary, adjust the voltage regulator to cut out at this voltage (14.5).

Input and Output Connections

The linear is equipped with two standard female coaxial connectors (SO-239). Connections to these should be made with 52 ohm coaxial cable terminated in standard male coaxial connectors (PL-259). The coaxial connectors are available from Lafayette Radio Electronics under the stock number 40-2596.

Connect the coaxial lead from the transmitter/transceiver to the RF input jack. Connect the coaxial line from the antenna to the RF output jack.

Figure 2: Displays the rear panel of the HA-250. It features two standard SO-239 coaxial connectors. One is labeled 'RF INPUT CONNECTED TO SPECTRUM (TRANSMITTER)' for connecting the transmitter, and the other is labeled 'RF OUTPUT CONNECTED TO ANTENNA' for connecting the antenna.

Band Changing

WARNING: HIGH VOLTAGES ARE PRESENT IN THIS UNIT. BE SURE TO REMOVE ALL POWER BEFORE ATTEMPTING TO MAKE ANY WIRING CHANGES.

The HA-250 is supplied with the final tank coil tapped for operation on the low end of 10 meters (this will also cover the 11 meter Citizens Band, but is illegal for this use in the U.S.A.). Refer to Figure 3 for the proper setting of the tap for 15, 10, or 6 meters. If your band of operation requires a wiring change made on the final coil, be sure to resolder the tap to the point indicated, using NON-CORROSIVE SOLDER.

Figure 3: Is a schematic illustrating the band changing mechanism for the tank coil. It shows the coil with a tap point for 10 meters, another tap for 15 meters, and a point where the connection can be entirely disconnected for 6-meter operation. The diagram emphasizes using non-corrosive solder for any tap adjustments.

Tuning Instructions

The transmitter and the linear can both be tuned using the linear's RF power meter as an accurate indication of maximum power. Switch the linear off and tune transmitter for maximum deflection on the linear's RF power meter. This is made possible by the switching circuits in the linear which allow the transmitter output to be fed directly to the antenna circuit in the linear (which contains the meter). Now switch the linear on and, after warm-up, adjust the Plate Tune and Ant Load controls for maximum output reading. The linear is sufficiently broadband to permit operation over each amateur band without retuning. To calculate the DC input power (quiescent conditions), refer to the following section.

Measuring DC Input Power

WARNING: HIGH VOLTAGES ARE PRESENT IN THIS UNIT. BE SURE TO REMOVE ALL POWER BEFORE ATTEMPTING TO MAKE ANY WIRING CHANGES.

Connect a 50 Ω dummy load to the RF output jack. Tune the plate control until a maximum value is indicated on the relative RF meter. Adjust the antenna loading control until a maximum is indicated on the relative RF meter. To determine internal DC plate input, refer to Figure 4. Open the plate circuit temporarily by removing the red lead from the end terminal of the strip (as shown) and place an ammeter in series to read plate current. The plate voltage may be obtained by measuring between the end terminal of the strip and ground. The product of plate voltage and plate current is the DC input power to the final.

Figure 4: Depicts the internal circuitry for measuring DC input power. A label points to a specific terminal on the strip, indicating 'PLATE VOLTAGE MEASURED HERE'. A red lead (labeled B+) is shown connected to this point. The text explains that to measure plate current, this red lead should be temporarily removed, and an ammeter placed in series.

Operation

Operation is the same for all services. After the linear has been turned on, it will operate automatically whenever the transmitter/transceiver emits an RF signal of 1 watt or more. This is accomplished by a relay circuit which "senses" the presence of an RF input signal and automatically switches the linear's B+ and antenna circuits into operation. When the transmitter/transceiver is placed in the receive position, the B+ circuit is deactivated, but the antenna is still connected to the transceiver "through" the linear. If the linear is not energized (off switch in the "off" position), the output will feed directly through the linear without amplification. Thus for local work, you may operate your equipment with a minimum of interference to others using the same frequency. In this mode, the rough RF meter will still operate and may be used for rough tuneups with only the low power transmitter/transceiver in operation.

Returning the Unit for Repair

If you wish to have the unit repaired, we recommend that you return the unit to the Lafayette store from which it was purchased. If the unit is to be shipped to our main office for service, please read the instructions which follow.

Shipping Instructions

Pack the unit very carefully to avoid damage in transit, preferably in its original carton. If the original carton is not available, use a sturdy carton with at least 3 inches of shredded paper or excelsior around the unit. In the latter case, wrap the unit in paper first to avoid particles of packing material getting into it. Include with the unit a letter explaining exactly what difficulties you have encountered (remittance to add on extra postage and indicate on the outside of the carton that First Class Mail is enclosed). Ship by prepaid express if possible and mark ELECTRONIC EQUIPMENT -- FRAGILE. Clearly address the carton as follows:

SERVICE DIVISION
LAFAYETTE RADIO ELECTRONICS CORP.
111 JERICHO TURNPIKE
SYOSSET, L.I., N.Y. 11791

Schematic Diagram Description

The schematic diagram details the internal electronic components and their connections. It shows the input and output coaxial connectors, power input terminals (12V DC), the two 12JB6 power output tubes, two CGT-1110 transistors, relays (K-1, K-2), transformers, tuning controls (Plate Tune, Ant Load), an RF power meter, switches, and various capacitors, resistors, and diodes. The circuit is designed to amplify RF signals from a transmitter, with automatic activation via an RF sensing relay.

Addenda

The RF sensing circuit of the HA-250 has been modified and now utilizes type IN914 diodes for D1 and D2 instead of type IN34A. Also, capacitor C1 has been changed from 20 µF @ 25WVDc to 10 µF @ 50WVDc. Please note this correction on the schematic diagram in your instruction manual.

Important RF Input Power Limits

IMPORTANT: Minimum RF input power required to drive the HA-250 is 1 watt. Maximum RF input power that can safely be applied to the HA-250 is indicated below.