ARB-704 Interface Buffer Instruction Manual
Ameritron ARB-704
IMPORTANT !
1.) This manual is for advanced users, who want to wire their own
cables. Detailed information in this manual describes input
requirements and output limitations of this device.
2.) Ameritron also offers prewired plug-and-play cables. If you use
prewired plug-and-play cables, you can ignore this manual
and use the simple instructions that are included in the plugand-play cable assembly for your radio.
3.) Many radios do not require a plug-and-play cable assembly,
and can use basic cables included with the ARB-704. These
radios are listed on the plug-and-play sheet included with this
unit.
INTRODUCTION
The ARB-704 advanced interface is compatible with all common radios and
amplifiers even though r adios and amplifiers do no t have standardiz ed voltages,
accessory plugs, or wiring. This interface is designed to work with any
amplifier/radio combination. The input is designed to be compatible with any
transmitter or transceiver, and the output is compatible with AC control lines or
DC positive or negative amplifier control lines having up to 200 volts peak open
circuit voltage and 300mA of operating current.
Ameritron offers several plug-and-play cables that interface more common
radios to the ARB-704. Plug-and-play cables for various radios as well as radios
not requiring a special cable assembly are listed on a loose page enclosed in this
manual.
WHY YOU NEED THE ARB-704
Amplifiers can damage radios if the amplifier has too much relay control
voltage, voltage spikes, or excessive current on the relay control line. Such
damage often appears as “stuck” or “sticky” transmit relays in exciters or shorted
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ARB-704 Interface Buffer Instruction Manual
transistors on transmit control lines. In most cases where the radio is damaged,
the system transmits normally but the external amplifier stays locked in a
“transmit” mode. This prevents receive signals from coming through the
amplifier with normal levels whenever the amplifier is in the “ON” or
“OPERATE” position.
One protection or buffering system requires installation of a low-current
isolation relay between the radio and amplifier. Unfortunately an isolation
relay’s field coil can still draw appreciable current, and relay coils produce a
voltage spike when unkeyed. If a back-pulse canceling diode is installed to
suppress back EMF or coil back-pulse voltage, relay release time increases.
Most amplifiers are already marginal on pull-in time. Even if current, release
time, and back-pulse aren’t excessive, closure time is always noticeably
increased because of the pull-in time delay of mechanical isolation relays. If the
external “isola tio n r el ay” ad d s e nough p ull -in d el ay, the rad io ’s outp ut signal can
appear before the amplifier’s internal relay completely transfers the antenna to
the amplifier output. This is called “hot switching”, and it can cause serious
damage to the amplifier and radio.
Hot switching can destroy an expensive bandswitch or tuning capacitor in the
amplifier, as well as cause annoying “clicks” or “pops” outside of your operating
frequency. Besi des b eing r ough on e quip ment and a nnoying to peo ple ope rat ing
close to your frequency, hot-switching can also mutilate your sending. On CW,
the entire first “dot” might disappear…making a call like AA4MM sound like
TA4MM.
The ARB-704 has many electrical advantages over conventional relay buffer
systems. The ARB-704, since it has no moving parts, switches almost
instantaneously. The ARB-704 is noiseless, has very long life, and will not
develop dirty contacts. It operates with negative, AC, or positive amplifier relay
control lines.
The ARB-704 works with any relay voltage (AC, positive, or negative) up to 200
peak volts and any relay current up to 300mA. The ARB-704 is fully compatible
with conventional amplifier relay systems. It is especially suited for buffering
vacuum relay QSK amplifiers, since the ARB-704 does not affect attack or
release times in a deleterious manner.
The ARB-704 operates with transmitters or transceivers that pull amplifier
control lines to ground, or output a wide range of positive or negative control
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ARB-704 Interface Buffer Instruction Manual
voltages. The transmitter only switches currents under 0.35 milliamperes and
very low voltages when using the ARB-704.
WARNING:
1.) Never use this unit with relay coils operated directly from power
lines (such as old amplifiers with external 120 VAC line
powered relays). AC coil relays can be switched if operated
from a small isolation transformer.
2.) This unit is NOT ground isolated. It electrically connects the
amplifier relay closure terminal to chassis when activated.
3.) Equipment damage and shock hazards will be present if you
fail to properly connect ground leads to the amplifier and/or
radio. Always be sure to connect the additional safety ground
lead to the station g round buss from the ARB-704. Always be
sure the amplifier ground return and radio ground return leads
are connected.
TECHNICAL DESCRIPTION
The ARB-704 uses four transistors and one operational amplifier. It has a red
LED indicator that illuminates when the relay control line is activated. The
ARB-704 accepts either traditional low = transmit (TX-SEND) or less common
high=transmit (TX-SEND) amplifier control outputs from the radio.
The ARB-704 RADIO input current and voltage requirements are minimal.
Only a few volts and much less than one milliampere is required for normal
operation. Virtually any radio can directly operate the ARB-704.
The ARB-704’s unique output circuit pulls either positive or negative amplifier
relay control lines to ground. Nearly any amplifier relay system is compatible
with the ARB-704. The ARB-704 can safely handle open circuit voltages up to
200 volts DC or peak AC, and carry relay currents up to 300mA.
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ARB-704 Interface Buffer Instruction Manual
CONNECTIONS
Connect the amplifier control line (often called TX-SEND line) of your radio to
the RADIO jack or to pin 3 of the ARB-704 MULTI-PORT jack.
JMP1 and JMP2, located inside the ARB-704, must be positioned to match the
type of output from the TX-SEND radio control line. The following control
voltage jumper settings are available:
RADIO (TX-SEND) = Low systems 2.5 volt threshold (most common system
used) 5 volts open circuit
This system activates whenever the RADIO (TX-SEND) line pulls below
approximately 2 volts. Only 100µA of current appears at the output terminal
when pulled low, and open circuit voltage is ~5 volts. This line is diode
blocked, so the radio can pull up to any voltage above ~5 volts without any
current being drawn from the radio. A pull-down resistance of less than 15kohms activates the system.
RADIO (TX-SEND) = Low systems 2.1 volt threshold (IC-706 series) 2.5 or
more volts open circuit.
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