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Automatic Amplifier Selection for the ICOM IC-746, 736 and 706MKII Transceivers.

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I new multiband radios have simplified many a radio amateur's I station. Unfortunately, the simplification doesn't extend to interfacing multiple non-ICOM linear amplifiers to that new radio. The ICOM IC-746 and IC-736 have only one buffered key line, and the delay its switching relay introduces creates problems for those who operate full-break-in CW.1 The IC-706 series transceivers don't have any buffered key lines for interfacing amplifiers. If you use separate amplifiers for HF, 6 and 2 meters, band changing involves manipulating the operate/standby switches of the individual amplifiers each time you change bands—not something I want to have to remember after 18 hours of VHP contesting!

Automated Switching

The simple interface described here resolves the aforementioned difficulties by providing automatic key and ALC-line selection for HF, 6 and 2-meter amplifiers. It also bypasses the slow internal reed relay in the IC-746 and IC-736 transceivers by providing electronically buffered key lines for all three ICOM transceiver models. All of this is accomplished without the need for Notes appear on page 36. another power supply or any internal transceiver modification thanks to the ACC(2) jack on the rear panel of each transceiver. Provisions are also made to control an external coaxial relay giving the IC-706MKII-series transceivers three programmable antenna ports for HF, 6 and 2 meters, eliminating the need for a manually operated coax switch when using three separate amplifiers or antennas.

Circuit Details

Figure 1A is the schematic of the interface and the two key-line buffers. Some of you might recognize the circuit as an adaptation of Nigel (KG7SG) Thompson's automatic antenna switch.2'3

The IC-746 ACC(2) pin-out (Figure 2) shows that in addition to the two unbuffered key lines (vsend and hsend), band logic is also provided and is used to bandswitch ICOM's solid-state linear amplifiers. The band logic is in the form of specific de-voltage levels between 0 and 8 V, corresponding to the various bands as shown in Table 1. The ACC(2) jack also provides an 8-V dc regulated reference source and a 13.8-V, 1-A supply (transceiver supply voltage for the IC-706MKII series). With the exception of vsend on the IC-736, all of these outputs are available on the IC-736 and IC-706MKII series radio's ACC(2) jacks. The original IC-706 does not have a separate vsend line and requires manual key-line switching of any 6- and 2-meter amplifiers. This interface also works with the IC-756.

Band Selection

Because the 2-meter-band position has its own key line, we can pass it to its own buffer and then out to a 2-meter amplifier. This means we need decode only the voltage differences between the HF bands and 6 meters. If we divide the 8-V reference voltage into voltage levels corresponding to the bands of interest, simple voltage-Table 1

ICOM Transceiver Band Voltages

Frequency Output Voltage
(MHz)  (V)
 1.8   7.0-8.0
 3.5   6.0-6.8
 7     5.0-5.8
 14    4.0-4.8
18,21  3.0-3.8
24,28  2.0-2.8
50,144 1.0-1.9

ALC-Line Selection

ALC-line selection between HF and 6 and 2 meters is included to make use of keying sequencers that apply a negative

Figure 1A—Schematic of the interface. Unless otherwise specified, resistors are 1A-W, 5%-tolerance carbon-composition or metal-film units. Equivalent parts may be substituted; n.c. indicates no connection. At the right, modification for 30-meter (10-MHz) amplifier control.


Paul Hewitt, WD7S, has enjoyed being an electronics experimenter since age 8. He wasn't licensed until 12 years ago when time finally allowed it. Paul currently owns and operates a construction company that specializes in building custom, beach-front homes.

You can contact Paul at PO Box 1735, Bandon, OR 97411; WD7S@arrl.net

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Прислал  И.А. Доброхотов (UN7GM),
Республика Казахстан, 480000, г.Алматы,
Главпочтамт, а/я 178. un7gm@qsl.net

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