Never apply positive voltage to the rear-panel ALC jack!

by Adam Farson VA7OJ/AB4OJ

Based on a discussion with Matt Erickson, KK5DR

This caution applies to all Icom HF transceivers.
Fig.1: IC-756Pro3 External ALC Input Circuit (p/o Main-A Unit)
Fig.1: IC-756Pro3 External ALC Input Circuit (p/o Main-A Unit).

Note on Fig.1: The partial schematic below shows the external ALC emitter-follower in the IC-756Pro3.

(Y) is the external ALC line (EALC) from the ALC jack (0 ~ -5V, negative-going). (W) is the internal ALC line (ALC). (X) is the -5V rail.  

If EALC goes positive, Q521 (Main-A Unit) will not conduct until its 5V reverse base-emitter voltage (Vbe0) is exceeded. Then, current will start to flow, and the base will pull the emitter (and the internal ALC line) positive. This will pull the ALC line voltage above the value preset by the RF Power control, and increase the gain of the ALC-controlled TX IF stage. As a result, Po will exceed the preset level. An ALC meter reading below zero is a warning sign.

If a positive voltage well in excess of +5V is applied to the ALC jack, damage to Q521 may result.

For these reasons, it is recommended to measure the amplifier's ALC output voltage before connecting the ALC line to the exciter's ALC input jack. Although a negative ALC voltage is the de facto industry standard, a few older amplifiers still have a positive ALC line. A fault in the amplifier's ALC circuit can also cause the line to go positive.

  • IC-756Pro3 Service Manual, p. 10-7, Main-A Unit Schematic
Text copyright  2006 A. Farson VA7OJ/AB4OJ. Image courtesy Icom Inc.
Last updated: 25/09/2019.