The Undocumented IC-756 Transmit Audio N-W Switch

by Adam M. Farson VA7OJ/AB4OJ

Fig.1:  MAIN board, showing 455 kHz filters and W-N switch.

Description:

1. IC-756's with serial numbers 01900 and higher are fitted with a transmit audio W-N switch. Radios with serial numbers below 01900 do not have this feature.

2. To gain access to the switch, remove the bottom case cover. The switch is located on the MAIN UNIT board as shown in Figure 1, and is clearly marked "W<=>N". To select a wider SSB transmitted occupied bandwidth (TOBW), change the switch setting from N to W.

3. A good-quality microphone such as the Icom SM-8 or SM-20, or the Heil HMi or iCM, should be used when W is selected. A "cheap" mic will produce very poor transmitted audio quality.

4. The MIC GAIN control should be set for an ALC meter reading in the centre of the ALC zone. Compression should never exceed 6 dB on voice peaks.

5. The operator can observe the transmitted spectral content on the Spectrum Scope, with "Scope during TX" ON.  MIC GAIN and COMP should be adjusted to limit displayed bandwidth to less than 5 kHz (1 horizontal division, SPAN = 25 kHz) at -10 dB (1 vertical division below peak).

Copyright © 2006, A. Farson VA7OJ/AB4OJ. Image courtesy Frank A. Ellis, W3UHF. Last updated: 07/20/06.