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A third method iii) is to increase the input damping of the final amp (there’s already a 3R-pi input attenuator). Two alternatives curing this ICOM implied design flaw are i) replacing the antenna relais and ii) replacing the decoupling diode by a better one. Even the antenna tuner changes the phase relation and can cause several go/no-go situations. When one of the attenuators is on (for sure not the default for 6m!) the decoupling might be ok. Depending on part tolerances the IC-756 oscillates on 6m or does not. On 6m the decoupling of both the antenna relais and the decoupling diode D131 is no more sufficient.

Hereafter the RX path joins the TX small signal route via IC151 to the final amp.

This is caused by a loop-back from the antenna relais (TX/RX decoupling) via the RF board, passing the persistently switched input attenuator (2 relais) and the RX/TX isolator diode D131. Some IC-756PRO (-2) tend to oscillate on 6m. Icom IC-756 PRO II (IC 756 PRO II IC756PROII) technical specifications Icom IC-756 PRO II (IC 756 PRO II IC756PROII) technical specifications : Icom IC-756 PRO II (IC 756 PRO II IC756PROII) TX mods 100 Khz - 60 MHz Icom IC-756 PRO II (IC 756 PRO II IC756PROII) TX expand on IC 756PROII #23 EU models Icom IC-756 PRO II (IC 756 PRO II IC756PROII) 6M oscillation problem Mods for Icom IC-756 PRO II (IC 756 PRO II IC756PROII) : Schematics for Icom IC-756 PRO II (IC 756 PRO II IC756PROII) : PDF Adjusments Procedures Manual with schematics for Icom IC-756 PRO II (IC 756 PRO II IC756PROII) : PDF User Manual for Icom IC-756 PRO II (IC 756 PRO II IC756PROII) : Icom IC-756 PRO II Instruction Manual Programming software for Icom IC-756 PRO II (IC 756 PRO II IC756PROII) : Programming interface schematics for Icom: Icom IC-756 PRO II (IC 756 PRO II IC756PROII)