After having made a study of the IC-703 manual I see that there is a high pass filter in the TX path , cutting off at 1.6 MHz.
This is most probably for avoiding the IF frequency (455kHz) reaching the TX output.
A switch - switching in a 500kHz bandpass filter (alt 136kHz filter for LW) or bypassing the filter - would probably be possible, but the *tiny* SMD components at this place in the radio makes me think twice about making the attempt.
It *may* be possible to tap the signal out to an external amplifier, but then there is still 30mW out of the TX at 500kHz, so that could as easily be used.
Conclusion : 500kHz looks possible with the 703, but I doubt the internal filter mod is worth the risk of ruining a perfectly good TRX.
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