The R71 series of Icom HF receivers are one of the best ever made. Having tolerated the tiring AM audio on mine for 20 odd years, finally decided to fix it!
The problem is centered around the switching diode D75 and emitter follower Q26. The current through the diode is around 300 microamps when Q26 is enabled in AM mode, insufficient to turn D75 fully on, the dynamic resistance then being modulated with the audio signal, causing distortion in the audio. The fix is very simple. Replace R202 (100k) with 15K , replace R128 (1k) with a link. The diode current is now around .5 mills, and distortion reduced dramatically. While you have it apart, replace L28 with a germanium diode, anode to ground. This will improve AM weak signal demod by a few dB. For us older listeners with high frequency hearing loss, the removal of C127 (.1) and C122 (.0047) will help further with muffled audio. These simple mods are far more effective than replacing the capacitors with "better" types.
The problem is centered around the switching diode D75 and emitter follower Q26. The current through the diode is around 300 microamps when Q26 is enabled in AM mode, insufficient to turn D75 fully on, the dynamic resistance then being modulated with the audio signal, causing distortion in the audio. The fix is very simple. Replace R202 (100k) with 15K , replace R128 (1k) with a link. The diode current is now around .5 mills, and distortion reduced dramatically. While you have it apart, replace L28 with a germanium diode, anode to ground. This will improve AM weak signal demod by a few dB. For us older listeners with high frequency hearing loss, the removal of C127 (.1) and C122 (.0047) will help further with muffled audio. These simple mods are far more effective than replacing the capacitors with "better" types.
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