Having suffered for years with tiring AM audio on my R71, decided to fix it. These are one of the best radios ever made, well worth hanging on to. The problem appears to be the switching diode D75. The Emitter follower Q26 is enabled for AM reception, the 3 volts or so being used to forward bias D75, thereby passing the audio to IC6a. The current through the diode being only a few hundred microamps is insufficient to turn it on fully, causing the forward dynamic `resistance to be modulated with the audio, causing distortion. The fix is simple. Replace R202, (100k) to 15k, and replace R128 (1K) with a jumper. This increases the current to a more satisfactory.3 mA or so. The results are great, AM audio now not fuzzy and muffled. While you are in there, replace L28 with a germanium diode, anode to ground. This will convert the AM detector to a voltage doubler, gaining a few dB better weak signal performance. The complete removal of C122 (.0047) and C127 (.1) will also further help with the muffled audio, especially with older listeners with higher frequency hearing loss. These simple mods are much easier, and far more effective than just changing the capacitors in the signal path.