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.

triode wrote:
Having suffered for years with tiring audio on my R71, decided to fix
it. These are one of the best radios ever made, worth hanging on to.
Hello Triode:

With your permission, I would like to post your solution and schematic
from Mods.DK on the R71 yahoo group. OK?

Thanks,

Dave

The problem is 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.

Hello Dave. Yes, thats fine. Have added a bit more info for you too.

Triode:

Thanks. I haven't been able to reply on RadioBanter; don't know why. I
took your description from your submission to worldwidedx.com. My ref to
mods.DK was a mixup with another post, sorry. Why don't you join the
yahoo R71 group? We have tried and pretty well succeeded, I think, in
making it the place to go for all things R71. Plus a bunch of really
helpful people.

Thanks again,

Dave

triode wrote:
dbird;698914 Wrote:
triode wrote:-

Hello Dave. Yes, thats fine. Have added a bit more info for you too.