Ryuji, your thoughts coincided with my assumptions. Several years ago on QRP.RU there was an active discussion about distortions in the audio amplifier in this transceiver. The manufacturer paid attention to the comments of active users, including well-known specialists in the field of sound and signal processing. The fact of the presence of distortions was acknowledged by the manufacturer. Measures were taken to reduce THD and intermodulation. The manufacturer reported on the regulation (reduction) of the signal level coming to the input of the low-frequency amplifier and on the adjustment of the modes. All this brought a positive result. But, perhaps, did not solve the problem completely. In my opinion, it was necessary to simply abandon the outdated IC. Put something more modern and high-quality. True, it would have been necessary to change the production process and have costs. And now the problem of counterfeit, “gray” ICs has not gone away.
I tried to describe in detail how the radio failed in my story about the unsuccessful activation.https://reflector.sota.org.uk/t/unsuccessful-activation-of-r9u-so-030-video/36345?u=rx9wt
I will no longer be able to add anything or clarify the reason. But I can say that I also thought about the possibility of damage to the low-frequency amplifier from the common-mode high-frequency currents. When I was diagnosing this radio, I did not see any chokes in the output circuits of the low-frequency amplifier. In the place where they could be, there are only 0 Ohm resistors. Just jumpers.
Perhaps there are ceramic blocking capacitors. But this is not enough a priori.
73!
Vlad