There are a number of cases on Chinese EC. I found the case just matched the R80 kit. It costs about USD10 with shipping on Aliexpress. Please take care of selecting the item because there are several lengths of the cases and L=110mm is popular. R80 PCB is asking L=120mm.
At first, I would like to simulate the circuit of the Chinese receiver kit. They clump the input signal by a diode to the ground. When switching to mute, there is a large noise occurred. Therefore I would like to improve. To find a better way I investigate the circuit by simulation.
Please look at the res is ult of the simulation. When turning off to mute, there are several Helz noises. (The blue trace is the MUTING signal of AGC, and the green trace is 386 SP output.)
BTW there is scarcely noise by using the recommended method of Mr.Chiba. Please refer to the simulation result. I think that the circuit shall be saturated when the input terminal voltage of 2 or 3 is more than 0.5V. This is affected by the input current, so we shall adjust the value of resister by experiment.
As a result of these simulations and practical circuits working, the way of 386 muting by Mr.Chiba is very cool.
スケルチのLM386のMUTE切り替え部分に着目し、LTspiceに回路入力します。LM358のAGC増幅回路、AF増幅回路、LM386のAMP部にしぼって 入力します。 To simulate the squelch circuit partially, I gathered LM386 and LM358 spice models for the audio and power AMP portion.
There are some issues to edit the circuit diagrams. 1. I determined the power supplies for LM358 of 5V, and for LM386 of 12V. 2. I'm using AC signal of 1kHz50ｍV as receiving audio sound. 3. Squelch volume setting will be 0.5V by the resister's deviation. 4. I determined AGC voltage by the pulse signal source.
そうすると、キットオリジナル回路ではmute時にすごいノイズ（数ヘルツで数秒間）が出ています。青トレースがＡＧＣ電圧、緑がスピーカー出力。 As the result of the simulation, there is a very large and low-frequency noise after the squelch turns on. The blue trace is AGC voltage, and the green trace is SP output.
Same time I got a comment on the old blog article from Z_MIKLOA san about JRC datasheets. He told me that there are several methods for squelching on LM386. Depending on this information I shall study more for LM386 squelch technic.