I bought D-47fET Microphone PCB, where is the tests points for signal injection for Bias setting ? I know there is present on the 87 PCB but I didn't see on the 47 FET PCB.
To inject a test signal to the FET, you can simply add a 100 pF capacitor to the gate of the FET and feed the test signal through this capacitor.
In principle there is no need to decouple the microphone capsule.