MK-312BT MIC input flaw


Capacitor C21 is connected at the wrong side of resistor R7.
The error was made while reverse enginering the original box.
This capacitor is in the original box (ET312B) connected to the input jack.
The MIC input of the MK-312BT is now considerably muted by C20.

Easiest solution is to solder the left leg of C21 to the other side of R7 on the component side.


Regarding the proposed solution.
The op amp is configured as a non-inverting op amp operating on a single power supply rail. One would typically bias the + input to 1/2 Vcc.

you might replace R7 with C21 and change R8 to 100K, and remove C20.
However V1.3R has this fixed. Not sure what R7/R8 with C20 filter network is actually doing - (low pass filter?).

however in the schematic, U4 pin 7 is not connected.


The circuit R7/R8/C20 provides power to an electret microphone, C20 acts as a smoothing capacitor. The bias voltage at pin 3 of U4A (LM358N) in an original ET-312 box measures 4.6 mV, so the pull-up resister is missing there. Output pin 1 is mixed with the line input.


I’ve been meaning to take my et-312 apart and check it out. Yes Line In goes to output pin 1 and input pin 6 of U4B. However the output of U4B (pin 7) is not connected, hence line input doesn’t go to the processor.

Not familiar with electret michrophone so will have to google that.


U4B isn’t doing anything useful.
The output of U4A (pin 1) is mixed with the right line-in channel, the combined signal goes through the multi-adjust pot to pin 3 of U2A, which is connected with analog input PA6 of the processor.