Oktava MK-12 original

The pictures below depict the PCB with the original components. The one with the most components is vertically flipped, make the top of both images the same.

One interesting observation is that the PCB contains radial capacitors, but the PCB itself expects axial capacitors, which is more logical, considering the available space. A lot of different versions of the internals of this microphone can be found on the internet. It seems that the first versions of this microphone did contain axial capacitors. These original versions are infamous regarding the flimsy PCB. The new PCBs looks more robust, but they didn’t bother to change the layout of the circuit for the new radial capacitors.

The picture above shows the signal flow. The capsule is polarized via XLR pin 2. The actual signal is send back via XLR pin 3. XLR pin 1 is the ground pin. The PCB contains two 1G ohm resistors. One resistor is used to polarize the capsule, the other sets the bias for the gate of the FET transistor.

The first guess was that one of the large capacitors was faulty, but it actually was the 1G ohm resistor at the gate of the FET transistor that was faulty. The picture below depicts all the components.

Comp	Value	Description
Q1	2SK30A-Y N-Channel FET	voltage controlled
Q2	2N5401 PNP BJT	current controlled, voltage source
C1	750pF Ceramic	Signal input AC coupling cap
C2	1uF Elco 50V 4x8 mm	Signal stage AC coupling cap
C3	68uF Elco 50V 6x12 mm	Polarization filter cap
C4	68uF Elco 50V 6x12 mm	Signal output AC coupling cap
C5	33uF Elco 50V 6x12 mm	Supply voltage filter cap
C6,C7,C8	10nF Ceramic	Filter cap
R1	1G Thick film	Polarization resistor
R2	1G Thick film	Bias resistor for Q1
R3	30M Thick film
R4,R5	13k	Impedance matching resistors
R6	50
R7	39k	Bias resistor
R8	82k	Bias resistor
R9	36k 5ppm?	Bias resistor
R10	7.5k	Signal coupling resistor
R11	39