Oktava MK-12 replacements

The two 68uF and the 33uF capacitors are changed as a first guess as the faulty parts. No changes: still a lot of noise. But changing the 1G ohm bias resistor did result in a working (and silent, no self noise) microphone!

Below the before and after pictures. Getting components of the right size was a bit hard: most capacitors (for at least 50V) were to bulky. Some axial capacitors did the trick. As there was more space at the back of the PCB, the new 1G ohm resistor was placed at this side.

Replacement components:

Spec	Component	Link
68uF cap 63V axiaal	Vishay MAL202138689E3	Mouser
33uF cap 63V axiaal	Vishay MAL202138339E3	Mouser
820pF cap	TDK FK24C0G2E821J	Mouser
10nF cap	TDK FK28C0G1H103J	Mouser
1G resistor	Ohmite SM102031007JE	Mouser
30M resistor	Ohmite SM102033005FE	Mouser

The other resistors are commodity. The two transistors are not made any more, but can be found at specialised webshops and might be replaced by newer versions.

Comp	Value	Spec
Q1	2SK30A-Y N-Channel FET	Should be low noise, low gate capacitance, Idss»2mA and Vth not large
Q2	2N5401 PNP BJT	Most PNP transistors will probably be OK

In the blogpost a 2N3819 is mentioned as a OK replacement of Q1.