Reviving two arcane Dual MC312 microphones

[antipattern]

Lots of fun!

Thanks everyone for the contributions, in particular MicUlli for the original schematic.

Curiously, the MC312 mics that I picked up appear to have two 1 GΩ resistors, so maybe there's been a change in the configuration during the production run.

 

[MicUlli]

That is fine! I did not see any MC312 fitted with 1G before, must be one of the latest production lots ever (1978?).

 

[Gus]

Maybe I should have picked a smaller project to get started

I must admit I have had no formal training so my understanding is very limited and my drawings a little bit odd
But the current != voltage was a rewrite error on my part, oops

The pair of diodes could indeed be something else, but showing up as diodes in my tester.

From my understanding, a transformer would be recommended to isolate the microphone's internal circuit and provide a balanced XLR input.

I have a tester that looks a lot like picture. they can have issues with identifying parts

Looks like a Source follower circuit

 

[MidnightArrakis]

Unfortunately the JFET pinning is wrong. You have to exchange D and S. And YES, more is not needed..

Is everything OK here???


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[MicUlli]

Is everything OK here???

View attachment 104361
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YES