Right now i’m repairing 2 AKG D12 microphones. AKG started building these in 1953. I have with me a one whose serial number is 0582!!! (At least as old as my mother)
There are many versions of the D12, produced over a long period of time. The main variations were in the capsules and transformers, which allowed for different impedance ratings (60Ω, 200Ω, 250Ω and ‘high’ impedances). Some versions had no transformer, and many in circulation today have been modified to be that way. With nearly six decades of use, abuse and maintenance, it doesn’t take long to see why these microphones can sound quite different from each other.
These are 200Ω, no transformer.
These are the screws you have to mess with to get the thing tuned. That was quite a challenge! After tuning i fixed the screws with some nail polish.
So this one was particularly bad, and i didn’t manage to get it 100%. It had a spiky thing (like dryed weed, or some nature like thing) stuck between the capsule grid and the capsule. It did not appear to have damaged the capsule, but with this sort of sensible things your never quite sure.