I hope this thread still bears some interest.
I have had the chance to fix quite a few of the D12-family mics and test/compare them with each other.There is just too much to say, but I try to start somewhere.
D12 was issued in 1952 according to the "History of AKG".What always impresses and amazes me is the variety of them and as Oliver noted: they do the same job in the end. Jet, there are some minor differences in performance.Those without a trafo have a better bass-responce(like a deeper range even), which is not that audible, but is there.You can test it by rewiring one of two "matched" 200ohms d12:s with a trafo used to step up the impedance from 60ohms to 200ohms, that you bypass the trafo.Anybody who tried it too?
Then Oliver,you mentioned "different"membrane" styles.Could you explain, because I only have seen one style? Some of the oldest have a very thin and light membrane more like paper or pergament,and the coil ends are pierced through to the top side of the membrane and the piercing points are glued making it impossible to pull out the coil if it brakes.However the wire was slightly thicker then on those 60ohm coils, and you hardly see them brake like the later ones.They have a bit less high-end,but the bass-end sounds as it should, which is also comfirmed by AKG specs(they do not show that "bass-hump", but my ears do).
Now the most common problem with the "D12-family" that I have encounterd is caused by the huge magnetic barrel.There is signal, but weak and no bass at all. The membrane has collapsed, because the magnet has absorbed iron-dust and it then gathers as a ring around the dome thus nailing it down so that it cannot move.Add a constant humidity and even the particles inside the barrel(there seems always to be some)rust and "glue" the coil to the barrel.Remove with a magnetic screwdriver that iron-ring around the dome and the mic should work nice again.Note that the screwdriver has to be supported firmly on the mounting ring, while gently picking the partickles away.They will stick to the tip and you just slowly pull them away until the membrane is clean.Many times in the beginning the screwdriver slipped from my fingers and the membrane was gone forever.But I learned my lesson.Same with the screws,careful not to loose them, because they will easily hit through the membrane and there are no replacements.Often the older ones have a pierced membrane, because the magnet has pulled slightly bigger partickles through it.Small holes can be patched with a thin instant-glue, but that very seldom works without causing coloration.Some glues would also soften or even disolve the material.
The loss of bass has often been discussed on this forum. First always better check the cable. A bad cable may also be the cause for the weak bass-responce. D12-and D19-families use paper-rings to adjust the height of the membranes.The older ones might need a hight-adjustment, because the paper has been compressed within the time and the membrane can be deformed too by the iron-ring around the dome or just beeing too long collapsed.It is sometimes necessary to do the opposite, so it is not that simple. But tuning the membrane will finally be the crowning work, that all depends on in the end.
I will post some pictures later and anyway try to continue this another time, because this is getting a bit long now.
Best regards,
Esa Tervala