AKG D12 repair attempt...

[s2udio]

I removed the diaphragm, carefully lifting it off against the tiny wires and I ran a magnetised screwdriver and some compressed air over the magnet surface to make sure there were no rust particles left in there. I reseated the diaphragm and did the sine sweep between 50 - 5000Hz. The diaphragm gives an even tone that fades in volume to just barely audible at 100Hz and below. There may be very minimal buzz at around 200Hz or thereabouts, but that could even be 'normal' for the small diaphragm.

That diaphragm needs to be fully removed........hope you did using the comp air !...... and the Gap needs to be fully scraped clean to the bottom.......as in the procedure
simply running a magnetic screwdriver and a puff of air will not do the job !...as the gap will be full of oxide .....non magnetic...
The sweep should be audible down to about 40 hz........with no buzz at any frequency...sounds like you may have to try again !
As for the paper in the hole.......dont recall any in mine..........from what i can gather from your post it is still not right

 

[Vik]

Hi, thanks for the quick response!

That diaphragm needs to be fully removed........hope you did using the comp air !...... and the Gap needs to be fully scraped clean to the bottom.......as in the procedure
simply running a magnetic screwdriver and a puff of air will not do the job !...as the gap will be full of oxide .....non magnetic...

I wish I had taken some pics. The capsule isn't so badly oxidised on mine as it was on yours, but I'll clean it anyway (although I was really hoping I won't need to...) On my mic, the diaphragm and the spacers are all stuck to the grille, meaning that I could easily pull the whole thing away as far as the tiny wires would allow (and that was when I applied the air, with the diaphragm at a 'safe' distance).

How did you get the paper shims to separate? Mine are firmly stuck together, and this makes it impossible to clean the grille, or to get a good view of the diaphragm.

The sweep should be audible down to about 40 hz........with no buzz at any frequency...sounds like you may have to try again !

It is audible, but quiet -- it's quiet enough that I have to put my ear next to it when the laptop is next to the mic with its fan turning on 'low' (the laptop is needed as it's generating the test signal, unfortunately).

As for the paper in the hole.......dont recall any in mine..........from what i can gather from your post it is still not right

Did you listen to the sample or are you just taking my word on it?

Anyway, I'll pull it to bits again, either tonight or next weekend. I've got some really thin wire so I'll try to practice soldering it first I'll be taking some pics this time.

Cheers,
Vik

UPDATE: I did pull it to bits and put it back together. It's too late for testing it with the bass amp, but it does not appear to sound better to me. Will test tomorrow.

 

[s2udio]

The procedure needs to be followed as per the instructions............there are no shortcuts....the diaphragm must be unsoldered and removed !
seperate the gaskets with a scalpel......patiently......
If the diaphram wires are not routed and clamped correctly.......no go....this is why capsule needs to be completely stripped and cleaned..
without cleaning the gap properly and carefull alignment you will be wasting your time.
If you cannot hear the capsule sing,........... carefully increase the laptop sig gen output...but not to overdrive and burn the coil......
unfortunatley there is no way i could tell you at what point that would happen.......you are turning it into a speaker so
dont over do it.........maybe i should have taken a voltage measurement............all 3 i have repaired were quite audible over
any laptop noise...........proceed with patience

 

[Vik]

Hi there,

A bit late, but here's the writeup of what I did:

First I pulled out the transformer and attempted to remove the piece of paper in the bass chamber. I was unable to pull it out with tweezers, but I got a better look of it. It appears to be some kind of spongy material with self-adhesive backing. It must have been glued to the back of the chamber originally for taming the resonance. It's probably not too good to have it detached though. Problem is, I think I'll need a hammer to open the chamber up, and I really hope I won't break it. Maybe I should get a spare chamber instead, mine was damaged already anyway...

This is the best picture I managed to take through the little hole at the bottom of the bass chamber:

For reference as I have not seen pics elsewhere, this is how the transformer looks like:

And this is how the front of the capsule looks on mine:

Next I took apart the shockmount and removed the capsule. Note: somehow I got lucky the first time and failed to notice how annoying it can be to put the shockmount back together. Especially that there is a hungry magnet ready to swallow the screws through the went holes on the capsule's side, into tweezerland. But I'm getting ahead of myself

The unsoldering and re-soldering of the tiny wire was actually easier than I tought it would be. I did practice with a similar gauge wire beforehand though. The plastic tie-wrap is definitely handy for moving the wire in and out of the solder.

I took off the capsule with grille, brass ring, paper shims & all. The coil gap appeared to be clean but I used 1000 grade sandpaper really gently to make sure it really is, and blew it out with compressed air afterwards. I made sure I can see the brass shining at the bottom of the gap and that there is no oxide stuck in there.

Here's some pics of the mic as disassembled:

I used a thin screwdriver for separating the shims from the brass. This is the part most likely to increase your grey hair count. Be *really* careful with that screwdriver / scalpel / whatever!

Once I got the brass and the membrane to separate, I noticed that the grille comes out as well -- it's held in by the edges of the same six screws that hold the diaphragm assembly in place. In fact, the grille was probably the source of the misterious rattle I heard earlier. I have already loosened the screws a bit, and it probably caused the grille to come loose. Either that or it had something under it that was rattling against the diaphragm. I'll never know, but the noise is gone. This is what the other side of the membrane looks like. There appear to be some fine oxide particles embedded, but nothing serious. Whatever's stuck in the membrane ist there to stay though, I will not risk trying to remove it:

So I tried to make sure that the gap is clear, the coil is clear, and the membrane is as clean as possible. I then reassembled everything. I did not have the holders for the solder joint. Whoever last repaired the mic just added some chewing gum to hold it in place. I replaced this with some blu-tac which may not last as as long but is much less disgusting

I tried the sine wave test with no rattle but no improvement either. I then tested the mic on the bass cab this morning and again I did not notice any improvement. I will try to realign the capsule but I'm not too confident that it would help. In any case I'll use the opportunity to mike it up and record the sine sweep, to make sure I'm not missing anything. Any suggestions on what to try next?

Cheers,
Vik

UPDATE: I forgot to add that larger versions of each pic are available at the same place here: http://www.avatarzenekar.hu/files/d12/

 

[s2udio]

Check this thread......thats not the D12 i know  :-\
and may not be what you think it is...........never seen one like this myself....??
http://recforums.prosoundweb.com/index.php/t/13463/0/

 

[Vik]

Check this thread......thats not the D12 i know  :-\
and may not be what you think it is...........never seen one like this myself....??
http://recforums.prosoundweb.com/index.php/t/13463/0/

Well, what I see and hear seems to relate to what folks say about the older D12 mics. Mine is definitely one of the older types around. It's (re)branded as an Echolette, the metal plate around the base says "D12/200", serial number 1302. The writing faded a lot but the serial number is punched into the metal.

Have the ones you've opened include a transformer? If yes, was it stuffed into the front part of the bass chamber? Maybe I'd get more low end by bypassing the tranny... I just looked at the pics of the assembled mic from your writeup. It looks as if the newer mics (including yours) had their bass chamber made out of plastic instead of metal and did away with the transformer as well. If this really was the case, that would explain a lot about the sonic differences. On my mic, the resonant area of the bass chamber is tiny as the entire front is used for the transformer. And even the space at the back is damped (even more now that the damping material got loose).

If you happened to have a pic that shows the bottom half of the mic in more detail, I would much appreciate it.

Thanks again!

Cheers,
Vik

 

[andre tchmil]

A few weeks ago I posted here my problem with a D12.
Both tails of the membrane wire are broken off.
Now I'm looking for a replacement membrane.

 

[s2udio]

That bass chamber is not a bass chamber......its the transformer can.....the early ones were not fitted with a chamber !
and did not have a pronounced bass response as the later ones ....
the D12 service document
http://www.akg.com/mediendatenbank2/psfile/datei/85/d12443e17c7c62d7.pdf

 

[Vik]

That bass chamber is not a bass chamber......its the transformer can.....the early ones were not fitted with a chamber !
and did not have a pronounced bass response as the later ones ....
the D12 service document
http://www.akg.com/mediendatenbank2/psfile/datei/85/d12443e17c7c62d7.pdf

Excellent document, thank you! I don't see the transformer inserted anywhere on the diagrams though. In any case, on mine the transformer can appears to have been hacked into the chamber after the fact. I've noticed the sloppy assembly (the outer can has been cut open slightly and the inner can hammered into it with a piece of rubber or soft plastic between the two. The hole through which the spongy damping material is visible is in the bottom of the inner can.

Should I try to get the inner can out, close up the chamber and rewire the bypassing the transformer?

UPDATE: I've put up some more pics to the same place ( http://www.avatarzenekar.hu/files/d12/ ) showing all aspects of the mic (transformer holder, serial number, etc) if anyone is interested.

Cheers,
Vik

 

[s2udio]

If it was a bass chamber it would connect to the capsule at the back as in the service document to allow airflow into it!
as i said......i think its just the transformer shielding can....from what i can see.....any pics of the back of the capsule ?

 

[Vik]

If it was a bass chamber it would connect to the capsule at the back as in the service document to allow airflow into it!
as i said......i think its just the transformer shielding can....from what i can see.....any pics of the back of the capsule ?

Well, it was a bass chamber and it's connected as it should be. But it probably stopped functioning as such when some git hammered another can into it and reduced its volume to about the quarter of what it was before.

I've put up a ton of pics, here's one of the back:

The tube coming out of the back of the capsule is somewhat broken, so it has been soldered to make it stay in place. But I have checked the airflow and even blew through it with compressed air.

Couple more pics from other aspects:

On the original large versions, you can sort of see how the transformer can was hacked in, although these pics are not very good. This was the best I could do in a hurry, and without taking it apart again. (larger pics here: http://www.avatarzenekar.hu/files/d12/)

Cheers,
Vik

 

[s2udio]

Ahh now i can see ......low Z D12 with a transformer fitted in can......?
remove trannie ....seal chamber......and make sure airflows from the small hole in the centre of the can in to it
That transformer is the hack ......remove the screws on the back of the capsule , remove the chamber and restore it ..
looks like that solder job may be blocking the airflow ?

 

[SSLtech]

Oh yeah... I have one here of that vintage also...

One of the wires is shortened... and the paper spacers are largely disintegrated.

I'm plucking up courage to go back into finishing the job off...

...still note enough courage, yet!!!

Keith

 

[Vik]

Ahh now i can see ......low Z D12 with a transformer fitted in can......?
remove trannie ....seal chamber......and make sure airflows from the small hole in the centre of the can in to it
That transformer is the hack ......remove the screws on the back of the capsule , remove the chamber and restore it ..
looks like that solder job may be blocking the airflow ?

I'll try to just pull out the tranny and seal the chamber up to see if it helps first. And I'll try to get the chamber sorted out. The solder job does not block the airflow (I've checked). the chamber didn't break off completely just partially, so the solder is holding it from the outside. The impedance (resistance actually) of the transformer is actually *very* high, I measured just over 2kOhm with a multimeter. The mic says D12/200 -- 200 ohms. Does anyone know if that's meant to be with or without a transformer? I think they made a 60Ohm version as well.

 

[Vik]

I'm plucking up courage to go back into finishing the job off...

Hi Keith,

Personally I expected the soldering of the fine wire to be harder. I never tried to solder anything like that before, so I got a similar gauge wire from an electronics supplier and practiced on that first. I had the iron on 300degC, touched it to the thick wire and pulled the thin wire away with a plastic tie-wrap like Paul suggested. Having one end shorter means you got more of a challenge, but I've found that actually fiddled a lot more with putting the shockmount back together. So I'd say go for it!

Cheers,
Vik

 

[s2udio]

I measured just over 2kOhm with a multimeter. The mic says D12/200 -- 200 ohms. Does anyone know if that's meant to be with or without a transformer? I think they made a 60Ohm version as well.

Yup spot on 200ohm........older ones 60............dump the trannie i am sure thats not meant to be there !
Nice pics btw.........sure to help the more ancient D12's to rebirth !

 

[emrr]

If you're getting 2K resistance on that transformer, its either 1) failing in the direction of open, or 2) a hi-Z (50K for feeding an instrument input) step-up someone hacked in.  Try it into a direct box and see if it sounds better, before you do anything.  That would confirm or deny hi-Z step-up intention. 

 

[Vik]

OK, I have removed the transformer and reconnected the wires directly. Output resistance is now 50Ohm. I have attempted to restore the bass chamber to it original purpose. To this end I heated the bass chamber with the soldering iron until I was able to remove the insert. It turned out to be an old school transformer housing -- the little hole was originally there for the cable to enter the case. In fact, whoever soldered it in just cut the cable off because it was easier than unsoldering it.

In the bass chamber behind the transformer housing there was a piece of spongy foam and what I thought to be paper turned out to be a small piece of insulating tape. I cut and filed a disc of the right size from a thin steel sheet, to replace the front of the bass chamber. I temporarily soldered it in place and gave it a go.

Unfortunately there is still no bass... The only difference was that the output signal got about maybe 10dB quieter. On the other hand, if I hook it up to my sound card output and feed it a sine wave, that's noticeably louder -- naturally the sound card is better at driving 50Ohms than 2000. This way it is now noticeable that from about 125Hz down there is some buzzing happening at higher volumes (above that it's fine). It appears the loudest around 90Hz. Moving the diaphragm doesn't really help. I wonder if I should add another paper shim (if I loosen the screws, it gets a little bit better). Or perhaps I should check the gap again... Any ideas?

Cheers,
Vik

 

[Vik]

If you're getting 2K resistance on that transformer, its either 1) failing in the direction of open, or 2) a hi-Z (50K for feeding an instrument input) step-up someone hacked in.  Try it into a direct box and see if it sounds better, before you do anything.  That would confirm or deny hi-Z step-up intention.  

I forgot to add that I did try the DI-box, but it didn't make any obvious sonic difference. After desoldering the transformer, I measured it again and got the same resistance 2kOhm. It was wired with a common ground, I made the measurement the same way.

UPDATE: Could it be that the transformer was there to boost the output level (voltage)? I think apart from this it also contributed some odd-order harmonics which helped to bring out the retro mid-range growl of the mic (which actually sounds cool on a bass amp but doesn't help with the low end).

 

[andre tchmil]

Don't dump the membrane !!