AKG C414 P48 EB

I am working on one of these, and it has lower than expected gain and higher than acceptable noise, but otherwise sounds pretty nice.

I checked the ohms from pin 2 to 3, and am seeing 50 ohms.  I have identified all of the tant caps, and made a cart in mouser.  I have also measured the resistors in circuit, and am seeing some anomalies:

R2 - 113K ohms (barely in spec, will change)
R3 - 113K ohms (barely in spec, will change)
R4 - 500k (due to transistor? I can't ever identify the circuits that cause resistors to measure incorrectly)
R10 & R11 - 250K
R20 - 8M

I can tell someone has worked on it before because there are metal film resistors in some of the places, and I haven't checked out the cleanliness of the boards on the backside (was going to take it apart when I get my parts in so I don't have to go a couple weeks between taking it apart and putting it back together)

Sound like I'm on the right track?  I did some searching and there is a thread with a similar problem, but that transformer tested open

Check tantals, especially C11 (if you have an option, replace it with MKT - evox rifa mmk or wima mks2 (smaller)).
Much more durable and there's a sonic improvement (for me of course - it's subjective).
If you will be replacing tantals, use for higher voltages than specified.
On transformer you have five windings, check if all are ok.
Measuring resistors in circuit doesn't give you  proper results.
I swapped most of resistors in my unit, but there was no need to do that.
Check do the all positions on each switch works correctly - these connections are messy, so you can have an issue there also (aspecially if you removed pcb before).
Your output DC resistance looks ok. My has 55Ω.
Make pictures of both PCB and capsule from both side.
How dirt the capsule is?
Check all connections again
There was a some grounding wires and RF caps which were added after first run, so yours unit may be not updated.

Thanks for the ideas, I was busy over the weekend repairing a couple tube amps but I hope to come back to this during the week.  Here are some pictures:






All of the switches work, and it actually sounds good, it just has lower output than it should and higher noise.

Looks like capsule fault.
It's dirty enough to give the symptoms which you discribed.
Good cleaning should bring back proper output, but anyway first check tantals .
It's updated version, so it's ok.

Noise could also be caused by a dirty PCB, especially close to the FET (T1) as it's soldered direct on the board.

Check C13&C14 as well, I think they are styroflex ones and sensitive to heat. (If sonone worked on the board)

Sredna said:

Noise could also be caused by a dirty PCB, especially close to the FET (T1) as it's soldered direct on the board.

Click to expand...

Maybe... but much more possible is the dirty capsule.
These capsules aren't dying so fast by dirt (like the rode NT 1/2) but they lose sensitivity what can cause the noise.
In that microphone is much more easy to clean capsule than pcb - believe me.

To be honest...
Some time ago i did tests in two different cheap microphones - LDC and SDC.
I put on the pcb tracks, fet etc. large amount of old stinky rosin paste flux and some other old crap, heated by the soldering iron - and there was no hum, noise or any other weird behaviors.
I worked on many old microphones, which had dirty pcbs and there was no any weird symptoms.
Some chinese mikes (mostly older ones) have dirty pcbs also. They work without problem.
When i work on some microphone, first check is always before cleaning the boards - gues what
Never had the problem with dirty pcb.
I don't say that dirty pcb, especially in high impedance section, can't cause the problem (even if i didn't noticed that in my work), because it's possible even in theory, but first i will look for completely different issues.
Most LDC which i repairing is not working or not properly working by dirty capsule.
Rode NT1/NT2/NT1a are most common dying microphones which i had. Too high polarization voltage and too much saliva on the diaphragm Good cleaning brings  to life, but slightly reducing polarisation voltage gives much higher life-time.

Sredna said:

Check C13&C14 as well, I think they are styroflex ones and sensitive to heat. (If sonone worked on the board)

Click to expand...

Nope... These are the yellow Wima FKC shown in the picture.
I even marked them yellow on a drawing which you posted, so you can see position of it on PCB (and C13 C14 description)

It's indeed a lot of work to disassemble the PCB's and clean them but my comment is based on experience with this circuit. 

I had problem with noise and checked everything including the capsule, working point for the current source, leaky tants etc.
Finally I cleaned the PCBs (both on both sides) and got the noise down.  The First FET (T1) is soldered direct on the PCB, including the gate leg that's normally floating in the air om many mics.

If there has been previously mods/repairs on the mic (like in mine), there's a good chance that the PCBs are contaminated with rosin and fingerprints.

Personally I find it more risky to try to clean the capsule myself than the PCB's. 

On the caps I stand corrected, the styroflex ones are: C1, C2 and C3. 

Interesting information about the RODE mics, thanks!

Thanks guys!

I replaced all Tants and cleaned the boards well with flux cleaner, and it sounds great now!

Here's a little clip with the C414 on my Martin HD-28, and an SM7B on my vocal:  https://www.dropbox.com/s/i57hi8arlxp36bi/MicDemo.wav?dl=0

I have worked on many, many 414's. They are prone to problems with dirty pcb's and switches as well as bad capacitors.

Thanks Tim, this one falls in line with your experience. 

I'm looking forward to using it more, I think I like it better than what I was using on my acoustic guitar recordings

You have a very pleasant voice

Try it on vocal, it's really good vocal mike.

if this is a EB shouldn't it have a original CK-12 capsule?

JessJackson said:

if this is a EB shouldn't it have a original CK-12 capsule?

Click to expand...

No.

1 - it's "EB P48" - there was never "brass" CK12 in this model
2 - even first "EB" had changed capsule from "brass" to "nylon" during production years
Only available online service manual for "EB" is with "nylon" CK12