AKG C414 EB P48 trouble

[Rossi]

Hi folks, I bought an AKG C414 EB P48 (the black one without a DC converter circuit) off Ebay. It was supposed to be in top condition but on inspection was only cosmetically so.

The mic's signal is very low and noisy. It requires about 60 dB gain for speech and the noise level almost as high as the signal level. The switches seem to work okay. The back diaphragm is a quite dusty, the front one is reasonably clean. I don't think it's a humidity problem. I put the mic under a desk lamp for about 12 hours, and the problems persist.

I noticed that the mic seems to give more level at higher impedance mic inputs (such as a Mindprint DTC @ 5k). I also tried a low impedance 600 ohms mic input, which introduced a pretty loud hum. There was a little hum with other mic inputs, too, but barely perceptible with all that noise on top.

The previous owner claims it worked fine the last time he used it in August. His Ebay profile is 100% positive, and he sold various pro audio items in the past, so I'm inclined to believe him. Still, the mic is unusable.

Anyone have an idea?

 

[Rossi]

Here's some pics:

 

[Gus]

did you check the transformer with an ohm meter?

Voltages at the drain and source and voltages at the NPN base collector and emitter

Check the tants with a meter set to ohms

 

[BYacey]

judging from the pictures, the diaphragms are pretty dirty. This could bleed off quite a bit of charge creating noise and reduced gain. I would start there.

 

[burdij]

You might want to check out this article before doing any service work:

http://mixonline.com/basics/education/audio_mysteries_vintage_mic/

 

[ioaudio]

check c11.

 

[rodabod]

And it's common to check the switches too.

 

[Rossi]

Thank you guys! I'm still checking things, but it looks like its not the ominous C11 tant. I also don't think it's the switches, as they do what they're supposed to do. The mic's just very very noisy.

I should add that the two capsule pictures show the same diaphragm (the dirty back one) from different angles. The front diaphragm is reasonably clean for a mic that old.

I'll keep you posted.

Cheers,
Andreas

 

[Terby]

I got a C414 P48 some time ago wich was completely dead. Phantom voltage went down to ~ 20V as soon as I plugged the mike in.

C11 and some other tants were short, after replacement all tants voltages looked OK, but I am expierencing the same problems as Rossi ... low output, if I crank the amp it gets noisy

My switches are working as well, you can clearly hear the difference between the various pad, HP and characteristics. The overall sound is
quite normal, just the output is very low ...

Andreas.

 

[Rossi]

Should have listened to Gus' advice first. Looks like it's the transformer; there's no continuity between pins 2 and 3. Only one half of the (center tapped) secondary is working...

 

[seavote]

always listen to gus first :grin: :grin: :grin: :grin:

 

[Gus]

I had a transformer open up I guess from a drop. You will get a very low output with almost no bass at all.

It might not be the transformer I forgot to add something. Did you check with a DMM or analog meter.

Some DMMs in ohms do not seem to work right with a transformer with any inductance. Anyone else ever note this?

 

[Terby]

With a DMM I can measure a DC resistance of ~54 ohms between pin 2 and 3 of the XLR connector (this includes both inductors)

My voltages at the transistors are:

T1 D 7,64V S 4,69V G 0,04V
T2 C 23,2V B 8,07V E 7,64V
T3 D 22,8V S 3,11V G 2,69V
T4 C 43,2V B 23,2V E 22,8V

Schematic is at http://www.akg.com/mediendatenbank2/psfile/datei/94/C414P48_di441fa6845b49e.pdf


Any hints from the voltages above ?

Thanks,

Andreas

 

[Rossi]

Just wanted to report back: I was able to fix the mic, eventually. It was definitely the transformer. I was about to check for a replacement, when I thought, well, what have I got to lose and took apart the broken one. While I broke another wire in the process, I was able to resolder the broken connections. The mic now works and sounds quite nice.

I also replaced the cap in front of the transformer. The original tant showed somewhat strange behavior. The measured capacitance was about twice its rating, and DC resistance was about 10 Meg - not dangerously low, but I don't think that's up to spec. For now I put a lytic in there, but I wanna put a fresh tan in there. Have to find a source for higher voltage tants. This one is 50V, the usual places only sell tants up to 35V.

I'm thinking about recapping all tants next time I open the mic. Those 414 EB are really a pain to service. You have to unsolder lots of wires to get access to anything. Anyway, I'm glad I got it working again. Thanks for your help, guys. :thumb: :guinness: :guinness: :guinness:

 

[BYacey]

Good going Rossi. You're right in AKG country there, why don't you try and source the tant from AKG directly?

 

[Rossi]

Nope, I'm in Germany, AKG is in Austria. That's pretty much like the USA and Canada. :grin: That said, I've been told that AKG service is now handled by Audiopro in Heilbronn, Germany.

Anyway, I'll try a local electronics dealer first. Perhaps he can order some 50V tants. If that fails, I might contact Audiopro.

 

[Salossi]

Hi Rossi,

Just had the same problem with recapping an old neumann km88i.

You can get these 50V tants at Schuricht Elektronik.

Here you are:
https://www.distrelec.com/ishopWebFront/search/luceneSearch.do?dispatch=find&showImage=on&artnrPhrase=810800&typePhrase=&keywordPhrase=&vendorPhrase=

Greets, Sascha

 

[Rossi]

Thanks, Sascha! :thumb:

Hmm, KM88 - never used one, but I've sometimes fancied owning one. Seems like an cool mic to have around.

 

[Sorr]

Sorry people, when I see a tant, I just have to remove it!

 

[Rossi]

Well, I guess it depends on your standpoint. Tants are troublemakers, but if you wanna keep the original sound signature, you better stick with the original parts.

BTW: I once repaired four TAB 372D dual preamp modules, which were full of tants. Before I even started, I bought some (expensive) replacement tants. As it turned out, not a single one of the tantal caps was broken but many aluminum lytics in the PSU section were and, as a result, some transistors.

The tantals in the said C414 are okay, too. I just feel it might be a good idea to replace them before anything bad happens. They are quite old, after all. About 25-30 years, I guess.