1. If you can afford to replace it, I would try myself.
2. If no, I would send it out to a professional
3. sell it as vintage and replace it with something new.

I think I'm going for option 2, and I think I'm gonna go with Richard Land, who has been trained at AKG in Vienna in the repairing of vintage AKG mics:

http://akgmicrepair.com/

I've just seen this thread, and would agree, that's the right path, Donny. I doubt that dirt on the diaphragm would give a simple 3dB loss of amplitude without extraneous noises (see Rode FoodMic video), but something like a backplate voltage loss could do that. Either way, it probably needs a pro to look at it.

Rather than a diaphragm problem, it occured to me that the backplate voltage is varied according to the pattern set on the switch, so if some of the electronic switches were not operating correctly for your standard position (cardioid?), that could be an explanation.

Boswell, post: 449048, member: 29034 wrote: I've just seen this thread, and would agree, that's the right path, Donny. I doubt that dirt on the diaphragm would give a simple 3dB loss of amplitude without extraneous noises (see Rode FoodMic video), but something like a backplate voltage loss could do that. Either way, it probably needs a pro to look at it.

Rather than a diaphragm problem, it occured to me that the backplate voltage is varied according to the pattern set on the switch, so if some of the electronic switches were not operating correctly for your standard position (cardioid?), that could be an explanation.

It's funny you mention the pattern set being related to the problem, Bos. I'm seeing the reduction in output on the cardioid setting, but the hyper-cardioid setting is fine... a strong, clear signal.
I'm assuming that this points more towards an electrical issue than the dirty diaphragm?

This article tells me at least three things plus my $0.02 worth:

1). The gold sputtering on mic diaphragms is so thin that perhaps ANY physical contact with the membrane with a brush, etc. could cause "flaking" from the mylar membrane of (especially) an older capsule.
2) The insulation between the membrane and clamping ring(s) may absorb the distilled water or water/alcohol cleaning solution. At hundreds of megohms (or gig ohm) impedance, the water may contribute to "popping" or "squeal" noises when the mic is fired up.
3) Over the years, a stretched membrane (especially mylar, etc.) may lose its tension. Fact of life. There is an electrostatic attraction between the polarized back plate and the grounded or otherwise charged diaphragm. Cleaning won't fix the popping or sputtering found here. The mic owner is probably looking at a new capsule or a reskin done by a reputable tech. Shell out some cash and get your gear working again.
4) Always use a good quality wind screen or wind shield with vocals near condenser mics. Let the vocalists "kiss" an RE20; it's easier (and less expensive) to replace, compared to a Neumann U47, U67, or U87...
5) Use a reputable mic tech when necessary. The capsule may not be the source of all mic noise issues. Certain components in the mic's impedance matching circuitry may be noisy while the capsule is good. Let the mic tech make that call.

DonnyThompson, post: 449049, member: 46114 wrote: It's funny you mention the pattern set being related to the problem, Bos. I'm seeing the reduction in output on the cardioid setting, but the hyper-cardioid setting is fine... a strong, clear signal.
I'm assuming that this points more towards an electrical issue than the dirty diaphragm?

Well, for me, the fact that the hypercardioid levels are normal but the cardioid are low moves the emphasis strongly away from there being a diaphragm issue to there being an electronic issue with the switching of the plate polarisation voltages.

Are you in a position to make relative level measurements for all of the pattern settings? If it's just the cardioid that is low, we might be able to focus the problem more acutely. It would likely still take a trip to an AKG accredited service centre to fix. However, if it turned out to point to a switch chip, there's the chance that could be fixed locally.

We've already talked about the guy I'd send it to, if I wanted it done to factory spec.

AKG Mic Repair

When AKG had an HQ in Nashville, Richard was the head mic repair guy (nationwide). When AKG consolidated its operations to Northridge, CA, he chose to stay in TN. He'd probably be among the best qualified anywhere, that doesn't have a thick Austrian accent.

Hats off to this guy in the video though, for an exceptional instructional video. Well lit, good audio, and in focus (if not always perfectly in-frame).

Thanks Guys... you've all confirmed for me the wisest move, which is to send it out to Richard Land.
I'm not going to risk doing anything myself - and considering it's most likely an electronic issue, I couldn't do it anyway - it's not that I don't understand how to replace a cap or a resistor, but I have neither the hands or the eyes for that kind of detailed work anymore; and, the bottom line is that I don't want to create another problem - or make the current problem worse - which will only result in jacking up the cost of the repair by a specialist.

I'll keep you in the loop on this; for right now, I'm going to have to continue using hyper-cardioid for mono work, until I can afford to send this out. At least it's still operational, it's making no noise that I can hear - there's just that -3db (or so) attenuation on the cardioid switch.

Thanks again for your insight. :)

Donny, Google comes up with very many schematics for the C414, so, when you have spare time, could you possibly post a link to which one is most likely to represent your problem microphone? I mean simply based on full microphone model number and/or age, and not the circuit details.

The pattern switching appears to be done in a pretty constant way in all the schematics I have looked at, but it would be nice to know.

Thanks for your insight Bos ( Boswell ). I'll search for - and if found - I will research the schematics and see if I can find my EB model's details.

hey donny did you ever get it fixed by that land guy? I have an akg 414 b-uls that I need to get repaired and want someone trustworthy. curious if you got yours fixed?

Donny - I've wrecked a 1st gen NT-1, and a cheap Chinese large diaphragm trying to clean them. I used Colclene - a cleaner I've used for years on other products - the can actually says good for cleaning guided missiles!

The diaphragm on the NT1 was very stained looking - I figured moisture condensed from breath. So I gave it a light spray - the staining didn't shoot so I gave it a closer and heavier burst - and the staining dissolved and washed away and that was the last peep from the mic at the treble end. I can only assume I stretched the membrane as it responded to voices as a low frequency mumble - like when you blow out the corrugations in a ribbon. I tried it on the Chinese mic a couple of years later in desperation, and the similar marks to the NT1 just seemed to have eaten into the surface. The cleaning did nothing for the dull tone, but didn't make it any worse - just not better.

With a precious mic like a 414, I think the money to get an export on it would be well spent. I don't worry about trying it on a cheap mic, but .....................

paulears, post: 456637, member: 47782 wrote: Donny - I've wrecked a 1st gen NT-1, and a cheap Chinese large diaphragm trying to clean them. I used Colclene - a cleaner I've used for years on other products - the can actually says good for cleaning guided missiles!

The diaphragm on the NT1 was very stained looking - I figured moisture condensed from breath. So I gave it a light spray - the staining didn't shoot so I gave it a closer and heavier burst - and the staining dissolved and washed away and that was the last peep from the mic at the treble end. I can only assume I stretched the membrane as it responded to voices as a low frequency mumble - like when you blow out the corrugations in a ribbon. I tried it on the Chinese mic a couple of years later in desperation, and the similar marks to the NT1 just seemed to have eaten into the surface. The cleaning did nothing for the dull tone, but didn't make it any worse - just not better.

With a precious mic like a 414, I think the money to get an export on it would be well spent. I don't worry about trying it on a cheap mic, but .....................

Paulears.... I may catch flak here, but cleaning any thin membrane is risky, especially condenser microphone capsules and more specifically the double membrane ones such as the Neumann or AKG. I don't use my condenser mics for vocal (only record classical or instrumental jazz). In the event there is a vocal, they get a EV666 or AKG D24E. Considering condenser microphone capsule loading in the 100 meg ohm to 1 gig ohm range, it doesn't take much leakage across insulation to cause noise. Usually, if a capsule becomes noisy or degraded to the point it requires cleaning, it won't be much longer that capsule "circles the drain". Murphy's law says the mic will fail during a paying session. Sometimes the sputtered mylar capsules begin to shed the gold. The other failure mode is insulation breakdown, especially if aggravated by a cleaning compound. Steam distilled water and the finest artist's brush and no pressure on the membrane may work in a pinch, but still risky. Pour a sample of the distilled water in a glass dish and check the resistance with a megohm meter for high or infinite resistance before attempting a cleaning.

Hi guys... Sorry for the delay in responding to this; I've been busy mixing for the last few weeks...
I didn't attempt to clean the mic myself, and haven't yet sent it out to be looked at yet, either.
I've got a list of things I need to do, and only so much money...
I've used the 414 since, and have been adding some top end to it ( a little shelving around 12k by about 2db).
Actually, after I acquired the ISA One pre and was able to get close to matching the impedance, it sounded better, and upgrading to an Apogee converter didn't hurt, either .
I'm sure I'll send it out eventually, just to have it gone over by a pro, someone experienced with vintage mics.
But I'm not gonna touch it myself. I'll let a pro handle it when I can afford to have it done .
:)

I like to try most things, but mic capsules are now on my 'don't mess with' list. I'm too ham fisted now too!

Late to party....Check in with Klaus Heyne's mic forum on the REP. He addresses this without question. He has cleaned my vintage U87.