AKG 451 E - issues

google "AKG c451e schematic" one of the first links has a schematic with some voltages marked.

Left side is a source follower into an emitter follower to the output transformer
Right side is the regulated 10VDC supply and DC to DC converter for the capsule voltage

Power the circuit up from a phantom supply and check you voltages. Nice simple circuit should be an easy fix.
 
Of course - it's so simple! :D

(and I must spread some reputation around before giving it to dgatwood again)

I have found one bad capacitor so far - a tantalum 33uf. I had a replacement ready, but for some reason I'm getting a lot of noise when I put it in, and now the old one's noisy too -- :( messed up I guess. Well it's late, and this little sucker's going in the toolbox for the time being.

Make sure you read that number right. 33 uF sounds awfully big to be in a microphone (unless maybe it's a power supply filter cap), and if you use an oversized part in certain spots, it may dramatically cut the output level, in which case things would be noisy when you add gain to compensate. Most of the SDCs I've dealt with had parts that ranged from single-digit pF up to about 0.1 uF....

*shrugs*
 
33 uF sounds awfully big to be in a microphone
I know it's weird, and my measurement of the existing capacitor isn't consistent, but it's also passing DC, so I figure it's bad. I'm pretty sure it's a dip tantalum capacitor, due to its size and shape, though.

But look at this first PDF that Ritzy linked to:

http://www.akg.com/mediendatenbank2/...4627a7b1f0.pdf

The schematics in there are close to what I'm looking at, and on the parts list, I swear it specifies a 33u capacitor ("Ta-Elko") for C7, which is the one I'm talking about. There are two other tants, a 10u for C9 and a "3u3" for C13, which I guess is a 3.3u - I haven't gotten that far yet.
 
I know it's weird, and my measurement of the existing capacitor isn't consistent, but it's also passing DC, so I figure it's bad. I'm pretty sure it's a dip tantalum capacitor, due to its size and shape, though.

But look at this first PDF that Ritzy linked to:

http://www.akg.com/mediendatenbank2/...4627a7b1f0.pdf

The schematics in there are close to what I'm looking at, and on the parts list, I swear it specifies a 33u capacitor ("Ta-Elko") for C7, which is the one I'm talking about. There are two other tants, a 10u for C9 and a "3u3" for C13, which I guess is a 3.3u - I haven't gotten that far yet.

Wow. Yeah, that's what the spec sheet says. Is the cap marked 336 on the side?

Oh, and yes, as best I can tell, 3u3 does appear to mean 3.3 uF.
 
I just says "33" and has 6.3 (the voltage), and a bunch of++++++ on one side (indicating polarity, I guess).

I replaced it, and the gain problem seems mostly resolved, but I still have an issue with bass response. I think I identified one of the other little poly ones as having a problem, and so I'll try and replace it -- I don't have an exact match (on both capacitance and voltage) for each, so I'm going to keep looking.
 
I just says "33" and has 6.3 (the voltage), and a bunch of++++++ on one side (indicating polarity, I guess).

I replaced it, and the gain problem seems mostly resolved, but I still have an issue with bass response. I think I identified one of the other little poly ones as having a problem, and so I'll try and replace it -- I don't have an exact match (on both capacitance and voltage) for each, so I'm going to keep looking.

If a cap says 33 on the side without any unit marking, that's equivalent to a three-digit code where the third digit is zero. That means it should be 33 pF.... If you put a 33 uF cap there, that could be why you're having gain problems.... :)
 
heh -- I didn't put the "33" in there, it was there - stock I think. It's the same one in the other mic, which works fine (at least, I *think* so :) ) I'm sure it's the higher value, since it's clearly a dip tant, and the schematic says what it is - it's pretty old -- could the numbering system have been less deterministic back then? (like the 1960s, I think). My wife ran off with the good camera, otherwise I'd post a mug shot -- maybe soon.

Does anyone know where I can get a 15pf 160volt polystyrene?
 
heh -- I didn't put the "33" in there, it was there - stock I think. It's the same one in the other mic, which works fine (at least, I *think* so :) )

I understand that. What I was saying was that if the original part says 33 and doesn't say uF after it, that should be 33 pF, not uF. If you were to use a 33 uF part there, you'd probably have significant loss of gain.


I'm sure it's the higher value, since it's clearly a dip tant, and the schematic says what it is - it's pretty old -- could the numbering system have been less deterministic back then? (like the 1960s, I think). My wife ran off with the good camera, otherwise I'd post a mug shot -- maybe soon.

You sure that's tantalum and not a multilayer ceramic? They look very similar. Are the other parts numbered correctly?


Does anyone know where I can get a 15pf 160volt polystyrene?

I can find you 10 and 22:

http://www.rapidonline.com/producti...e&tier4=Polystyrene+capacitors&moduleno=62504

Ah. Actually, yes, I did find 15. Thank you, Froogle.

http://www.newark.com/jsp/search/productdetail.jsp?SKU=88K0668&CMP=AFC-GB100000001
 
thank you dgatwood!! (and indirectly thank you froogle!) -- that Newark place is awesome -- also picked up some 470pf capacitors, even though I don't think any of mine are bad yet. As an aside, they also have the BC441 and BC461 transistors that you're supposed to put in the ACMP-81s, so I got a bunch of those, too. Those little things add up when you buy a bunch of them :o

I actually got a significant gain in gain when I put in my honking 33uf, but that's because I believe I was replacing a bad (and very old) 33uf capacitor. I'll try an get a picture sometime soon (when my wife comes back with the camera), and maybe it'll make sense at that point. Anyway, I'm well on my way to having this mic completely back in action - thanks!
 
What ever happened to your 451? Did you get it back to 100%? I'm working on a dead one right now and curious how it turned out...
 
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