Trouble shooting vintage AKG 451E small diaphragm condenser. Works for 10 minutes then just noise.

[777funk]

Actually it used to do this, now it seems like it doesn't pass sound at all. Where do I start?

So far, I unscrewed the capsule and don't measure any voltage from the center terminal to the case ground. I would suspect I could also troubleshoot the amplifier section like any amplifier as in probe from the end of the circuit and listen for pops working my way to the beginning of the circuit until I hear the pops stop.

But does this apply to mics? Any other ideas to trouble shoot?



p.s. side note, It looks like at some point some of the transistors had been replaced. I don't have the actual part numbers for the originals to look up and cross reference. But it seems to have worked at times since I've owned it (and sounds great) so I would think the replacements are ok... if they are replacements.

 

[ZHOUJUN]

Check all capacitors.

 

[MagnetoSound]

First, check the voltages are there according to the schematic. Luckily they are all marked.

Start with the 10v rail, after R13. 6v at T2 emitter.

T3/U36 is an oscillator that polarises the capsule. You need to check for capsule voltage around R12. There should be 30v AC at D1/C5 and 38v DC or so at D1/C4.

Tell us what you find.

 

[777funk]

First, check the voltages are there according to the schematic. Luckily they are all marked.

Start with the 10v rail, after R13. 6v at T2 emitter.

T3/U36 is an oscillator that polarises the capsule. You need to check for capsule voltage around R12. There should be 30v AC at D1/C5 and 38v DC or so at D1/C4.

Tell us what you find.

Thank you, I was at a little bit of a loss as to what the right half of that schematic did. Old school 1960s tech DC-DC converter.

 

[777funk]

Ok tested the mic and it's working. All voltages look to match (within 10-20%) the voltages on the schematic. The only strange things I'm noticing are:

1. Pins 2 and 3 (referenced to ground pin 1 or mic case) measure 50VDC before plugging in the mic and only 12VDC after plugging in. Should the mic sag the phantom power that much?

2. I'm seeing no substantial voltage at the capsule. Shouldn't I be able to measure a polarizing voltage? I'm getting around 60VDC at one side of R1 but on its other side (feeds the capsule), I get 0.7VDC without the capsule screwed on.

 

[MagnetoSound]

1. T4 and T5 regulate the voltage after R13. I don’t know the value of R13 but given the quite large voltage drop across it, it is quite probable that the regulator pulls enough current to drop the voltage through the external 6k8 phantom resistors as well.

2. is normal. R1 is extra high value (like 100M or more). Your multimeter has a low impedance relative to the resistor (probably about 10M and divides the voltage accordingly before it can be read by the chip.

You say it’s working, are you getting a healthy signal now?

 

[dmp]

R13 is probably 330. So it pulls quite a bit of current to get the 10vDC. Your voltages look good. Do you have a different capsule to try? It might be a capsule problem.

Attach files

 

[777funk]

It's odd with this mic. It went into high noise and low output since I posted the update (which the update was only an hour or so ago). I pulled it apart and everything seemed normal and sound came back. Put it back together and again worked great and is still working great (for now).

I should mention that there was one difference noted in my measurements vs the schematic. I have no 30VAC at the C5-D1 junction (at least that my meter was able to pickup in AC mode). But I do have 43VDC at the R12-C3 junction so it must be a problem with my meter (maybe too high frequency AC for the meter to read it).

All seems to be well for much of the time, but the sound seems to be coming and going. When it is gone, I can't tap the mic and get it to come back (like a bad connection etc).

Thank you dmp for the cleaner schematic and with values! My 451E doesn't have the HPF switching... would be nice! But otherwise I think the same.

 

[dmp]

The oscillator is 2-3kHz if I remember right. Your AC meter is probably looking for 60 hz. So not a surprise you couldn't measure it.
Those gremlins can be the toughest to find. It might be the connector at the capsule. Maybe clean the pin and socket with 99% IPA. Wear gloves and don't touch anything up in the high impedance area.
I have a pair of the old version like yours and they are fantastic mics. I wouldn't trade them for the later versions

 

[MagnetoSound]

It might help to put the capsule somewhere warm and dry like an airing cupboard overnight, get rid of any moisture trapped behind the membrane. If that doesn’t fix it, at least you’ve eliminated that possibility.

Also do what dmp says and clean the capsule area with IPA.

 

[777funk]

It might help to put the capsule somewhere warm and dry like an airing cupboard overnight, get rid of any moisture trapped behind the membrane. If that doesn’t fix it, at least you’ve eliminated that possibility.

Also do what dmp says and clean the capsule area with IPA.

I will try the capsule from another mic (won't thread on but can hold in place and ground with a wire) and see what happens if it quits again. Hopefully not! But not holding my breath that it won't.

When all is working, it's a very nice sounding mic to my ears. This is it through an M-Audio Profire610 that has been modded with an Edcor 600/10k transformer at the input (removed the 47uF electrolytics on the XLR inputs to be replaced with said transformer) and out of the THAT1510 chip (the stock preamp chip in the interface) I removed the 47uF electrolytic on the chip output (pin 6) and replaced it with a variety of switched poly caps (HPF based on the size output cap switched on). This recording is a 0.1uF in series with a 0.047uF which I guess comes out to 0.033uF approx on the output of the THAT chip.

The attached mp3 file uploaded from my PC doesn't seem to go. Here that is in link form:
https://gearspace.com/board/attachm...o-interface-acoustic_mic_testing_akg_451e.mp3

 

[cyrano]

My C451 EBs don't work with phantom power from USB bus powered interfaces. The Profire 610 is Firewire, so that shouldn't be a problem. Have you tried with another preamp or interface?

Mine take up to a few minutes to start working on some preamp's phantom power. And if fed by the USB bus, they make all sorts of noises (sputtering, motorboating...), or rest completely silent.

 

[777funk]

My C451 EBs don't work with phantom power from USB bus powered interfaces. The Profire 610 is Firewire, so that shouldn't be a problem. Have you tried with another preamp or interface?

Mine take up to a few minutes to start working on some preamp's phantom power. And if fed by the USB bus, they make all sorts of noises (sputtering, motorboating...), or rest completely silent.

A year or so ago, I also tried it on my Neve build with a pretty strong phantom power supply and it didn't work on it either (when it was in non working mode). I checked the other XLR input on this interface and it is still at 48V. So at least it's only saging the resistor on the one channel and not the entire phantom supply in the interface. It does have its own power supply (12V 1A I believe) and I use it since my laptop doesn't supply power through Firewire as a desktop would.

 

[cyrano]

I fear your 451's are in for a recap then. They already use a lot of the available phantom power, but if the 10 uF cap is leaky, they never stabilise.

I don't know if yours has removable capsules, but mine do. The thread that lets you screw the capsules on needs cleaning if they haven't been used for months.

 

[CalavoBob]

I have several of these vintage C451's and I used to have the problem you describe, but I found that they are very susceptible to humidity. They will crap out if you speak directly into them. Always use a windscreen if used for voice or testing them by talking into them. My solution is I store them in a sealed mason jar with a silica gel packet inside. They will sound great after a day in that jar. These are fantastic mics on acoustic stringed instruments, snare and overhead drums. If you can score the omni Ck22 capsule for yours, it becomes an excellent acoustical measurement test microphone. I found that my vintage C414s also sound better after storing them in a sealed mason jar with silica gel. I love the sound of old C451s.

 

[dmp]

If you can score the omni Ck22 capsule for yours

Good to know, I only have the CK1 capsules.
I've wanted the screw on pads for my pair but parts for these have become extremely expensive

 

[cyrano]

Capsules for the 451 have become unobtanium. I'd love some more, but I'll have to do with what I have, I'm afraid. And what I have is one CK1 and one CK8...

 

[777funk]

It's too bad that what we have today is mass china capsules more than you could number and dirt cheap vs the good stuff at a somewhat reasonable price. Good repros would be super nice.