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R/E/P => Klaus Heyne's Mic Lab => Topic started by: fffaustino on May 11, 2018, 05:34:13 am

Title: Neumann km54 faulty
Post by: fffaustino on May 11, 2018, 05:34:13 am
Hi Klaus,

I have recently adquired an original Neumann km54 with nickel capsule and selected ac701k.
I correctly tested it and it was working good with no issues but after a few days of use the mic suddently lost it sound and began to generate a loud “fire, crackling” noise. The noise seems to go down a few dbs after waiting for tube heating but it does not disappear and the mic does not work at all..

Any advice really appreciated..thanks!

Alfredo
Title: Re: Neumann km54 faulty
Post by: Kai on May 11, 2018, 04:17:02 pm
Check PSU voltages on the mic side (this checks the cable too).
Remove the capsule and screen with alu foil or a dime, and check if noise is still present.
Identify the version, check all the voltages according to the schematics below.
Check (-) side of C6, DC voltage should be 0mV, else C6 is broken.
I calculated and noted the anode voltage according to specs, but usually it's closer to 40V, not 24V.
If anode voltage is off and everything else is correct R4 or the tube might have failed.
To test R4 bridge it with any high Ohms resitor (>=4.7MOhms) that is lying around.

If that all sounds strange to you, send microphone back to seller or to Klaus for repair 8)
Title: Re: Neumann km54 faulty
Post by: Kai on May 11, 2018, 04:20:27 pm
Schematic Neumann KM54a
Title: Re: Neumann km54 faulty
Post by: klaus on May 11, 2018, 04:44:30 pm
(After I wrote this, I got distracted and forgot to send it off. 2 hours later, Kai had already responded. We obviously agree how to trouble shoot this issue)

If the supply voltages fed to the mic are correct, you may want to test the plate voltage measured directly at the plate (filament with red lacquer dot on the glass), to see whether the tube is shot. A healthy AC701 plate voltage will read anywhere from the high 30s to low fifties, depending on KM54 version.

If you can also knock the housing tube hard with your knuckle, and see whether the noise spectrum or level changes, that would be a further indication that it's time to take out a loan.
Title: Re: Neumann km54 faulty
Post by: fffaustino on May 12, 2018, 06:16:51 am
Kai, Klaus,

Thanks so much for your advices, i will try to follow them correctly and let you know..

thank you and best regards from Spain!

Alfredo
Title: Re: Neumann km54 faulty
Post by: fffaustino on May 16, 2018, 05:14:00 am
Hi again Kai and Klaus,

I have checked all the things i could, considering my absolutly lack of knowledge about vintage mics..., here you are:

- Voltages on the mic side: 119,7V B+, 3,85V heater
- Plate voltage measured directly at the plate: 33,5V
- Noise with capsule removed: different noise, a "hum" noise instead of the "fire, wind" noise when capsule installed (i screened with alu foil, maybe i did not do it correctly..)
- C6 meassured correctly
- I knocked the housing tube and no changes in noise spectrum or level, maybe i not knocked hard enough...

After all this checkings i power off the psu, waited half an hour aprox, reinstalled the capsule and housing and power on the mic again, and surprisingly the mic worked perfect during 5 minutes or so, then started the "fire, wind" noise again and no signal incoming..

Any clue?

Thank you very much, in any case, and sorry for my english

Alfredo




Title: Re: Neumann km54 faulty
Post by: Kai on May 16, 2018, 06:24:56 am
- Noise with capsule removed: different noise, a "hum" noise instead of the "fire, wind" noise when capsule installed (i screened with alu foil, maybe i did not do it correctly..)
... reinstalled the capsule and housing and power on the mic again, and surprisingly the mic worked perfect during 5 minutes or so, then started the "fire, wind" noise again and no signal incoming..
The cause for the hum should be insufficient screening, and is a sign of the tube working correctly.
If the "fire, wind"- noise has gone in that situation and comes back with the reinstalled capsule it's a clear hint that the capsule itself has a problem.

You can try to dry the capsule in an oven for 1/2h, not hotter than 90℃  (Celsius!, when does US finally switch to international units and metric system? :) ).
Really not hotter, cause there is plexiglass used inside which will deform if you go to hot.
Preheat the oven, control with a separate thermometer before putting the capsule in.
This will very likely not finally fix the problem, but it will give a hint what's really wrong.
The problem with nickel capsules (besides dirt killing the isolation) is build up of dendrites (fine metal spikes) between capsule and backplate under the influence of humidity and the polarization voltage.
They even suffer from diaphrag corrosion, showing up by little pinholes on the surface.
The edges of the holes are not straight and make more or less contact to the backplate. If you dismount the capsule you can see them under a microscope or strong magnifying glass.

I once fixed a capsule by closing a pinhole with something special.
This is about 25 years ago and the capsule is still working fine.
Title: Re: Neumann km54 faulty
Post by: uwe ret on May 17, 2018, 12:21:18 pm
To confirm the capsule as your problem simply exhale gently a few times at close proximity and monitor the noise. If the humidity causes a significant increase/change in the noise you will have a defective capsule, or, if you are really lucky, just a capsule contaminated with dirt and spittle residue, which may be able to be cleaned by an experienced microphone specialist.
Title: Re: Neumann km54 faulty
Post by: klaus on May 17, 2018, 12:44:44 pm
Agree with Uwe, and let's get on with the show. It is not that complicated to troubleshoot at least the general area of a malfunction- capsule or amp.

By the way: your plate voltage at 33.5VDC is low, especially if it's the newer (cathode biased) version.

Title: Re: Neumann km54 faulty
Post by: fffaustino on May 18, 2018, 06:06:07 pm
Many thanks Kai, Uwe and Klaus,

my mic is a km54a model converted to "c" (at least there is a c stamped following the "a").

I will do the breath test carefully and see what happen with the noise, i hope it is just a dirty capsule and it is possible to clean it..let´s see

thanks and best regards!
Title: Re: Neumann km54 faulty
Post by: klaus on May 18, 2018, 08:41:31 pm
Now that you confirmed that the mic is c-cathode biased, I am not liking that low plate voltage you reported.

A defective tube will have a plate voltage that low. The fact that the frying sound is fairly constant and never goes away also would support a defective tube, at least as a secondary if not primary cause for your noise.