R/E/P Community

Please login or register.

Login with username, password and session length
Advanced search  

News:


Author Topic: Neumann km54 faulty  (Read 1182 times)

Offline fffaustino

  • Newbie
  • *
  • Posts: 16
  • Real Full Name: Alfredo Diego
Neumann km54 faulty
« 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

Offline Kai

  • Sr. Member
  • ****
  • Posts: 292
Re: Neumann km54 faulty
« Reply #1 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)
« Last Edit: May 11, 2018, 05:42:24 pm by Kai »

Offline Kai

  • Sr. Member
  • ****
  • Posts: 292
Re: Neumann km54 faulty
« Reply #2 on: May 11, 2018, 04:20:27 pm »
Schematic Neumann KM54a

Offline klaus

  • Moderator
  • Hero Member
  • *****
  • Posts: 1548
Re: Neumann km54 faulty
« Reply #3 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.
« Last Edit: May 12, 2018, 06:37:29 am by klaus »
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline fffaustino

  • Newbie
  • *
  • Posts: 16
  • Real Full Name: Alfredo Diego
Re: Neumann km54 faulty
« Reply #4 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

Offline fffaustino

  • Newbie
  • *
  • Posts: 16
  • Real Full Name: Alfredo Diego
Re: Neumann km54 faulty
« Reply #5 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





Offline Kai

  • Sr. Member
  • ****
  • Posts: 292
Re: Neumann km54 faulty
« Reply #6 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.
« Last Edit: May 16, 2018, 08:05:58 am by Kai »

Offline uwe ret

  • Newbie
  • *
  • Posts: 42
  • Real Full Name: Uwe Sattler
Re: Neumann km54 faulty
« Reply #7 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.

Offline klaus

  • Moderator
  • Hero Member
  • *****
  • Posts: 1548
Re: Neumann km54 faulty
« Reply #8 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.

Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline fffaustino

  • Newbie
  • *
  • Posts: 16
  • Real Full Name: Alfredo Diego
Re: Neumann km54 faulty
« Reply #9 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!

Offline klaus

  • Moderator
  • Hero Member
  • *****
  • Posts: 1548
Re: Neumann km54 faulty
« Reply #10 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.

Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline fffaustino

  • Newbie
  • *
  • Posts: 16
  • Real Full Name: Alfredo Diego
Re: Neumann km54 faulty
« Reply #11 on: May 28, 2018, 12:33:17 pm »
Hi again,

I replaced the ac701k with a good one but the problem still remains the same. The mic worked great for 2 or 3 minutes (with better noise floor than with the old tube) but after those minutes the loud frying noise appeared again.

I have not try the capsule´s drying method Kai wrote (it scares me a bit), but i will probe it next days and let you know if the mic goes better...

Thanks much!!

Alfredo




Offline klaus

  • Moderator
  • Hero Member
  • *****
  • Posts: 1548
Re: Neumann km54 faulty
« Reply #12 on: May 28, 2018, 01:55:53 pm »
Before you attempt capsule cleaning, please substitute the capsule with a 40-50pf capacitor. That will determine whether it's a capsule of amp problem.
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Offline Kai

  • Sr. Member
  • ****
  • Posts: 292
Re: Neumann km54 faulty
« Reply #13 on: May 28, 2018, 05:14:09 pm »
I have not try the capsule´s drying method Kai wrote (it scares me a bit), but i will probe it next days and let you know if the mic goes better...
Your reply #5 99% confirms a faulty capsule for me. Drying it will improve isolation for some time and the noise should be gone until humidity comes back.
If you fear to break it go for 70°C for one hour, then you're absolutely on the safe side.
Of course, plugging in a 50pf cap is a simple quick test that should be done before.
 If drying doesn't doesn't help or the noise doesn't change with the dummy cap:
- does the other tube show the same voltages as before?
- Did you connect pin 6 to ground? If yes, open the connection to check for a faulty C3.  If no, connect it in the female plug.

@Uwe:  I don't think the breath test isolates the problem in this case because it only adds a little to an already strong fault.
« Last Edit: May 28, 2018, 05:33:32 pm by Kai »

Offline klaus

  • Moderator
  • Hero Member
  • *****
  • Posts: 1548
Re: Neumann km54 faulty
« Reply #14 on: May 28, 2018, 05:55:55 pm »
There you have it: three cooks in the kitchen. Which is the right recipe? Any bets?

I'll put

50 cents on dish #2: electronic issue,
20 cents on #1: dirty capsule and
30 cents on: none of the above
« Last Edit: May 30, 2018, 04:18:29 pm by klaus »
Klaus Heyne
German Masterworks®
www.GermanMasterworks.com