mic experts help needed- Neumann U-47 Fet fault

[maxkriza]

Strange fault on this lovely vintage U-47 fet mic. Every 5-10 minutes there is a loud pop that last very
briefly, otherwise mic sounds great. Any idea what may be the cause of it? I have no experience with these
rare mics. Help much appreciated!
Max

 

[radardoug]

Most likely capsule. To prove it, remove the capsule from the headamp, it unplugs after you take out a few screws. Cover the top with a peice of aluminium foil which is grounded, don't short out the socket. See if the preamp is quiet.
Also you can do the breath test. Hold the capsule close to your mouth, breath out gently with lots of moisture in your mouth. If the mike stops working or makes lots of noises, its a dirty capsule.

 

[Steph.B]

I agree with the tips given by Max, breath test would be particuraly helpful to determine if the problem comes from the capsule (probably the case here)

 

[maxkriza]

Did the breath test, mic output is much reduced now. does this indicate a fault?
does it mean the capsule is dirty? (it looks pretty clean).
I left the mic to dry out and will check again later.
your help much appreciated
Max

 

[RuudNL]

It is 'normal' that condenser microphones are sensitive for high humidity.
In the radiostation I used to work for, we had a lot of Neumann U47i microphones.
During the summer, when the air humidity was high, it was sometimes impossible to use them, because they only produced a very loud random noise.
The only solution was to use Sennheiser MHK416's. Because of the HF working principle, they had no problem with the high humidity.

The fact that the output is reduced after the 'breath test', proves that some leak is introduced as a result of condensation.
I know exactly what you mean: loud pops and a temporary reduction in sensitivity...
You could put the microphone in a closed box with a bag of silicagel or some order moisture absorbing material to let it dry.

 

[maxkriza]

Thanks, loud pops indeed every 10min or so.
Mic is currently in a dry space so I will check it tomorrow.
all the best
Max

 

[radardoug]

Your tests indicate the capsule needs cleaning. There are several very expert people who do this round the world. It may also need reskinning, cleaning is sometimes not possible because of pinholes in the diaphragm.

 

[maxkriza]

Hi
Managed to clean the capsule, there is no change and the mic still popping every few 10-20 minutes.
I decided to test it with a new capsule, got the same fault! so now quite sure its the electronics.
Any idea what can it be and how to test for it ?
I suspect the tantalum caps from past experience but would like to hear some other opinions
thanks

 

[tubestation]

Hello !!
I once had a Problem with my Km54.Every 5-10 Minutes it did put out a 0 dB pop.After checking the capsule(I did put another one to the Amp) witch wasn t the Problem ,and it is not easy to change the Ac701to try another one,I messured all Resistors,et voila!! Kathoderesistor was faulty.I put a new one and the Problem was gone.I hope that you have the same Problem,good Luck!!
Greatings

 

[maxkriza]

thanks!

my mic is the U47 FET version, I will test all resistors and see

all the best
Max

 

[pucho812]

I would be more suspicious of caps. Check your caps.

 

[maxkriza]

how do you test those tantalum one's?

 

[maxkriza]

on a close inspection there is a 100m resistor (R3) which has a black ring all around the middle.
multimeter can not verify it is faulty. how can I test this and where can I get one in the UK?
thanks

 

[pucho812]

you have to desoler one side so that it's out of circuit and can give an accurate reading on a multimeter. If it's shorted it will not give an ohm reading at all. If it's fault you will get a reading way off.

 

[zebra50]

And you will struggle to get an accurate reading on a 100 Meg resistor with a standard multimeter. Normally they go up to about 40 Meg or less.

Radio spares and Farnell both sell 100 Meg resistors.

 

[mjrippe]

If you have a 10M resistor on hand, put it in parallel with the 100M and it should read ~9.09M if the 100M is good.

 

[bockaudio]

I would be more suspicious of caps. Check your caps.

seconded.

 

[poctop]

I would be more suspicious of caps. Check your caps.

seconded.

I thinka good thing to do is to get a cheap chinese K47 style capsule and put it in  ,  a not too bad looking chinese capsule will most likely tell you if it is a capsule problem wich is seem to be imho, Measure all reference voltage  and make sure they are stable , then have a look to the drain voltage and if you see the voltage swings very much when the pops occurs then it is probably the capsule. looks like something is charging up and discharge.
Checking the caps would complete this work.
hope this helps,
Dan,

 

[maxkriza]

100m resistor is probably ok.
I've replaced the capsule with 70pf cap and it still pops every 10-15 minutes or so.
it's tricky to catch this on a scope. NOT AN EASY ONE ....

 

[tubestation]

Hello!!
Looking at the schematic and after the troubleshooting you have already done,i would have a closer look to the high z capsule polarisation section. R1, R2,18p cup etc. I agree with Poctop that probably a  static charge/discarge of a (high z) resistor or a condenser cause the pops.. I rember i once had a problem with a Geithan Transistormicpre.It made also a very strange brizel and poping time by time.The unit caries 2 Pcp cards .one is the powersupply the other the amp.I swapted all carts with working one s. the problem stayed until i soldert new all conections on all plugs .good your capsule isn t faulty.in the worst case,recap the whole mic?
Greatings