Phantom Powered Mic with problems on some PS

[DerEber]

Hey guys,

I am having problems with an Condenser Mic recently.
It doesn't work on some 48V PS and give strange noise.
It works on all of my small mixers, but it shows problems on my main desk were some Neumann 24V->48V cards take care of the phantom power.

What I did was to meassure the Voltage on the Mic while plugged in.
The Voltage on the mic drops to 22V. I know it is normal to have a voltage drop due to the 2x6,8k resistors.
But I tested another working condenser and the Voltage drops only to 42V.
But of course it is a different circuit.
The circuit on my maybe "borken" mic looks somewhat like a simple 1 transistor km8x like thing.
So my question is now if such a voltage drop is reasonable for a circuit like this. If no, then maybe some tantal is gone bad and sucks all the amps.
The other possibility is that there is something wrong with my desk PS. As the mic is working on other mixers.

I just want to sure if my measurements look like a broken mic befor I start soldering.
None of the tantals measure 0 Ohm. But maybe they are just not yet completly gone.

best greeting,
Stephan

 

[Gus]

A very good link for checking phantom power
http://shure.custhelp.com/app/answers/detail/a_id/3789/~/how-to-test-phantom-power-voltage-and-current

 

[DerEber]

So 24V/3400Ohm (2x6.8k) = 7ma
This is maximum that can be delivered, so my mic is broken or very very hungry.
A Km84 is 0.4mA.

Ah...there is also a series resistor on the mic power rail.
Its 3.2k. So this is the other half of my voltage.
I'll check for shorts tomorrow.

Just why does it work on other mixers?

 

[DerEber]

Ok, I swaped the cap that could lead to a supply voltage breakdown. Nothing happened.

The cap was rated for 16V. That would only fit if it is supposed to knock down the phantom voltage like it does,
But I can hear the 48V card "singing". So maybe it's the card. Some later channels on the desk work. They are fed by another 48V card. Time to swap the cards.
I also have problems when patching the outputs of my converter and leave 48V on.
But I always thought it was the converter not liking the phantompower and never bothered.

 

[Gus]

Did you check your different phantom supplies voltage and current it can supply using the Shure link writeup?
The meter set to current and measuring 2 to 1 and 3 to 1
The phantom supply might not be able to supply 10ma.

What microphone uses more current?

Do you have the  specifications of the microphone? It might list the current.

 

[DerEber]

I only measured the voltages. Thanks for the link Gus.
As the voltage drop is equal on 2 pin and pin 3 I suppose it delivers the same current to both pins...
Wait....... this point is tied together over the output transformer. So maybe I better measure it.

The mic is an old rebranded MBHO.
Maybe I can get informations via Haun.
But what much can be done with only 1 Fet.

 

[[silent:arts]]

Replace those old Neumann 48V cards (with something else). Those are from the beginning of switched PSUs, and known to make problems.
Since the Mic is working with other Phantom Power sources ...

 

[DerEber]

Oh, good to know.
Hm..... any drop ins for those?
Wait, I have to look at them.
It is possible, that they make 12V->48V as my desk was equipped with 12V Phantom.
And there is also already a PS before that to get the DC from the mains. I never serviced them.
Maybe something wrong here too.


Now the biggest task is to move all the stuff sitting on the side board that hold those cards.
;D

Thats when you buy something from ebay that even works.

 

[[silent:arts]]

Are those GW2448 cards?
They convert 24V DC to 48V DC.

 

[DerEber]

No ...... no GW2448.

The whole card is labeled N393.0
It is there to replace the RFT LS 690/691 that provide 12V Phantom power for two mics derived from another 24V supply.
It's a 700er desk.

The N393.0 holds two caps, a small coil and a small Neumann SMD card.
The SMD card is covered in red paint and labeled "DCH 148  #AD"
It could be that the caps on the N393.0 are from 1989. But thats a guess. It says *89*.

I have a GW2448 somewhere.  I am sure it locked at least very similar. I'll take a look if I find it.

Now I suddenly remember many situations were we had strange things happening with condensor Mics.
:-\

best Greetings,
Stephan

 

[[silent:arts]]

Go and build yourself a linear 48V PSU to replace all those old switching PSUs.
You will not regret it.

 

[DerEber]

Thats a good, straightforward advice! Thanks Volker.

You mean throw out the 27 cards that handle phantom-power and build it on one PCB and some resistors?

Sounds like "2019" anyway, but I will consider it.
I still have some FET 80 PS for my patchbay, so this can cover most of the situations until then.

Problem solved. Thanks a lot!
8)

 

[DerEber]

Ok, I moved cards around and some seem to work on some channels, but not on others.
Two cards make strange noise.
Seems like I will have to redo the whole thing soon.

Would be cool to get some people together using the same desk, but maybe I am the only one around still using a 700 RFZ.
Anyone knowing the measures of the "Zeibina-Buchsenleiste".......no?