Schoeps CMC641 120Hz hum

[camarada78]

Hello,

I've recently "fixed" a Schoeps CMC641 that had a hiss problem. Changed the caps with new Nichicon ones and the hiss was gone, but now i have a 120hz hum very audible in some preamps (not so much in others but its there). I have no idea where to start. the hum was not there before and its twin pair that has no noise at all.

I've tested with these settings:

Avid Carbon + Neve Shelford = very audible 120hz hum (peaking around -65dB with gain around +48dB)
Avid Carbon + Avalon 737 = not that audible but its there (peaking around -70dB with gain around +42dB)
- first it thought it needed a good clean. didnt solve it.
- i changed the caps again. didnt solve it
- put old caps back. hiss is back and now with the new 120hz hum

i think the hot air might damaged something on the voltage stage that started to give me this hiss.

Thanks, have a good day

 

[camarada78]

just to point things I ruled out:

- its not the pre, cable or the system, all other mics are dead silent, no 120hz or any noise whatsover with the same system.
- its not the capsule. i've changed the capsule with its twin and the problem remained.

 

[NoisyIndividual]

Only thing I can think of is the body is not grounded properly and it is picking up hum from outside.
Try connecting a flying lead from a good ground and connecting it to the capsule body, or anywhere where you can actually get a good connection to the metal.

Or check there isn't some ground cable/screw inside you didn't put back.


Or, it is the capsule itself that particularly needs to have a good ground to provide the needed Faraday Cage to prevent hum ingress. This is normally just screwed onto the body to make the ground contact. Maybe this is bad.

 

[radardoug]

120 Hz is the clue. Its not the capsule picking up hum, that would be 60 Hz.

 

[NoisyIndividual]

How do you think a mic might generate 120Hz from the phantom?
Quite a conundrum I feel.

 

[NoisyIndividual]

OP, do you really mean 120Hz hum, or did you make a mistake and it is actually 60 Hz?

 

[Gus]

What are the DC voltages at pin 2 and pin 3 to ground?

 

[NoisyIndividual]

These are going to be 48v or thereabouts.

The OP has probably misassembled the mic, but fortunately has a good one to compare.


I would carefully check each mic assembly to make sure something as silly as a insulating washer has not been left out or put in the wrong place.

 

[Gus]

I did not think I need to add with the microphone connected they should be about the same
CMC6 4mA with P48
https://schoeps.de/en/products/colette/microphone-amplifiers/cmc-6.html3.4K x 4mA = 13.6VDC so the pins should measure about 34.4VDC and be very close if one or both are much lower that is a hint.

 

[NoisyIndividual]

So how does that generate a 120Hz signal?

 

[camarada78]

here is a picture I took (couldnt printscreen at the time). notice the clear 120Hz peak there and no peak at 60Hz.

 

[camarada78]

I will check the assemply for errors.

 

[Khron]

So how does that generate a 120Hz signal?

Not so much "generate", as "pick up".

 

[Gus]

If one of the output transistors has an issue(open or shorted) and the output is unbalanced the CMRR gets real bad
and if one is shorted the power supply will be loadeed down more with most of the current in that leg