RCA 74B humming

[echorec]

Hi, I just got a dead 74B. I replaced the internal cabling and fixed a cold solder joint and now it sounds quite nice. But it is very sensitive to magnetic fields, screens etc. It is like playing an electric guitar with single coils in a studio control room... You have to search for a silent spot...

I have another RCA which does not behave like this so obviously something is wrong here.

Maybe I connected the wrong outputs from the transformer? I wired it the same way it was when I got it.

There is a sticker inside the mic with instructions for different impedances but the (four) output pins on the transformer have no markings so that is of little use to me.

Any ideas?

Thanks in advance,

Gunnar

 

[rodabod]

Is the case and grill properly earthed? Does the interference change when you touch the body?

 

[echorec]

Thanks for the reply.

Yes they are properly earthed and nothing changes when I touch the body.

 

[emrr]

I read this at first thinking you meant a BA-74B power amp; sorry.

 

[Marik]

Those things ARE sensitive to the fields.

Does it have a proper hum-bucking structure?--the two thick copper wires coming off the top of the ribbon along the sides?

A couple years ago I worked on one and it did not have a mu metal transformer enclosure--look at that.

 

[echorec]

Thanks for replying Marik.

The humbucking structure looks like the one you are describing.

Here is a picture of the enclosure. Does it look right?

 

[Marik]

It is completely open on one side. I'd try and cover it completely and see if it helps.

Another thing to try is to ground one of the ribbon's clamps.

Did you try other ribbons at your space? There are some rooms notorious for that problem and no amount of shielding helps and all the ribbons tend to hum (like one in the previous house I used to live).

Does your mic hums in other places (known good with other ribbons)?

 

[echorec]

Hi, I´ll try your suggestions.

I have not tried this one at another place but I have a 77D that does not hum at all.