U87 noise problem

[valco]

Hello !

I hope everybody is  surviving the quarantine  and has time to do some DIY 

I´ve got an U87 with the 930 - 18 Schematic which just makes a lot of noise. It sounds like some kind of grounding noise. I can´t quite figure out what the problem might be ,  I got a couple of other u87 ´s  that has the newer schematic that works fine and i have compared the guts of them. It seems like this U87 have had some kind of modification around r17 , but I´m not sure.

Is there some knowledgable people in here that can pinpoint me to where i should start fault searching ?

 

[gyraf]

What do you mean: "grounding noise" ?

The circuit board looks like it could benefit from some cleaning around the modification..

/Jakob E.

 

[RuudNL]

Add a 68 pF capacitor between the gate of the FET and ground.
Remove the capsule head and place the body tube.
Screen the open end (where the capsule head used to be) with a piece of aluminium foil.
Check again for noise.
Is the noise gone: the cause of the noise is a leak current in the microphone capsule. (Pollution)
Is the noise still there: check and clean the high impedance parts and check electrolytic capacitors.

 

[valco]

What I meant by grounding noise was a noise that sounds like something is not connected to ground.  A loud hum.

I tested RuudNLs method. Thank you very much and it seems to be something with the capsule. The noise also changes when i move my hand closer to the capsule. So its most likely a current leak in the capsule.
Is there anyway to repair this or do i have to send it to someone with more extensive knowledge of  the capsule ?

 

[gyraf]

probably it's a lack of shielding  in the head basket, that is, poor ground connection?

/Jakob E.

 

[RuudNL]

Yes, it sounds like the microphone body (and/or headbasket) isn't connected to pin 1 of the XLR.
Check the continuity with an ohm meter.

 

[valco]

Thank you very much for the advise.

I went over several of the ground connections and measures for resistance . After some cleaning its working again without the ground noise.
The cardioid function doesn´t work , but the omni and figure eight does  .There is also a loud pop when i change between these configurations. So after a bit og investigation it seems that the front skin is not responding as well as the back skin, its hard to tell if its completly silent.

Do you guys know what might be causing this ?

 

[Khron]

Some photos of the capsule (with the headbasket removed) might help..?

 

[Gus]

Most likely the front of the capsule is dirty. The front is used for cardioid and both side are used in fig 8 and Omni so you will pickup sound if the back side is clean.

I would guess the figure 8 and omni pattern are not correct.

 

[valco]

Thank you very much for all the help. 

I noticed when i took the capsule out of the headbasket that the solder on the connection point on the capsule had loosened.  That was probably part of the problem the whole way , but it looked good when i first inspected it . With no indication that it was bad. So i resoldered that one and  cleaned the capsule with destilled water afterwards and now it works perfectly.