Help with U87

[chownmander]

Original DIY-U87 build thread here - https://groupdiy.com/threads/neumann-vintage-u87-clone-build-thread.48030/

I need some guidance here. I've build the mic and tested it - works in my DAW.
However, the audio is steeped in a (quiet but noticeable) white-noise interference sound beneath the recording audio.

I plugged in another DIY-U87 into my DAW straight after, and it sounds crystal clear. No white noise.


So far, I've had assistance from someone who has previously built a few DIY U87.
- changed the transformer for a known working one.
- swapped transistor with a new one
- checked all leads from board-to-board, and capsule-to-board
- checked switch PCB connections
- general tidying of all solder connections


Not sure where to go from here. Any suggestions?

Many thanks

 

[micolas]

Original DIY-U87 build thread here - https://groupdiy.com/threads/neumann-vintage-u87-clone-build-thread.48030/

I need some guidance here. I've build the mic and tested it - works in my DAW.
However, the audio is steeped in a (quiet but noticeable) white-noise interference sound beneath the recording audio.

I plugged in another DIY-U87 into my DAW straight after, and it sounds crystal clear. No white noise.


So far, I've had assistance from someone who has previously built a few DIY U87.
- changed the transformer for a known working one.
- swapped transistor with a new one
- checked all leads from board-to-board, and capsule-to-board
- checked switch PCB connections
- general tidying of all solder connections


Not sure where to go from here. Any suggestions?

Many thanks

Did you clean the high impedance area with isopropyl alcohol? (capsule connections, attenuation pad switch , pattern switch pcb, jFET input circuit, etc.)

 

[Khron]

Not sure where to go from here. Any suggestions?

Photos of the build? Especially the JFET and capsule connection areas...

 

[Voyager10]

Also, if you could post a copy of the actual schematic & component values you used, that would help.

 

[chownmander]

Did you clean the high impedance area with isopropyl alcohol? (capsule connections, attenuation pad switch , pattern switch pcb, jFET input circuit, etc.)

Hi, yes it was cleaned

 

[chownmander]

Photos of the build? Especially the JFET and capsule connection areas...

 

[chownmander]

Also, if you could post a copy of the actual schematic & component values you used, that would help.

This is the BoM I used from Mouser - https://bit.ly/3WsYbj3
The obsolete items which are listed, I replaced with -

1
RoHS1660-GS1/4LC686J
GS1/4LC686J
KOA Speer Metal Film Resistors - Through Hole 1/4 WATT 68M OHM 5%

2
RoHS2594-2222-021-28109
MAL202128109E3
Vishay Aluminum Electrolytic Capacitors - Axial Leaded 10uF 63V 20% Axial

3
279-H4P560RFCA
H4P560RFCA
TE Connectivity Metal Film Resistors - Through Hole H4P 560R 1% 50PPM

4
RoHS172-T93YA-25K
T93YA253KT20
Vishay Trimmer Resistors - Through Hole 3/8" SQ V/ADJ 25K


I mainly followed the steps taken to build on this page, with it being same PCB and microphone - https://www.vintagemicrophonepcbkit.com/Neumann U87 Build.htm


Hope this helps?

 

[Khron]

Hi, yes it was cleaned

Was it? Then what's all the brown(?) stuff on the PCB, in the 4th photo?

 

[ElSmurf]

The high amount of flux on the back of all three PCBs is not helping. I suggest you start by completely cleaning all of them, especially around the high impedance sections mentioned above, and then check if your problem still persists.

And as @Khron mentioned, that brown stuff is problematic too, looks like a lot of overheating went on during your build.

 

[micolas]

Hi, yes it was cleaned

On the contrary, there is a lot of dirt in sensitive areas. Repeat the cleaning procedure as many times as necessary until the PCBs are perfectly clean.

 

[Delta Sigma]

Is this thread locked?
https://groupdiy.com/threads/neumann-vintage-u87-clone-build-thread.48030/

If not, you should be asking this question there. If someone has the same problem as you, they will be searching the build thread and not finding the answer.

 

[micolas]

The thread is active,the last post is a month old. U87 related answers are there.
@Delta Sigma is absolutely right, the same problems keep coming up in new threads and it's difficult to search for a specific topic.

 

[Drosselmeier]

The photo shows that the FET gate is soldered directly to the board
The arrow indicates the place where the fluoroplastic turret should be located for soldering the gate

 

[ElSmurf]

The photo shows that the FET gate is soldered directly to the board
The arrow indicates the place where the fluoroplastic turret should be located for soldering the gate

Good catch!