U87 high frequency problem

[samgraysound]

Let me ask the obvious - Is the capsule original?  If it was changed at some point (to a clone perhaps) it may have a different response.

I think it's original?  It has as little tag on the underside with the Neumann logo and says:

Georg Neumann GMBH
KK 87 A 10575

 

[samgraysound]

820pF for -3dB at 16KHz is a very unusual value. (In my U87's there is only 160pF for -3dB at 16 KHz.)
I suppose something else is wrong.
The 2N3819 is fine, Neumann used this type in the U87.

Ok. Any suggestions on what to check? I have checked value and continuity on basically everything in this part of the circuit, the only reading I got that was off was C2 at 8900pf, but putting a 1000pf cap in parallel to that did nothing.

 

[RuudNL]

It has as little tag on the underside with the Neumann logo and says:

Georg Neumann GMBH
KK 87 A 10575

That sticker doesn't say anything!
It is just the serial number of the complete headbasket assembly. (KK= Kapselkopf).
But that doesn't mean the capsule inside was never changed!

 

[samgraysound]

That sticker doesn't say anything!
It is just the serial number of the complete headbasket assembly. (KK= Kapselkopf).
But that doesn't mean the capsule inside was never changed!

I swapped the capsule from a working unit and the frequency response stayed the same

 

[samgraysound]

I think I found it!

C5 had been changed to 0.015uf and R9 had been changed to 1.2k.

 

[RuudNL]

It seems your U87 is an 'early U87us' version, pretty rare...

 

[samgraysound]

It seems your U87 is an 'early U87us' version, pretty rare...

well shit. The components didn't look original though. Do you think I should leave it be then?

Sam

 

[Khron]

Why not either rebuild it with original-value parts, or with U87(i)-value parts? Or... what was your initial intention?

well sh*t. The components didn't look original though. Do you think I should leave it be then?

Sam

 

[samgraysound]

Why not either rebuild it with original-value parts, or with U87(i)-value parts? Or... what was your initial intention?

My original intention was to get it back to stock working order. It orginally had a problem in the regulator circuit which I fixed, then I did a frequency response test and started trying to figure out why it didn't match the other one I was comparing it to.

Sam