old u87 question

[Gus]

I have an old u87 on my bench. I have a few questions. The person told me it is from 1972 and has not been cleaned or seviced. It has the clear plastic not the grey weird looking resistors, look almost like mica box caps with color bands at the edge.

I has a low output and was very dirty I cleaned it with distilled water and an optics brush. It had the dropping out with moisture problem you should have seen the dirt at the edges. The dropping out is gone but it still has a low output. Does this era U87 have low output. the other battery U87s I have been in have had the gray plastic and a k170 fet

The 2n3819 fet has a 2.2 pf cap soldered from D to G it looks like it was a factory job the solder all looks like the same person worked on the micrphone at neumann. I want to disconnect the cap because it drops the gain being a feedback element. Does anyone know if it has another purpose like a RF filter etc?

 

[Bo Hansen]

Gus,

The old U87 issues shall have same output levels, but the new U87Ai have aprox. 8 dB higher output because the higher capsule polarization voltage (increased from 47 to 60 volt)

Check the output connection for the transformer secondary side, if it is strap for 50 ohm, you have lower output gain.

Check also the electrolytic capsitor from the FET source to ground.

--Bo

 

[Gus]

Thanks Bo! it was strapped for 50 ohm.