Neumann U87 Service docs? Component layouts?

[mrclunk]

I've a pair of U87 P48's on the bench.
A 70's model (circuit diagram 930-16) and an 80's model (diagram 930-21)

i've found the schematic for the diagram 930-16 but can't find one for the 930-21. Anybody have it?

A component layout would be handy too... I'm not sure which cap is which, some are labeled some aren't. The 70's mic has colour coded tants without writing.


I've been reading the various posts on the 87 circuit and seems like it would be a good idea to replace the tants and electrolytics.
Also i'll try swapping C7 for a PET film cap too and possibly increasing C4 also.

Few questions:
Is there  particular type of tantalum cap i should be using to replace the originals?

Do i need to re bias the fet after replacing these caps?

I'll do some test recording before i change anything for comparison and post the results.
cheers

 

[volker]

Do i need to re bias the fet after replacing these caps?

No.

 

[Gus]

Maybe you should not be working on something of value if you can't trace the circuit of the u87 and know what effects bias.  This might come across as harsh but is not meant that way.  I suggest more reading and learning then maybe working on them.

What I would do is read the Vishay AppNotes about jfets.

Is there something wrong with the microphones?  Don't believe the web about changing tant caps for electros always being better sometimes you might want that Tant "sound".  It is more important to learn and know why certain caps can have a sound and what affects the sound by where it is in the circuit, then selecting the cap to use.  I often tune a microphone by not using the "best" type cap sometimes but sometimes I do.

The older 70's 87s I have worked on had more issues with dirt on the capsule membranes than part issues.  The tricky part is when the capsule is that old you need to know how to clean it correctly.  The stuff posted on the web about cleaning will often if not always destroy a older capsule.  I learned by buying capsules to learn on.

It is more important to have a known good capsule before tuning other parts of the circuit.

Are the microphones set as 50, 150 or 200 ohm output?

It is tricky to open the top when the plastic gets older as not to snap the pin on the two switch tops.  The first thing I try is an optic brush to remove the dust etc from the capsule you need a light touch

 

[mrclunk]

cheers guys, i appreciate your remarks.
no there's nothing wrong with the mics really.

i didn't think i needed to re-bias, just checking...

They both have the output resisters bridged for more output.

only reason i was looking to replace the tants is because their 30+ years old.

Gus from ur posts over the years i realize u know ur shit! just nt confident enough to mess with these without reassurance.

 

[Gus]

Tants tend to be good or very bad. in my close to 3 decades of repairing equipment I don't recall a leaky tant only good or shorted or burned ones.  Maybe others have seen different.

I am not posting not to change the cap you might like a different type maybe a PET maybe a polypro a polycarb etc.

The Vishay app notes about fets are a very good read and you can find them on the web.  Another good book is the 1982 NS transistor book.  Look for Vishay AN102

What you might like to note with your two microphones is the source resistor value and the transistor number.  The source resistor is soldered to the two posts from the top of the board bottom right of the jfet.  That is selected for the drain operating point, sometimes you will see two in parallel to get the needed value.  If you look at one of the older 87 schematic on the web you will see 10VDC at the drain and a note to select the source resistor.
http://recordist.com/ampex/schematics/neumann/u87sch.jpg

I just "repaired" an older u87.  It turned out the battery phantom switch was not making a good contacts and the microphone cut in and out, all I did was switch it back a forth a few time to clean the contacts.  Sometimes all that is needed is cleaning contacts and capsules on older microphones

 

[Mike Cleaver]

Here's some stuff I have on file.

 

[Mike Cleaver]

More....

 

[Mike Cleaver]

One more.
The instruction manual is available as a .pdf on the Neumann site.
It's too large to attach but with a little searching, it will turn up.

 

[SparkleBear]

Sorry to necrobump this thread... but did we ever find the u87 930-21 schematics? I have a troublesome u87 thats needing some loving..

 

[JerryPbury]

Sorry to necrobump this thread... but did we ever find the u87 930-21 schematics? I have a troublesome u87 thats needing some loving..