martthie_08

U87 P48 microphone electronics
« on: August 12, 2008, 07:55:14 AM »
My trusty U87 has been giving me a little trouble lately, I am getting random pops every now and then, that can sometimes ruin a otherwise perfect take. The capsule has been overhauled by Mr. Thiersch a little more that a year ago and I'd like to give the electronics a general overhaul.

I haven't seen a lot of talk here about U87 electronics, but I'd like to dive into this a little. As my mic has the battery option I believe this schematic would apply: U87schematic

For a start I think it wouldn't hurt to replace all existing electrolytics and tantalum caps as they all original 1979. As I'm not using batteries with it, there would be lots of space in the mic to use larger film/foil capacitors, which is one part I'd like to  do some experiments on. If anyone has specific suggestions as to what one may try out with this mic I'd be very interested to hear about them.

Thanks, Marten


Gus

U87 P48 microphone electronics
« Reply #1 on: August 12, 2008, 09:55:51 AM »
There are a few things you can do but what don't you like?  

The pops can still be the capsule having dirt at the clear edge or the foam ring inside might be breaking down.

First check the drain voltage.

Also I take notes when I work on or see a older u87.  If you don't mind what number fet 2n3819, 2sk105 etc. and what value source resistor(s) R11.  Sometimes R11 is two resistors for the correct value

jensenmann

U87 P48 microphone electronics
« Reply #2 on: August 12, 2008, 10:57:04 AM »
Some semiconductor cases get leaky over the years. Oxygene enters and alters the conductivity of the semiconductor partially. This may be audible as pops. Changing the FET may be the next step when recapping gave no improvement.
As Gus said, the resistors around the FET are matched to this particular FET.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

Gus

U87 P48 microphone electronics
« Reply #3 on: August 12, 2008, 11:57:25 AM »
Interesting about the fet case I have not seen that yet with a FET Neumann.
I have worked on some early 70's ones that looked like original parts and solder all they needed was a capsule cleaning.

jensenmann

U87 P48 microphone electronics
« Reply #4 on: August 12, 2008, 12:43:20 PM »
One of my U87s has a replacement FET. The tech who did the repair for the foreowner soldered it in with a cold solderjoint. This caused some funny things. The owner decided to sell it for very low bucks because he thought there are seriously bad things going on. Good for me  :grin:
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

martthie_08

U87 P48 microphone electronics
« Reply #5 on: August 12, 2008, 01:47:15 PM »
Thanks for your input, here is a quick pic of the circuit board, it doesn't look to be all original caps after all, click for larger image:




Quote
There are a few things you can do but what don't you like?


Well, I like it generally as a general purpose high res mic. Often I find with old gear, maybe specific to german broadcast stuff, that there were design decisions that had to be made to fullfill specific obligations at that time. Using the old gear in studios today may not need to meet these requirements and as technology has advanced today there might be "better" or at least a wider selection of parts available.
That said, I like this mic on certain singers, although sometimes I wished it had more character or directness (as other mics I use from time to time, but don't own). On the positive side, I don't get strong silibance with this mic, and I'm usually able to tailor it to my needs with EQ.

nevertheless, if there was things like "this part of the circuit is designed rather poorly, try this" or try a cheap ceramic cap on this cap instead of a hifi WIMA or something I'd definately give it a shot while I am working on it...

Quote
If you don't mind what number fet 2n3819, 2sk105 etc. and what value source resistor(s) R11. Sometimes R11 is two resistors for the correct value


yes, R11 is at 2K2 ohms and T1 is a S2436 TIP940 transistor.


Quote
As Gus said, the resistors around the FET are matched to this particular FET.


yes, I have seen the annotation on the schematic. Is this something I can check on the designated "calibration input" across R10? What am I looking for here?

jensenmann

U87 P48 microphone electronics
« Reply #6 on: August 12, 2008, 01:57:32 PM »
I´m not an expert to judge this but from what I see the calibration point has nothing to do with the FET biasing. I would chose R11 that I get 10V between R12 and C9.
Jens
Quote from: PRR
The tubes of course don't care what frequency they distort

martthie_08

U87 P48 microphone electronics
« Reply #7 on: August 12, 2008, 02:36:36 PM »
Quote
I´m not an expert to judge this but from what I see the calibration point has nothing to do with the FET biasing.


yeah, I thought so... (edit: the part on the calibration point having nothing to do with the FET biasing :oops: )

Quote
I would chose R11 that I get 10V between R12 and C9.


yup, it's pretty much spot on: 10,19V

for the recap I'll order some caps tonight, any specific brands come to mind for the Tantals? banzaieffects, only have the generic yellow no-names...

martthie_08

U87 P48 microphone electronics
« Reply #8 on: August 25, 2008, 07:57:27 AM »
ok, a small update for those who might be interested:

the orange electrolytic (10uF / 63V) in the pic above is not original. It was a replacement for C12 (tantal 5uF / 25V), I put it back to stock value, but measuring the voltage across the cap I got readings of close to 30V so I should really replace it with a 35V rated cap soon.

The initial problem went away after replacing C7 which was a 1uF / 35V tantal cap. Would this be a good place for experimenting with some different type of cap?

JLM Audio

U87 P48 microphone electronics
« Reply #9 on: August 25, 2008, 08:56:29 AM »
The U87 in the photo up the page has a black round constant current instead of R17 (56k) resistor. This version usually will have a 33v zener instead of a 24v as shown in the normal original U87 battery version. The voltage rail is usually about 31v in this version. So yes the 5uF power rail smoothing caps will need to be 35v types. Changing all tants to new will usually bring the mic back to working with the original extended clear high frequency it had when new.
Joe

http://www.jlmaudio.com/shop
Capturing Audio without Injury


martthie_08

U87 P48 microphone electronics
« Reply #10 on: August 26, 2008, 05:08:24 AM »
Very good info, thanks :guinness:

What is in the inside of that black round thing anyways, I've been wondering about that all along?

JLM Audio

U87 P48 microphone electronics
« Reply #11 on: August 27, 2008, 06:17:13 AM »
Quote
What is in the inside of that black round thing anyways, I've been wondering about that all along?

Usually a FET with its Gate connected to its Source as this give you a constant current device with 2 pins.
Joe

http://www.jlmaudio.com/shop
Capturing Audio without Injury


 

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