Mic&Mod U87 help

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fredwreck

Member
Joined
Dec 21, 2015
Messages
12
Hey guys, I put together a U87 from Mic and Mod, and Im getting very very very faint sound.  Lots of white Noise.

Also, when I adjust the 10k pot to get it to 60v.  I have it all the way cranked and it only reads 55volts.  Any ideas would be appreciated.  Thanks in advance!
 
You could've edited your previous post, and remove / re-do the attachment.

Is the resistor at the bottom of the board, in series with the input voltage?
And is that diode next to it a standard diode, or a Zener?
Or can you snap a photo of the bottom of that board?

For the noise part of the issue, a photo of the JFET area of the circuit might help.

I don't suppose you have a schematic of this circuit, do you?
 
Thanks for the reply!

Yes the Diode is an 18v Zener. Ive attached the schematic of the DC Converter.  I have a schematic for the rest of it too.

 

Attachments

  • Screen Shot 2019-08-26 at 5.25.14 PM.png
    Screen Shot 2019-08-26 at 5.25.14 PM.png
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Kinda-sorta had a hunch. 

Well then, given that schematic, i see two choices - either swap out the zener (if not even remove it altogether), or reduce the value of that 6.8K resistor that's in series with the trimmer.

Easiest step might be the latter, by soldering another resistor in parallel with the 6.8K, something in the 4.7k-10k region.
 
By the way: the polarisation voltage isn't critical.
55 V instead of 60 V will only give an output that is 0.8 dB lower than with 60 V!

Also check if your phantom voltage is really 48 V. (Measure between pin 2 or 3 and 1 without anything connected.)
Many (cheaper) mixers have a low phantom voltage.
Do you measure 18 V over the zener diode?

Normally it should be possible to vary the polarisation voltage between 50 and 70 V.
 
On the note about absolute voltages, i posted this not long ago in another thread in The Lab, but sounds like it applies very well in this cas too :)

Khron said:
https://www.youtube.com/watch?v=bywYBtkfsWA

One major point is made starting around 2:31 ;) (but the whole thing is worth watching)
Click to expand...
 

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