Brauner Phantom V, I need a schematic!

[Nollman]

Well that seems to have done it! The previous owner got the rear membrane and the backplate leads reversed. Now it sounds great in all patterns. Thanks to all of you for the help in getting this thing cranking again. It sounds quite a bit like my Gefell UM70s which I like quite a bit.

 

[Gus]

What meter are you using to measure the voltages?
The reason I ask is the math does not work out at the J201.
There is a chance the meter is loading the circuit down.
Do you know the input resistance in the DCV setting?. This is if I traced the circuit correctly.
I traced a 19.1K from the Zener to the drain and you measured 3VDC at the red wire and 1.1VDC at the drain.
Is there any other markings on the Zener diode? I could not find it with a search for 79CX28
.4VDC/2.7k =.148mA
3VDC-1.1VDC/19.1k =.099mA
The source and drain current in the J201should be close to the same

 

[Tim Campbell]

So I was correct about this?

 

[Nollman]

My Multimeter is a Kilter. My talent with it is limited. All previous measurements were using ground as a reference. If I test across the terminals of the transistor I get the following values;
J201 drain to gate .06v
J201 drain to source .04v
BC264D Drain to gate 1.5v
BC264D drain to source 2.5v

 

[Nollman]

So I was correct about this?

Basically, yes. Thanks for helping out! It saved me a whole lot of shipping to Germany and back for a minor problem.

 

[Tim Campbell]

Yes. The capsule membranes were shorted as I suspected just through a miswired switch instead of by a fault. This is a lesson to remember. If you are getting strange polar pattern response check the capsule and all related wiring to the capsule first. By determining which lead was ground, which was app. 60v and which was variable the switch wiring would have been evident.

I cannot imagine a component failure or circuit fault that would cause your original pattern problem and then cure it by removing the back membrane connection.

 

[Gus]

Nollman

Can you check the drain and source voltages to ground again?
If what I traced is correct some of the ohms law math does not make sense
When I get some time I will check my trace again
What model number is the Kilter meter?

I am thinking about the low drain and supply voltages for the J201.
If you look up the curves 1.1VDC and around .15mA it is not in the "Triode" range it looks to just to the right of the "Triode" part. I guess I could measure a J201 at this voltage and current.

Is there any other markings on the Zener diode? I have not found that part number with searches. Sometimes the last numbers are the voltage X28 could be a 28VDC some lower voltage devices might have V in them like 3v3 for 3.3VDC.


the source being about .4VDC is kind of low however, it is not cap bypassed and the source resistance with the drain resistance controls the gain to an extent.
Unlike opamps single JFETs and BJT in basic circuits have a more limited open loop gain

Some diode links

https://www.vishay.com/docs/88912/diodesgroupbodymarking.pdf

https://www.vishay.com/docs/85607/bzx85.pdf

https://www.nxp.com/docs/en/product-selector-guide/75017631.pdf look at page 40

 

[Nollman]

The Zener Diode has the following marks on it
BZX
79C
15

 

[Nollman]

There are two other diodes that I can only partially see markings
PH
41
48

 

[Khron]

Those will most likely be a 15v BZX79C, and a 1N4148 made in the Philippines.

 

[Nollman]

Voltage at the zener diode is 4.68v and same at the adjacent C264D FET the center leg of that FET is 1.62V. The 550C measures .61 volts on the center tap and 5.95v on the leg that looks like it feeds the trimmer.

 

[Gus]

Thanks
it is probably a 4.7VDC Zener
The 550 is for the DC to DC this looks more like the real Schoeps DC to DC than the China built microphones DC to DC

 

[Gus]

Did anyone attempt to reverse the circuit?
I did and I expected more people to maybe post if they did.
A few interesting design choices.

How boring can this place be?

 

[Nollman]

So I now have a bag of J201 FET's and a healthy curiosity about transistor circuits. Can anyone suggest a mic project that would make good use of them?

 

[Gus]

If the J201s are leaded devices and you bought them recently check that you bought J201s.

There are testers for Vgs and IDSS posted on the web.

 

[Gus]

if you look at page 5 of the following link BZX79C15 appears to be a 15VDC Zener.
What preamp are you using to power the microphone do you know the phantom power supply max current it can supply?

https://assets.nexperia.com/documents/data-sheet/BZX79.pdf

 

[Nollman]

I was using a John Hardy M1. I can't find the specs for phantom power voltage on his website. I measured 30v at the mic's XLR.

 

[Gus]

Made a sim of the gain stage part of the circuit.

The following correctness depend on if traced the circuit correctly

Your voltages seem off if there is a 15VDC zener

I believe the voltage to the drain of the BD246D should be closer to 15VDC and the top of the 10K "above" the 19.1K drain resistors should be about the same
It might be something is shunting the current away dropping the voltage at the Zener. The first thing I would check is the 470uF(the one to the left under the BC246D) across the Zener. I would lift one end and check the voltages again
48VDC -30VDC(29.7VDC) =18VDC
18VDC/3.4K(phantoms)= 5.3mA I have not found a specification for the current needed for the PhantomV at the Brauner site.

I have not posted what I traced because I have not checked it again

I was hoping others would have traced it

Output SF about 1.6VDC/680 ohm(if I read the resistor colors correctly) about 2.4mA

There appears to be a 10K from CT to 15VDC Zener and you measured 30VDC(29.7VDC) - 15VDC =15VDC

Is there a 10k between the CT and the Zener?

10k x 2.4mA = 24VDC drop across the 10k

30VDC - 24VDC = 6VDC

you measured about 4.7VDC at the Zener that makes sense because the J201 uses some current(.4VDC / 2.7k for about .148mA)

Looks like the Zener is not Zenering if it is a 15VDC Zener

If what I traced is correct the CT splits to two 10Ks one to the gain stage and the other to the DC to DC

Does it look like any other parts besides the capsule wires have been changed or moved on the PCBs?

 

[Nollman]

There's was a good amount of flux on the circuitboard. That usually indicates that there were parts replaced by hand. I will check the voltages again tomorrow morning.

 

[Nollman]

Sorry to drop off for a couple of days, things got weird here. The Zener still gives me 4.6 volts and measures 9.93K CT to Zener. Here's a pic of the circuitboard with backlighting.