Please help me identifiy and optimize this mic circuit

[Mellowmusic]

Hello there!

I got hold of this microphone, which originally seems to have been a Fame SKEC 002/w.
Further research led to the "Apex 420", but the PCB looks totally different and I haven't found any more information about this mic.
Has anyone here ever seen this PCB before and can help me?

The capsule has been changed to a Peluso CEK 89 at some point. Sounds kind of harsh once you sing louder and the circuit seems to distort quite early. I can't tell if other parts in the circuit got replaced.

Some values I can identify:
Both transistors are A1015 GR01
C5 and C7: 07L 474
C8: nothing soldered in (?)
C6: 226 1G+
I can try to decipher more of the values, if needed.

Any help or pointers in the right direction is appreciated very much!

Best regards and thanks in advance,
Mellowmusic

 

[Khron]

Soldering looks pretty much factory, so it's likely no further mods were done.

JFET might use rebiasing though - no trimpot in sight, so you're at the mercy of (loose) manufacturing tolerances.

 

[daschnoz]

Hitting the google with "E257309 circuit board" led me here:
https://productiq.ulprospector.com/en/profile/204859/zpmv8.e257309?term=ul&page=1623

I don't have an account, so I can't dig any deeper in that page. Hell, what was returned might be a hair dryer for all I know... but it is a positive hit on what looks to be a UL number of some type from the circuit board (based on the symbol that it's directly next to).

Try throwing some of the other numbers on the board into google. It might return something useful.

 

[Mellowmusic]

Hey Guys, I compared the schematic of the MXL 2001 with the Fame C02. I attached a picture for anyone needing it in the future.
Parts marked green are identical although sometimes labeled differently. Parts marked yellow are different / changed values (factory).
The microphone also has a low cut and a polarity switch for cardioid/omni, which I left out of the schematic for now.

I have three of these mics and will mod them in the future. I plan to put Arienne Audio Flat K47s and a K87 in them respectively and change a few caps as well.
If interested I'll post some audio files in the future.

Take care!

 

[RuudNL]

I highly doubt if the value of R4 (750K?) and/or R5 is correct.

 

[micolas]

I highly doubt if the value of R4 (750K?) and/or R5 is correct.

@RuudNL , as always, you are right. It makes sense, NFB is copied from 87.
There is a single resistor of 7.5kohm or 2 in series that sums this value, IIRC mounted on the round pcb under the capsule.
Here I found a picture of one of
my 6 (or 7 ? ), only Cardioid with Lo-Cut inside on the PCB. They all sounded (stock) differently, but on the good side. Some have been modified: NFB, the BIAS circuit, elimination of the coupling capacitor of the capsule (and the inversion of the xlr pins to preserve the phase), and others received additional dc/dc converters that deliver between 55..65v compared to the stock polarization voltage of 33..36v.
Incredibly cheap. Very good value for money.
After the vacation I will draw the correct stock diagram.