MXL 990 mod question from Norway

[micolas]

@Frdfrode
What Khron said is correct. 151 and 620 are SMD inductors, 150uH and 62uH, D3 is the zenner diode, to find out its stabilization voltage measure the voltage across it.
To find the correct output voltage of the DC/DC converter, you need to measure after the voltage doubler, between GND and the D6+R12 connection (and it is slightly lower at the end of the 1Gohm coming from the converter). Possibly the voltage generated by the DC/DC is more than 24.5v. (It depends on the internal resistance of your multimeter too)
Maybe the phantom voltage is substantially lower than 48v. You have to measure it without the microphone connected and then with the microphone connected, on XLR, 2 vs. 1 and 3 vs. 1(GND).

 

[Frdfrode]

Ok, new measurements. First I must admit that I went ahead and removed D3, just to check how it is done...surely a bit premature. Maybe this screw up all measurements, but anyway, this is what I got with D3 out of the board:

Across D3 (with D3 removed) 14.7 V

Ground and D6+R12 55V (fluctuating) mic connected

Pins on xlr mic connected 37,8 v Phantom power coming from RME babyface fs pro

mic not connected I get 0 v across pins

Best regards
Frode

 

[micolas]

Ok, new measurements. First I must admit that I went ahead and removed D3, just to check how it is done...surely a bit premature. Maybe this screw up all measurements, but anyway, this is what I got with D3 out of the board:

Across D3 (with D3 removed) 14.7 V

Ground and D6+R12 55V (fluctuating) mic connected

Pins on xlr mic connected 37,8 v Phantom power coming from RME babyface fs pro

mic not connected I get 0 v across pins

Best regards
Frode

NO! Do NOT remove the zener diode from the circuit. You have to leave it in the circuit to measure over it how much it stabilizes.
Maybe this capsule only requires 24v of external polarization (maybe it's also an electret and its sensitivity has just been increased) or maybe the PCB is used for several models of microphones, with different capsules, and only minimal changes are made.
Some models from the Perception series, although they are only cardioid, have a DC/DC inside with + and - outputs
For technological reasons it was more convenient to make only one type of converter.

 

[micolas]

@Frdfrode
A zenner to effectively stabilize needs at least 2...3v more. Probably with a 12v zenner you will get about 45..50v, I don't know. The resistance that feeds the zener diode must also be changed. I made DC/DC using 18v zenner to get +60v for the capsule.
(to draw small current from the Phantom)

 

[Frdfrode]

I will get a zener to put in again. I buy some different voltages. Will put up an order from mouser for misc parts. first need to check what I already have, its not organized after 30 years storage.

regards
Frode