Alpha VHD-Pre – SSL – Phantom Power not working (SOLVED)

This an excerpt of a Portuguese post that you can find here.

I work on an Alpha VHD-Pre SSL 4 channels that has a problem on its phantom power. Except for this, all channels are working well.

The VHD-Pre has a switched power supply that generates 48V from 5V. To do this, an oscillator sends pulses on a MOSFET connected to an inductor. This procedure generates pulses of approximately 50V. This will be rectified by a diode and regulated by a capacitor.

I found this coments on a GeekLutz board. Appears to be a project fault … :O

They couldn’t send me the schematics for “intellectual property reasons”, but told me that this fault can develop on the first batch of Alpha Channels and instructed me to remove TR9 and replace it with a MOSFET N-CHAN 100V 0.3R. There was a BSP372 in place for TR9 and this unit is a 100V mosfet so I’m guessing that my Alpha Channels are not of the 1st batch. I replaced it anyway with a F9530N mosfet (TO220 instead of SOT223-3, but I made it fit in).
There must be something else going on in the 48V section because it still has no phantom. Anything else works fine.

https://www.gearslutz.com/board/geekslutz-forum/1007344-ssl-xlogic-alpha-vhd-pre-phantom-problem.html

Hello Fullscale.
SSL sent me the 2nd bulletin.
Assuming you know what they say in that 2nd one: I did what they told me, used a 1N4148 for the Silicon 200mA 75V Diode, a simple 10K resistor (not a 0.5% precicion one), a 100nF (very small ceramic cap) and an IRF830 (500V, 4.5A, 1.5ohm, TO-220) for the N-channel mosfet. The original one was a BSP372 (100V, 1.7A, 0,31 Ohm, SOT-223) that was shorted and I couldn’t find this IC in particular.
Now I get 16.4VDC for phantom on the mic in connectors insted of 48V. Before the modification I got 5VDC.
I’m not sure if the 1.5 ohm on IRF830 instead of 0.3ohm on the BSP372 has something to do, as “davidsrsb on diyaudio forum pointed out.
Also, I have no idea what caused the phantom to disappear in the 1st place.

https://www.gearslutz.com/board/geekslutz-forum/1007344-ssl-xlogic-alpha-vhd-pre-phantom-problem.html

I changed the capacitors with no evolution. Then I tried to take the TR9 MOSFET from the board to test them, but I broke it! :O

Bought an IRF840 (didn’t find IR830) and put on circuit. Success!

For your reference, I took measures of some points of the circuit, after the repair. Search for TP1, TP2 and TP3. TP3 is the 48V point. TP1 if the oscillator, and TP2 is the pulses.