[danilopezpradas]

Hello! I purchased and assembled two MP599 Microphone preamplifiers 4 years ago. I always used the preamps to amplify a synth. Now I was trying to amplify some condenser microphones and I realized that one works fine but the other has a problem with phantom. Both preamps work fine with a dynamic microphone such as Shure sm 58, but only one of them works with the condenser microphone. I guess the problem is something related to phantom. How can I fix this? Thank you very much in advance!!

[JPK]

This is probably because the phantom power doesn't reach the mic.
Check the solder joints of SW1, R1, R2, R3, R4, C3, C4.

Check with your voltmeter if you can measure the phantom voltage on the mic XLR, between pin1 and pin2 or pin3.

[danilopezpradas]

Hello JPK, I just Checked the solder joints of SW1, R1, R2, R3, R4, C3, C4 and they are fine. I have measured the phantom voltage on the mic XLR and it is just 2 volts. 
What can I do next?

Thank you very much in advance!

[JPK]

Can you report the DC voltages (against 0V) on both sides (L & R) of R3 and R4?

[danilopezpradas]

Hello again!
I checked out R3 and R4 I found that R4 is burnt. Should I replace it? Should it be 680 ohms as in the photo attached? Should I buy it somewhere nearby or can you send me one?

Thank you!!

[JPK]

This resistor should not burn in this position. You should first find out why it burnt before replacing it.
Check if there is any DC voltage (against 0V) on both sides.
Also, with the power off, check the resistance between each side of the resistor and the 0V.
This resistor is a metal film 1/4W 0.1% 6k8.

[danilopezpradas]

I just checked the voltage and it is like this:
L           R
0.07 R3 0.06
0.00 R4 0.06

Is this the information you asked?

[JPK]

There in no voltage across R4 so how could it burn?
Do you have any too long components wires touching the chassis plate?