[Solved] Mackie mixer with noise on two channels

[trondl]

Hi!
I'm a bit stuck on fixing a noisy cheapo Mackie mixer.
I've concluded that two of the channels with noise has a negative DC offset in the gain stage.
There is a chance that something has gotten damaged due to phantom power hot plugging.
See attachment for the circuit.

What I've found so far is that there is a negative 12V DC present on the red line when it should be at 0V compared to the working channels without an incoming signal.
I've swapped Q19 with Q18, no change.
U15 has been replaced, no change.
R228 doesn't measure 3k in circuit but closer to 2k, but R227 does.
R228 measures correct out of circuit.

Any hint is welcome.
Thanks in advance!

[wasedadoc]

Try removing D27. If that restores proper dc conditions, the diode is faulty.

[Audiorepair]

Yes, it's quite common for these protection diodes to go faulty, which either makes DC, or noise, or both.

I usually change all 4, 1N4148 are very cheap.

[trondl]

Thanks a bunch to both of you for your knowledge!
Immediately found one very broken (-12VDC) and one partially broken (leaky in circuit which gave about -1.5VDC on the second faulty channel) 4148 when desoldering.
0VDC restored