[Perry]

I just finished my second ab528.

There is a crackling, irregularly noise. 
The diode on the front panel is lit solid green.
I have done all other tests, changing cables, changing slot in my 500- rack. but still there. 

[JPK]

Does the preamp work apart from the noise?
Does the noise level depend on the position of the pot?
Does it depend on the gain switch position?
What is the DC voltage on Q5 collector, taken on the heatsink? Should be about -0.5V.
Check that all your electrolytic caps are in the correct direction.

[Perry]

Thank you for your reply!
After more testing of the AB528, the noise/crackling only appears when the v48 is on.


Does the preamp work apart from the noise? - Yes

Does the noise level depend on the position of the pot? - Yes

Does it depend on the gain switch position? - Yes, the noise increase when the gain is increasing

What is the DC voltage on Q5 collector, taken on the heatsink? Should be about -0.5V. - The DCV is 0.

Check that all your electrolytic caps are in the correct direction. - All are correct.

[JPK]

Check that the 48V is present on both microphone balanced lines (input XLR pin2 and pin3), against lunchbox chassis.
Check the solder joints on R1, R2, C1, SW1, RLY1, RLY2, RLY3.
Try removing C1, in case it is damaged.