Multiple Pre's built but one of my A12's has a hum.

[Garagedays]

Happy thanksgiving

I had time after the family left to try out some of my new SCA Pre's. I finished building 2 A12's, 2N72 and 1 C84. One of my A12's has a hum to it. The Hum increases with gain. Also, if I touch the power switch it seems to get louder so I am guessing it has to do with a ground somehow. I read a couple of posts already and tried to tighten the allen screws as well as wiggled the power harness connection for that Pre. I also tried to move it to different slots in the chassis. So far, nothing is fixing it. Since it is not happening with the other 3 I have mounted already, it is definately something wrong with that Pre. To start, I am going to go through and see if I can find some bad solder joints and reflow them. Hopefully that will be all it is. If someone could help me with other pointers, I would appreciate it

Tim Davis

[Garagedays]

OK, I have now run out of Ideas.... I tried swapping the SC25 from the one that is not exibiting the ground noise problem. I tried reflowing the power connector on the board and re-flowed aquite a few of the solder connections on the board as well. I am now out of ideas. It has to be something with the ground. The other 3 channels in the chassis do not make noise when I touch the power button but I am such a newb, I don't know where else to look. Please help


Thanks,
Tim

[tpryan]

Make sure the front panel screws are secure and the rotary switch lock nuts are snug. Don't over tighten them, though! Is anything connected to the input?

[Garagedays]

Thanks for replying Tim. I found most of the problem. I made a stupid mistake and had the jumpers wrong on J3 and J4. I had the shunts going across the jumpers so I jumpered J3 Pin 3 and J4 Pin 3 together and the same with Pin 2 instead of putting the jumper on pin 2 & 3 on each jumper block. Silly mistake. But I still get hum at full gain on this A12 and the other one I can have at full gain and full fader without a speck of noise. It also still seems to have the grounding issue where if I have this channel armed to record and I touch the power switch on the kit or touch anything metal , it makes the hum. I did some recording with it and it sounds great. I recorded snare and with the gain at 3 and fader all the way down, I don't think the noise will be the problem now but I worry about the fact that when I touch the power it hums because that means I have a ground problem somewhere doesn't it?

Thanks for your help.

Tim