Continuous low hum (all checks passed) [resolved]

[HirkEukvic]

So I got my Mainline assembled last night and it passed all checks that were in the instructions, but when I turn it on there's a constant low hum on both channels. Which terminal numbers do I need to check, in particular, for any grounding issues?

Details: The hum doesn't get louder or quieter with volume changes, it's not loud at all, and it starts as soon as I flip the power switch regardless of if any inputs are connected (I haven't checked the XLR port but the 1/4" port definitely has the issue with my ZMFs plugged in). My transformer bell has continuity to the grounding lug but I'm unsure what other terminals I'd need to check for soldering issues.

Thanks!

[Paul Birkeland]

I would start with tightening the hardware on all the transformers. 

[HirkEukvic]

Tightened all transformer hardware and rechecked resistance and voltages. All passed. Tubes biased appropriately. Also resoldered one loose joint (all others look and feel good and solid). Same issue, a low 60Hz buzz, and I confirmed using a test tone that is is indeed 60Hz. I'll try swapping out my tubes but I'm guessing that's not the issue.

Any ideas?

[Paul Birkeland]

It could possibly help to post some build photos.

-PB

[HirkEukvic]

First batch of photos attached. I already checked and resoldered a few of the XLR pins and the grounding lug on the 1/4" out jack. The C-side output transformer brown and blue (solid) cables now have a bit of extra solder too. Other than that, photos are as it stands now (and those adjustments didn't fix the issue). I will note that the coarse attenuator switch arrived very corroded, and I used 320 grit sandpaper to remove a bit of corrosion to help the solder take on its terminals. There was no anti-corrosion sheet in its box and it looked like the box had been opened--if that's potentially the source of the issue, I might need a new switch.

[HirkEukvic]

Additional photos attached.

[Paul Birkeland]

If the hum appears immediately upon powerup, that eliminates pretty much anything except magnetic coupling.  Do you happen to live in a country with 50Hz mains?

How is the noise level with the impedance selector on low and the output set to unbalanced? 

Is the noise identical in both channels?

[HirkEukvic]

I live in the US (California), in an apartment. Amp is connected to a typical surge protector. My Crack amp (which I used on the same power strip) hasn't had the same issue, though I know it's a lot less sensitive and maybe that makes a difference. The noise seems the same across both channels and is pretty much the same at any volume level. Would you suggest a power cleaner?

[Paul Birkeland]

If you set your Crack right next to your Mainline and power up the Crack, do you get hum out of the Mainline in that situation?

[HirkEukvic]

Tried that just now. Still humming on Mainline with the Crack powered up (and the Crack doesn't hum with the Mainline powered up, I will add). Since I fixed the balanced out jack solder points, it's humming as well. I also swapped out the Mainline's power cord to a different power strip, with the same results. My building is properly grounded (built in 2015) so I'm a bit stumped at this point.

[Paul Birkeland]

So with the Mainline unplugged and the Crack running (with your HPs plugged into the Mainline), see if the hum disappears as you move the Crack further away.

[HirkEukvic]

Sorry, I actually misunderstood your first question. The Mainline does not hum unless it is powered up -- proximity to the Crack doesn't seem to make a difference if the Mainline is powered down, but I'll try moving the Mainline to a different room in case something nearby is interfering with it.

Edited: moving across the apartment and plugging the Mainline to a totally different circuit had no effect.

[HirkEukvic]

I'm still stumped on a solution to this. Are there any particular portions of the circuit or a board you'd recommend I recheck the soldering and assembly on?

[Doc B.]

Wire dress might be a issue. Check the connections of the shielded cable drain wires to ground.

[HirkEukvic]

Thanks, Doc B. The instructions called for the shielded cable to have the drain wire cut off only on one side when I stripped them. I was a bit confused how that would affect the grounding since at that point the drains aren't forming a complete circuit. Can you explain that for me?

I do notice that the foil remnants might be contacting the chassis at the edge of the remaining jacket on those shielded twisted pairs. Any chance that's affecting it?