the new V72 (pre order closed)

[FarisElek]

Left channel ( from overhead picture view) is dead quiet. Right channel has a slight 60 cycle (and harmonics) hum, even with input unplugged. The lorlin switch wires are there because no matter how much I try to measure pcb's when I get my front panel made, the holes are always a little off. That would be a nice addition for people who make the pcb's to share these exact dimensions. Perhaps my power switch is too close to the extender wires on the far right lorlin? The veroboard on center back is phantom slow start board. Mains transformer is outside of case on the back, 48v transformer is on back right side by power inlet.
[list type=decimal]
[*]I've swapped output wiring to the other clean output transformer > phase flip switch> xlr, and hum is still present.
[*]I've disconnected the switch board's star ground wiring and buzzed to check for another ground present causing a loop, and nothing.
[*]I've re-flowed solder to any suspect cold solder joints, checked other grounds, and nothing.
[*]I've routed power wiring for left board as far to the rear of right output transformer, no difference.
[*]I've disconnected power to the left board completely, and no difference.
[*]I've disconnected the 6vac to the indicator lamp, no difference.
[*]I've pivoted right choke, no difference
[/list] 
Anyone have any thoughts on what else to search?

The transformer on the back right is the 48v transformer I assume? I would either mount that on the back if it has leads or move the input and output transformers to the left and away from it. PT let out a fair amount of RF and the audio transformers will pick it up.


Ryan

 

[duantro]

Yes that is the 48v transformer. I’ll try unmounting it and see if it cleans up!

 

[FarisElek]

I have a very similar transformer in one of my builds that I keep on top of my rack. When I place my DI on top of it, as I sometimes do, my DI will noticeably pick it up.


Ryan

 

[aomahana]

Have you tried moving the phantom power transformer further away to see if the hum reduces.
If it does, you may need to replace it with a toroidal transformer.

 

[duantro]

I only had a few minutes, but pulled 48v transformer outside of case and it still is having a hum like before. Would the choke be too close to the output transformer? The build photos of Martin’s look to be positioned similar. And it doesn’t seem to be effecting the left side.

 

[FarisElek]

Is the choke mounted on the back right? If so, that looks fine. I'd double-check ground connection and swap tubes between channels to see if it follows the tubes. Check ohms between pcb ground and chasis on both pcb.

 

[duantro]

The choke is the sowter 9375 between the output transformer and 48v transformer. I have swapped tubes between sides, and the hum stays with the right board. I’ll triple check all of the star grounds tomorrow.

 

[duantro]

Another question, Am I correct assuming all the big ground holes on main pcb should run to star ground as well as the ground terminal between the heater ac input connector? Martin's photo appears to be this way, but all of the schematics I've seen just have a single ground point. I tried floating the ground between the heater connection, and no change. I've checked and resistance is 0r from ground connections to star ground.
I used an eico bench power supply to bypass my existing power supply as a culprit and it still had a hum at high gain, on variable gain boost mode. The hum goes away immediately when the power is switched off.