Ashdown EVO 500 III Hum

[rf7]

This amp has a bit of 60Hz hum coming from the preamp PCB. Engaging the EQ section with a low freq. boost or engaging the tube section increases the volume of the hum greatly. Swapping tubes does not help. I fixed a cracked solder joint on the preamp PCB and corrected a factory solder bridge too, but it still has the issue. The owner said it always had a bit of hum, but not this much. The inputs are isolated from the chassis. Any ideas?

All the power supply rails look fine on the scope. The amp works fine otherwise.

[Enzo]

Are you SURE it is 60Hz and not 120Hz? Use a scope.

What other controls have an effect? Flat or SHape switch? Pre gain control? Passive/active button?

[rf7]

Yes, it's 60Hz on the scope. Passive/active switch does not change it. Pre-gain control changes it slightly. Flat/Shape switch changes it too. I measure 5K ohms between signal ground and chassis ground.

[dugdiamond]

AND???????
So what became of this problem? Did this issue ever get resolved? What resolved this issue? I haave an ashdown with very similar problem and would like to know what others have experienced and the solution to this hum.
Thank you