Tube PHONO preamp interference - RFI, EMI, bad grounding?

@filipm

I think if I unsoldered the green wires from the pin and soldered them together, I’d just ground the board while the cage would be left absolutely unshielded.

Yes, but I’m not suggesting you leave it like this.

We want to get to a place where the chassis is grounded (no 33ohm resistor ) but leave the board floating with the 33ohm resistor. This I believe is the optimum solution.

If you can prove the chassis ground can be detached from the board ( by unsoldering the ground post earth and checking that there is no continuity between chassis and circuit board ground ) then you can safely ground the chassis and ground post to the mains ground pin ahead of the 33 ohm resistor with an additional wire. In other words ground the chassis and grounding post without the 33ohm resistor but still run the 33ohm resistor to the circuit board.

You could do as you suggest and just eliminate the 33 ohm resistor and leave everything grounded at 0. Only downside is it may be suboptimal for the circuit itself in terms of noise floor.

I think I am losing my mind here. The photo showing the two green/yellow wires both soldered to what is said to be a ground lug may not be consistent with your earlier verbal description of what you measured, namely a 33 ohm resistance between "ground and chassis". First, and most important, that ground lug you show in the photo I take to be part of a screw clamp lug on the outside of the chassis, which is meant for connecting the turntable and/or tonearm ground to the EAR. If that is so, then that lug MUST be isolated from the chassis, i.e., your meter should show an "OL" or very high resistance between that lug and chassis if you were to disconnect both of the green/yellow wires from it. In the current state, it should show a 33 ohm resistance between that (audio ground) lug and the third prong on the IEC where you plug a 3-conductor AC cord, because we see the 33 ohm resistor in series with that pathway. There should be very low or near zero resistance between the ground lug (that threaded screw) and audio ground because we see that one of the two yellow/green wires is soldered to the PCB directly, presumably to its ground plane. There needs to be another wire or contact connection between the third (ground prong) on the IEC and the chassis. If so, then this is is all good. Not the problem. Without de-soldering anything, just put one probe on the 3rd prong of the IEC, which is earth ground or AC ground, and the other anywhere on the chassis where there is bare metal. You ought to see continuity.

@lewm

Without de-soldering anything, just put one probe on the 3rd prong of the IEC, which is earth ground or AC ground, and the other anywhere on the chassis where there is bare metal. You ought to see continuity.

It’s very simple - from @filipm tests - both the chassis and board are floated at 33 ohms above ground. That is a problem as @atmasphere has pointed out.

If you look at the IEC earth pin there is only 1 33 ohm resistor attached from the earth pin to the board and ground post.- nothing else.

So the suggested fix is to isolate the chassis from the board and ground it to the IEC earth pin directly bypassing the 33 ohm resistor as suggested by @atmasphere and according to EU regulations.

In order to achieve this @filipm needs to be sure that there are no other ground connections between the board the the chassis, otherwise he will end up with other issues.

The ground lug should be disconnected from the board and earthed to the chassis.

If you go back and read my posts you will get it.

Believe me, I got it. I have a lot of DIY experience, as Atma-sphere will attest.

I just am not sure that all the reported information, by verbal communication and by photo, is internally consistent. That’s what I wrote. I was not contradicting you, but your approach seems to be dependent upon the verbal description only. What I see in the photo is that IF the 3rd prong on the IEC is directly connected to the chassis, somewhere not visible in the photo, and if the ground lug shown in the photo is isolated from the chassis where it perforates the chassis wall, then there may be no problem at all with the ground scheme. You and I are not in disagreement. I am only questioning the information we’ve received.

What I see in the photo is that IF the 3rd prong on the IEC is directly connected to the chassis, somewhere not visible in the photo, 

@lewm 

And thats precisely why I suggested he check the chassis for grounding after unsoldering the ground lug ( but keeping the link to the board through the 33 ohm resistor ).- there could be an unknown connection to the chassis from somewhere else - my fear is there might be another connection between the board somewhere other than the grounding post and if he grounds the chassis ahead of the 33 ohm resistor he will end up will multiple earth paths.

So in a nutshell bypass the ground lug, check the chassis is now ungrounded completely ( not 0 not 33ohms ), then he can earth the chassis and ground post directly to the mains input earth lug ahead of the 33 ohm resistor.

cc @filipm