Marshall Artiste 2040 Ground Loop

[bobloblaws]

I have this 1972 Marshall Artiste amp here that I've been fixing up. The last issue I have is apparently some kind of ground loop problem. If there is nothing plugged into the inputs there is no hum (input jack switches grounded) and If I'm touching the guitar strings there is no hum, but if I let go of the strings there is a faily loud hum. It is affecting both channels and is affected by the level of the volume and tone pots as well as the presence pot. It sounds like I'm hearing both 60Hz and 120Hz but the 120 is more prominent (more like a buzz than a hum is how I would describe it). I tried a few things like separating the ground rail (attached to the back of the pots) between the two channels, bypassing the tone stack on the normal channel and trying a different input jack on the normal channel. I feel like I have pretty much no clue what I'm doing when it comes to this kind issue and just taking shots in the dark. I could use some expert guidance.

Marshall-Artiste-50W-2040-Schematic.pdf

[g1]

Is the hum also gone when you turn down all the controls on the guitar?

[Helmholtz]

The last issue I have is apparently some kind of ground loop problem. If there is nothing plugged into the inputs there is no hum (input jack switches grounded) and If I'm touching the guitar strings there is no hum, but if I let go of the strings there is a faily loud hum.

Only hum/noise with shorted inputs is generated inside the amp.
Everything else must be entering the amp from outside. In your case the guitar seems to have poor shielding.

Certainly no ground loop problem of the amp.

[bobloblaws]

Is the hum also gone when you turn down all the controls on the guitar?

Yes.

Only hum/noise with shorted inputs is generated inside the amp.
Everything else must be entering the amp from outside. In your case the guitar seems to have poor shielding.

Certainly no ground loop problem of the amp.

I wish it were that simple, but I tried a bunch of guitars, single coil and humbucker, and different cables.They work fine with other amps. Turning the guitar volume down from max actually increases the low hum until a certain point. As mentioned above, turning the volume to zero eliminates it though.

[Helmholtz]

First of all check the safety grounding of the amp and measure resistance from amp chassis to the earth contact of the mains plug.
Should be below 1R.

[Helmholtz]

If I'm touching the guitar strings there is no hum

That's quite normal as touching the strings grounds your body, making it a shield.
It typically indicates a "noisy" environment.

Try in a different location.

[g1]

Also, do you have the amp assembled, or chassis open on a bench? Is there any kind of shielding inside the cabinet?

[bobloblaws]

That's quite normal as touching the strings grounds your body, making it a shield.
It typically indicates a "noisy" environment.

Try in a different location.

Yes, but this is more way more than typical. I'll check that mains ground.

[bobloblaws]

Also, do you have the amp assembled, or chassis open on a bench? Is there any kind of shielding inside the cabinet?

At the moment it is open but I had it re-assembled and it was the same.

[bobloblaws]

First of all check the safety grounding of the amp and measure resistance from amp chassis to the earth contact of the mains plug.
Should be below 1R.

No problem there, it is 0.3R max.