Strange Ampex 351 Ground Loop

[pinebox]

I have an odd issue with a 351 that I want to get another opinion on. The unit in question would have a ground loop and I bury the VU needle when I press on the right side of it, but only while I have it feeding either my interface or my little Yamaha mixer. When disconnected or connected to my Fluke, it does not happen (probes are XLR Pin 2 and 3)

Very slight pressure on the right hand side causes it to bury the VU meter from ground noise. If I remove the output XLR this goes away and the meter relaxes. This happens even after removing all the tubes. It happens if I lift the power ground, and I have pressed on pretty much everything with a wooden chopstick, wiggled every wire, every circuit board has been lightly flexed to see if it is a broken trace or cold joint. I have detached the front panel from the unit and I can cause the problem to come and go by flexing the front panel lightly with the power on. Again, I have been through and tapped all the connections with a stick and flexed everything I can. I have removed one side of the VU connection, but still have metering and the issues, so I am curious, could the meter itself be causing this? Wouldnt the meter be rendered inoperable by lifting one side UNLESS the meter is grounding somehow? And with it being directly in the output circuit, is it possible all this headache is being caused by the meter itself?

 

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Try removing the lamp supply wires from the meter and see if the problem goes away.
You might have an intermittent short inside the meter from the lamp supply to an audio when the pressure/flex is applied.
-Mac