I’ve got the 5 volt issue reasonably working. The Drok module combined in series with a LP filter has significantly stabilized the critical 5v line. Had to play with grounding but ended up grounding at the header and then on the chassis ground.
2.5-3 amps is just what’s needed.
Ive done some experiments with shielding the Pi4 and the ribbon cables.
Using copper foil tape or similar.
I used a bare HAT board wrapped in copper tape.
This is installed on the exposed bottom on the installed Pi4. Then carefully cut long strips of copper tape and cover the long internal ribbon cable. Use clear packing tape over the copper to insulate.
Still working on the main display RFI shield.
With the already suggested 5v DC-DC modifications and the promising results from my amateur copper tape shielding the internal RFI is being attenuated. So headed in the right direction.
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Kb4yho