The changes I made are:
- lift the Vo pin on the PPU (pin 21), you want to get it completely out of the hole and flipped up
- desolder the transistor Q1
- On the bottom of the board, add a 10uF tantalum capacitor as pictured, between the inboard leg of R6 (which is 5V) and the ground trace near it.
- Solder a wire across pin 20, the hole that pin 21 used to be in, and the negative of the tantalum cap you just placed.
- short the pads on the Q1 footprint for collector and base (these should both already be grounded, so this may not be necessary, but it was done on the unit I tested.
- take the transistor you desoldered and solder the base directly to the lifted PPU pin, the emitter to the outboard leg of R6, and the collector via a short wire to the center pin of its original footprint (which is now grounded).
To me the fact that this works so well indicates that the jailbar noise is being coupled in at the PPU output, which is presumably high enough impedance that it's vulnerable to this noise.
I don't know what the analogous mod to the HVC-CPU-07 board would be, since the transistor amplifier circuit is significantly different there (and the RF module maybe also expects a different output impedance/voltage level). I also can't presume that any such mod would actually improve that revision as it also has different routing and no third ground layer on top.
Attached pictures are the mod, and a comparison of the SMB3 1-1 with just the capacitor and ground jumper, vs the capacitor, ground jumper, plus the transistor relocation. Please excuse the other artifacts in the image, the setup was a bit hasty.