Famicom AV Mod Situation in 2019

[krzysiobal]

I've made a board that you can put in place of original DC/RF board (fits both famicom revisions).
No need to drill any holes in shell.
I can offer for 15$/board + 5$ shipping

[Persune]

since jailbars are kind of a function of memory access noise on the 2k SRAMs, i've wondered if we could just also decouple those in addition with the PPU.
after chatting with lidnariq and seeing GuidoAnchovy's noise vs RAM chip use, i've thought of an experiment to try.

given an AV modded RF famicom, measure jailbar noise with these combinations:

1. no additional decoupling caps for PPU and SRAMs
2. additional decoupling caps on PPU only
3. additional decoupling caps on SRAMs only
4. additional decoupling caps for both PPU and SRAMs

decoupling cap values for PPU:

• tantalum 2uF (targeting 2px jailbars)
• tantalum 10uF (targeting 4px jailbars)
• tantalum 47uF (targeting 8px jailbars)

decoupling cap values for SRAMs (i think the PPU decoupling caps will do most of the work, but it doesn't hurt to also add this to the SRAMs)

• tantalum 4.7uF for PPU SRAM (targeting 4px jailbars)
• tantalum 4uF for CPU SRAM (targeting 3px jailbars)

important: change values as needed, ESR and ESL will likely skew their effectiveness to filter out these exact frequencies.

[kikendo]

On my research, I found an AV mod I never seen before :
Limone only changes 2 resistors and adds a RC serial circuit to output composite
(s)he also adds a "missing" capacitor to fix the jailbars issue (!! I almost drop my jaw when I saw it the first time !!)

you can find more on his/her youtube video -> https://youtu.be/hIMtESSiX0M?t=239

I'm very interested by your advice / comments on this one

I just signed up interested in this topic because I found another mod that caught my attention and, similarly, does not make a mess of the motherboard, instead replacing R12 and R6 too. it's here:
https://www.youtube.com/watch?v=Cg8x_YePLd8
I am going to try this mod and then add the capacitor shown in the video you linked to.
This solution looks by far the cleanest to me. Just wonder how different results are.

What I couldn't figure out was the CPU/PPU decoupling, I fail to figure out which number pins I am supposed to bridge with the caps. Any help anybody?
The pictures I found online are bad, I would much rather have pin numbering on the board and more clear instructions.

[krzysiobal]

since jailbars are kind of a function of memory access noise on the 2k SRAMs, i've wondered if we could just also decouple those in addition with the PPU.

It is also function of how the traces on pcb, especially in nearest neighborhood of PPU are routed. Here is an example famiclone that has almost invissible jailbars. Its video circuit it identical to other famiclones.
A lof of empty space underneath PPU might be the answer.

[Persune]

A lof of empty space underneath PPU might be the answer.

would something like a pdip-40 leaf socket help? though it would block the eject mechanism on some Famicoms. perhaps a PCB between the PPU and the motherboard might help too instead of covering it with copper foil.

anecdotally, this might only help on GPM revision motherboards, where it seems that crosstalk is bad enough that isolating the PPU's body with copper foil makes a difference.