I finally got around to working on this today. I got very lucky (in this case) and found a Famicom with very bad jailbars. Here is the AV mod that I did (video only, no cut traces, no cut pins, no copper foil, etc, only the parts shown):
Here are the jailbars with this mod:
By adding a 47 uF Tantalum capacitor from pin 22 to GND, it makes an incredible improvement:
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(GND)
PPU.20 -----------+---------------------------------+
| | |
| | V (c) +----O } Composite
| PPU.21 --|< PNP (Reused the 2SA937) } Video
| | \ (e) 33uF +----O }
--- Magic | 100 Electrolytic |
--- Capacitor +-------/\/\/----+------|(--------+
| |
| (/RST) 150 |
PPU.22 -------------------/\/\/----+
I don't have any particular reason to think my values are any better worse than other mods, this is just the way that I have been doing it in the past. I notice now that my AV mod uses PPU pin 22 (/RST) as the pull-up rail. It seems to me that might be a digital signal and not really a 5V rail, that's interesting, I wonder why it is like that?? I should try using a true 5V, and also tweaking the resistors to be like other mods that are out there.
I tried various "magic" capacitors. I notice these capacitors are good, I can't tell any of them better or worse than each other:
- 47uF 35V Tantalum
- 15uF 35V Tantalum
- 6.8uF 35V Tantalum
- 4.7uF 35V Tantalum
These caps are
almost as good as the tantalums above, almost undetectable difference:
- 10uF 100V Electrolytic (might be a low ESR cap, doubtful but not sure)
- 3.3uF 35V Tantalum
- 2.2uF 35V Tantalum
These caps are are definitely NOT AS GOOD as the ones listed above:
- 1uF 35V Tantalum
- 100uF 16V Tantalum
- 1uF 100V electrolytic
- 100uF 35V electrolytic
- 22uF 16V X7R ceramic (much to my surprise, I suspected this would be a winner)
Referring to the oscilloscope, the jailbars should be measurable. Triggering on the first scanline, showing just the first few tiles, running Super Mario Bros. 1 demo (solid sky blue):
With tantalum:
I think that if it was perfect, all of the peaks would line up, so I can try experiments and measure the effects now. Also, when viewed in motion with the oscilloscope, the jailbar one has much more variation to it.