I have no issue with repairs to a console, where feasible. Some repairs, like recapping a Turbo Duo or replacing the CD lens of a Playstation SPCH-1001, are necessary to restore the console to working order. Recapping a board may require a certain advanced level of skill, but it may be necessary to return the console to something as close to the out-of-the-box experience as possible. For systems with ABS plastic yellowed by UV light, retr0bright and repainting may be the way to go. I modded my precious Famicom AV, which I bought new in the box around 2003, to support a NES Zapper Light Gun in Controller Port 2. The mod was simple, I only needed to solder two wires to connect the necessary bits from the 15-pin expansion port to the 2nd controller port. You would not be able to tell the board had been modified unless you completely removed the PCB from the shell.
However, certain mods I am very uncomfortable with. Modding a NES to display graphics above composite video quality is one of them. The true Famicom and NES used the 2C02 PPU that generated NTSC composite color signals, mixing the brightness, hue and saturation signals inside the chip. The video output is on one pin, making composite video the best video available from the chip. (S-video would require two pins). For the original Famicom and NES Top Loader, RF was officially the only output available. Composite video was available from the NES Front Loader and Famicom AV.
The 2C02 PPU has an official RGB variant called the 2C03 PPU which was used in the Playchoice-10 arcade machine and an identically functioning 2C05 PPU used in the Sharp Famicom Titler. You can replace a 2C02 with a 2C03 in a Famicom or NES, but desoldering the chip is not for beginners. The 2C03s are hard to come by and extremely expensive. However, the 2C03 PPU is not 100% compatible with its composite brother. Two of the grays in the composite PPU are missing from the RGB PPU, which will render them as black, resulting in a loss of detail in games that use them. Colors will look a bit off and rather garish compared to the composite PPU. Second, The Immortal, James Bond Jr., Just Breed, Magician, The Jungle Book, the Lion King, Noah's Ark and Felix the Cat use the color emphasis bits to darken the entire screen with a composite PPU, but with an RGB PPU, these games will show a totally white screen, making them completely unplayable. (Just Breed is one of the very few Japanese games to use color emphasis throughout the game, so the issue is not as pronounced for Japanese games.) A few other games, Final Fantasy 1 and 2, Super Spy Hunter and The Fantastic Adventures of Dizzy, use color emphasis for minor effects that do not affect gameplay. The Titler converts RGB into S-Video, it doesn't output RGB natively but can do so without too much trouble. Now there is one caveat to the rule that NES games should always use composite video. It would seem likely that Nintendo's in-house development hardware may have used 2C03 PPU.
Today there is a modification board called the NESRGB. This is a daughterboard which you mount the PPU into (after desoldering it from the NES mainboard). The FPGA on the NESRGB monitors PPU accesses, take information from the palette registers and combines them with the video output signal to digitally convert the color into RGB. This mod has the huge advantages of not requiring a rare 2C03 and does not perform an imprecise analog composite to RGB conversion like the French NES. Unfortunately, the mod is more difficult than the 2C03 mod because you must save the 2C02 PPU.
Kevin Horton (kevtris) and Jason over at Game-Tech.us are deep in development of a Hi-Def NES mod that installs similarly to the NESRGB. However, it outputs to an HDMI cable at 720p or 1080p. It also emulates the NES and expansion audio channels, because they cannot be captured digitally like the video can. It can also apply smoothing scalers and scanline filters.
At $499.00, it will hit the wallet really hard. Its case is made out of aluminum and it supports RGB, S-Video, composite video and stereo sound. It uses 2A03s and 2C02s reclaimed from Nintendo machines but the PCB is a custom design and the rest of the components are new. It has a NESRGB board built in. It has two NES controller ports properly spaced for the four player adapters and a Famicom expansion port. The latest revision of the PCB also supports the Famicom microphone, something not implemented elsewhere outside an original Famicom.
Bunnyboy of NES PowerPak fame has been devolping an HDMI NES. This is a complete clone, and should be far more accurate than the typical Taiwanese System-on-a-Chip designs. However, the hardware that will emulate the NES is far, far more powerful than the NES, so I ask how different is it than running a NES emulator and outputting it to a TV? One advantage will be that there will be less, if any input lag to deal with from reading the controllers or having the LCD interpolate the standard definition frame, since the system will output in 720p/60fps. The picture will be sharp as it can be on modern LCD TVs, hopefully the device will be able to get all the PPU and APU quirks right.
Another unnecessary NES mod is the so-called stereo sound mod. The 2A03 CPU contains an Audio Processing Unit that outputs audio on two pins. One pin contains the two pulse wave channels and triangle wave channel, the second pin contains the noise and PCM channel. These can be split into separate outputs easy enough, but the NES was not designed for stereo sound. Most games use the pulse and triangle waves for music and the noise and PCM for percussion and sound effects. I think the resulting sound is very unbalanced toward the waveform output. Earlier games tended not to use the PCM channel, so the output for it and noise would sound very quiet.
On the other hand, modding the original Famicom or Top Loader NES to output composite video is a worthy endeavor. The Famicom was RF only, and the Japanese RF channels are well-nigh impossible to turn perfectly to US TVs. Composite video is universal in NTSC countries, RF is not. When the Famicom was released, the Sharp My Computer C1 TV was also released and it used an internal composite video connection. Gaming magazines would take their screenshots from this TV because of the improved picture quality. The video output quality of a standard Top Loader NES is comparative garbage, but it can be brought into line with the Front Loader NES and Famicom AV with a mod. The Sega Master System Model 1 has composite and RGB output at its DIN, but the Model 2 has RF only and requires a mod to support either.
When you upgrade to the 4th and 5th generation systems, then RGB becomes available for all of them, many via a modification, but the original SNES, Playstation, NeoGeo, Atari Jaguar, Phillips CD-i, the Sega Master System, Sega Genesis Models 1 and 2, Sega Saturn and Dreamcast offer it on their AV connector. The N64, Turbo Grafx, CD-i and 3D0 can be modified to support it, one way or another. For those of us in the United States, true analog 15kHz RGB monitors were rare. Component video is the closest substitute, but RGB to Component video conversion requires a converter box. RGB is pushing it, especially with the Genesis. The Genesis was known to use dithering that NTSC resolution and decoding would not completely resolve, but would look extremely pixelated on an RGB system. The SNES games can also take advantage of the fuzziness of composite video when dithering. The early 3D consoles like the N64 and Playstation tend to take advantage of the natural anti-aliasing effect that composite video can produce. Viewing these consoles in RGB, where most of their 3D is in a low resolution form, shows sharp jaggies.
One last issue I want to address is how many people enjoyed RGB when these consoles were the current generation? RGB, even in Japanese (JP-21 connector) and European TVs (SCART connector) was strictly high end in the 1990s. The main TV may have had a connection in the more affulent homes, but many video game consoles tended to be relegated to the second TV. Hours of video game playing tended to tie up the main TV, so parents tended to insist that a video game system be connected to another TV. The second TV would be lucky to have composite video, and many people didn't know better and simply used their RF switch.