A while ago I picked up an original Famicom for peanuts at a retro games fair. It was sold as untested and I mainly picked it up as a museum piece. I tried to do a composite mod a while back but that didn't seem to go so well. Unsure as to what to do I put it on a shelf. Now I'm gonna try to fix it and mod it so I can use it more easily.
After some research I found out that the most likely culprit is a pico fuse on the bottom of the power board. Indeed there was still the original fuse on there that had no continuity so that seems like a quick fix. I replaced it, ran composite out through my awful wiring job and got a signal. Though, it seemed to disappear after jiggling it a bit to snap a photo unfortunately. I chalked that up to my composite thing shorting out or not having a good connection. The picture was pretty iffy looking so clearly something wasn't right. Unfortunately the original Famicom has no power light so figuring out if it still worked was a little tricky.
I wanted to replace it with a board that handles all the AV stuff and also allows me to use a more convenient power source. The Famicom runs on 5V internally, and all the power supply board on my unit does is step down 9V to 5V and make the picture into horrible RF. Since it was also rusty and would make mounting new boards a pain, I decided to get rid of it. We'll run the Famicom off USB power which gives us 5V and most mains adapters give 1A which is more than enough for the Famicom.
My Famicom is the later GPM revision which has this annoying RF cage, the GND for the RF is also a part of the cage so my only other option for getting a proper composite jack on there would be to saw it off. Since I don't know how long this power supply will last and pico fuses are getting pricey, this seemed like a good idea. I still didn't really know if the board is secretly dead but I was hopeful given earlier signs of life. And being able to run this off USB instead of rummaging for a 9V Centre-Neg supply means I'll probably use it more.
First I wanted to design the board. I used a guide off CTRL-alt-Rees which calls for the components to be soldered to each other along the case, which obviously wasn't working. I was going to combine that with a USB power mod as shown on Hakk's Lab (here). I got some perfboard and converted the instructions to a design - A little like this:
My first attempt was a failure but I learnt that soldering tracks like I used to see on 70s electronics doesn't work on perfboards. Each hole has a contact and solder only really adheres properly if that hole is filled. If you're a hoarder and save the component legs you clip off, you can bridge the connections like so. If not, then start doing that since it's useful.
And this is what we're left with as a rough draft. I make the mistake of not measuring the board beforehand so I'll probably have to redo this but it'll work for testing at least.
After plugging it into a USB port, the power LED came on and I got a steady 5V out on the USB. I won't be able to test the composite side yet. I think this'll probably work, but I need to remove the board and figure out how everything wires up to the motherboard.
The GPM has 4 pins that connect from the power board straight to the motherboard on this weird plastic bridge thing. I thought it might be some kind of IC but it seems to just bridge the connections while adding support. There's 'A' (Audio), 'V' (Video) 'VCC OUT' (Which is obviously the 5V out, goes directly to the power switch on the motherboard) and another pin called 'IN'. I couldn't really figure out what this was at first, It wasn't a GND, and it connected to a pin on the power board which I couldn't see due to the shield. I figured out after some troubleshooting that it's
We'll use the Audio pin to get audio (and convert it to stereo for convenience), and the VCC OUT for the power input. I'm going to take video straight from the PPU as per the instructions, running it through a replacement transistor straight to the new board where the rest of the booster circuit is. As for the IN, .
Getting the RF shield off is a pain because there are legs on the motherboard which are hard to get off - I had to delay the project after I ran out of desoldering braid. If there's a problem I can still use it by connecting the wires together but I am not sad to see this jerk go.
I also noticed that the switch had the wires coming off but also that it was 'burnt' - I definitely think someone plugged in the wrong power adapter, blew the fuse and figured it was dead. This scorching makes me think it was an NES power supply that inadvertantly delivered AC to the switch before the fuse could blow. Looks like it saved the console though, if the switch was open this thing would be fried. It even still works, that's the power of retro tech for you.
My mod means that we can actually keep the power switch in the same location, but one thing I neglected to think of is that my power LED only shows if the console is plugged in, not if the switch is open. But since the Famicom has a indicator on the power switch it doesn't really matter (I'll fix it later I promise). I'll just solder it back up for now. I think I managed to mix myself up because Hakk's schematic is for the earlier revision which has the switch on the PSU.
Finally I wanted to give it a jack for composite input and a 3.5mm port for audio (I normally use headphones so this in combination with an inline remote for volume control will be A-Ok). This was also what I had in my scrap bin so bonus points.
Annoyingly I managed to rip the trace for the original video transistor clean off 5v because my old leadless solder wouldn't come off. We love you RoHS, we do. I can just lift 5V from VCC on my new board so it doesn't matter much.
Wiring it up and giving it a quick test and we're left with a pretty good signal, good sound and consistent power on USB. Seems all good but I need to sort out my circuit board so it'll fit. I decided to mount it in the space between the 2 plastic beams, lifting it up to the same height as the motherboard using some cardboard and sticky tape. I needed to trim a bit of the case to mount LEDs / ports properly but I was able to make it fit quite well. Not too bad altogether.
Now I finally wanted to fix the LED thing. I had already soldered it in but I didn't want to leave it at this stage. So, I made the decision to move the switch to the power supply just before it gets outputted to the motherboard, and connect the positive pin of the LED just after the switch. I also decided to add another yellow LED to show if it's plugged in since I could; that also shows me at a glance if the voltage is being weird before I turn on the switch.
And finally bridge a connection where the switch used to be.