Successful Famicom AV mod

[phreak97]

Hi everyone..

So I took on the job of modding a red/white famicom for av output, figured it'd be easy and thered be heaps of guides online.

wrong!

the output of the existing video amp is high impedence, which means if you take video from the labled video solder pad the video gets pulled to 0v the moment you connect the tv's 75R input. I have no idea why people have had success doing this, their tv would have to be missing the 75r termination (crap tv).

anyway I got a friend of mine over and we worked out a reliable mod which actually works.

all you have to do is put in one wire and two resistors.
I used resistors based on what I had on hand, the right hand one in the photo should be 75 ohms and the left one should be about 1k. The 820 ohms I used was a bit too low and produced slightly dark video but it wasnt too bad, I decided it was acceptable.

NOTE: I disconnected the video line at the wires between the two pcb's because I removed the rf modulator connector to install an av port without cutting the case. I have no idea what results come from the rf output if you leave this connected, try it at your own risk.



The audio is still taken from the "SOU" pad




Basically this converts the high impedance amp to one which will drive 75R.

Have fun!

[kendrick]

Nice work, seems simple and elegant. Does pulling composite video this way cancel out the RF output like the old mod could have, or do both signals stay active?

[phreak97]

I just thought of that and rushed back to my pc to add a note.

I have no idea if the rf output will work, I disconnected it long before we worked this out, there were many attempts before this one and I'd simply forgotten it ever existed by the time I posted.

if someone is willing to try it I'd love to know the results (and may be able to help correct any problems). but I wont take any responsibility for a fried modulator due to the new current capabilities of the video amp, do this at your own risk. probably it would need another resistor somewhere, but I dont have a schematic of the modulator unfortunately

[Midori]

Interesting. Have you compared your results to this one you dug up in another thread? http://www002.upp.so-net.ne.jp/jsrc/densi/fcav2.html

Is there a difference? Is yours better?

I tried this one: http://www.h3.dion.ne.jp/~kuboken/famicom/famav2.html on a famicom and the result was somewhat mixed. Worked well overall but there were bright lines in the blue colours(all the others were fine), although that problem was fairly dependent on which TV it was connected to.

Doesn't the Famicom and the NTSC NES use the same PPU? What does the schematic for the NTSC NES composite video circut look like? Does anyone know?

[phreak97]

I didnt try those mods because both of them (when translated to english) bascially explain that the results are imperfect and seem to have been made with a bit of trial and error. the one with the cro image looks good because it looks like theye know what theyre doing, but the description goes on to say the diagram is incorrect and changing some resistor values helps to improve the quality.

in the end it wasnt that hard to make a new mod from scratch anyway.

the nes video section is probably better, but I didnt think I'd be able to find a schematic for it. since then I've actually found one..

http://www.freeinfosociety.com/electronics/schemview.php?id=2405

pin 21 of the ppu, its the same ppu as the famicom I think.

[Midori]

Nice find, doesn't look all that fancy.

I decided to do some work on my own Famicom over here and made progress. I must ask you, do you have vertikal lines in your video from the Famicom? I had severe problems with it, the blue sky in Super Mario Brothers was the best way to spot it. I read that it was a common problem in this thread: http://nesdev.parodius.com/bbs/viewtopic.php?t=601&postdays=0&postorder=asc&start=90&sid=63f8e4cfb020955bd3a092815072679c

Then I found after some reading this post from Moosmann:

To reduce the lines, lift Pin 21 from the PPU or disconnect the line from pin 21 to 2sc...transistor and solder the Video-Amp directly to the PPU. However, you always have some vertikal lines on some games (e.g. Bird Week), so please solder an 220uF electrolyt capacitor between PPU Pin 40 and GND. You also have to take the 5V for the Video Amp from PPU Pin 40 (...and 220uf capacitor) and not from an other 5V place from the PCB.

So I tried lifting pin 21 and soldering a capacitor from +5v to ground on the PPU and then connected all those signals to the simple amp I used on my last famicom.

The image quality was improved beyond all hopes, there are no lines in sight on most of my monitors. My plasma still shows them, but in a tolerable way now. The colours are decent, but not perfect. Using a better amp probably will improve the picture quite a bit. But I am satisfied at the moment.