Schematic for Famicom AV Mod

[Kefka]

Does anyone know where I can get a Schematic for the AV mod for a Japanese Famicom?  The only one I found was in Japanese and didn't show up properly.  Thanks in advance...

[NFG]

Some Japanese famicoms have clearly labelled AV outputs on the bridge connector from the PCB to the rear RF board, open yours and have a look.  If there's a hard black plastic chunk with (IIRC) five metal rods connecting the two, and on the RF side are labelled A, V, etc then just attach wires to that.

[Kefka]

Oh, okay.  Thanks.  

[Guest]

Are you sure that this would be sufficient to get AV out?
No need to tap and amplify the signal first?:
http://www002.upp.so-net.ne.jp/jsrc/densi/fcav.html

[NFG]

I've done it and can confirm it works just fine.  It may not be the most amazing signal, or it may be - I haven't had this Famicom for many years, so I don't recall.

[viletim!]

Kefka,
I know nothing about famicoms but I know that's a crappy video amplifier. It's very dependent on transistor gain and the components are poorly chosen. I think it'll only work with one in in every hundred transistors.

If you are going to build it then I suggest you reduce the value of the resistor between Vcc (5 volts ) and the transistor's base. I can't say by how much because it depends a lot on the transistor. 3.9k is a good value to try. The voltage at the output should be around a volt, idealy. Chaning the value of the coupling capacitor to about 10u might help a bit too.

[phreak97]

reviving an old topic yaaay


I'm attempting to mod a famicom at the moment, I tried simply tapping the signal from the labled points but it produced a black screen. examining the schematic for the famicom it turns out the famicom's own video amp is one transitor (npn) and two resistors (2.2k and 200R). it is also wired for high impedance, so the 75R termination in my tv just pulls it low. I got a working signal onscreen by lowering the resistor values drastically, but the colours were kindof washed out, so I might forget the inbuilt amp completely.

I dont understand why theres no good documented av mod for this, but it doesnt seem like it will be difficult to do anyway, I simply need to make a better video amp to amplify the signal from the ppu.

has anyone got a suggestion for me? I dont want to make anything too complicated.. I've already spent way longer than I'd intended on this mod.

thanks!

[phreak97]

I found this just now after adding japanese encoding to firefox so I could at least read the katakana.
apparently the same people went a little bit more in depth after getting crappy results:P

is this amp more up to scratch?

http://www002.upp.so-net.ne.jp/jsrc/densi/fcav2.html

[viletim]

is this amp more up to scratch?

http://www002.upp.so-net.ne.jp/jsrc/densi/fcav2.html

Definately an imporvement over the first one, though I would increase the output coupling capacitor from 10u to 100u.

[jpx72]

the same topic with schematics:
http://www.famicomworld.com/forum/index.php?topic=4713.0

[marqs]

the same topic with schematics:
http://www.famicomworld.com/forum/index.php?topic=4713.0

I made the circuit according to these schematics but the brightness drops down in a few seconds after powering on the FC, quite similarly as if you use a RGB cable with 220uF caps on PAL SNES. Wiring the PPU pin 21 directly to BC558's base (without the 10uF cap) fixed the issue for me. Also, it would be worth mentioning that to reduce vertical banding you can cut the trace from PPU #21 to Q1 and put 220uF filter cap between PPU's Vcc and GND. Otherwise, good tutorial and images 

[jpx72]

I made the circuit according to these schematics but the brightness drops down in a few seconds after powering on the FC, quite similarly as if you use a RGB cable with 220uF caps on PAL SNES. Wiring the PPU pin 21 directly to BC558's base (without the 10uF cap) fixed the issue for me. Also, it would be worth mentioning that to reduce vertical banding you can cut the trace from PPU #21 to Q1 and put 220uF filter cap between PPU's Vcc and GND.

Thanks for posting this, I 've made my AV-mod on a different famicom (Hvc-cpu-07, the first one was Hvc-cpu-gpm-02), and witnessed the same brightness dropping issue like you did. I think that the 10uF cap should really not be there. I am going to do some tests and post my results here later.
And thanks for the tip on removing the vertical banding!

[jpx72]

NEWS  

I have done some testing (thanks to marqs) and I have changed my schematics to work properly with any famicom. Also I love the idea of adding the capacitor on PPU - that's a signifficant improvement!!!
Check my webpage or look on the attached schematics only.
This schematics is universal, because different Famicom boards have different parts. Isolating the pin21 of PPU from the PCB isn't necessary.

EDIT: If you experience very bright image, try to experiment with the 2,2k ohm resistor connected to +5V. Try to exchange it with lower ohm resistor, but not lower than 500 ohm!

[phreak97]

I just did another famicom av mod, I tried a couple of different circuits, but the best result I got was the av famicom schematic from here: http://www.assemblergames.com/forums/showthread.php?24154-NES-2-TOP-LOADER-A-V-MOD

It's reverse engineered straight out of an original av famicom.

The jailbars are not gone but theyre not worse than a toaster nes.
Other mods produced very bright jailbars or a dark or washed out image (or all three).

Im aware that it isnt the best video amp in theory, but the results are what count.