Famicom AV Mod Situation in 2019

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Erai
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Re: Famicom AV Mod Situation in 2019

Post by Erai » Mon Feb 10, 2020 3:07 am

I just did the mod following this guide http://8bitplus.co.uk/projects/famicom-av-mod-nintendo/ the only difference is that i used a BC547 transistor, now i'm still waiting for my everdrive to arrive but in the meantime i tested the mod with a famicom to nes adapter, i get nothing by using PAL games with the adapter, is that just because of the games and adapter used ? i checked multiple times and the mod had been done correctly.

Ice Man
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Re: Famicom AV Mod Situation in 2019

Post by Ice Man » Mon Feb 10, 2020 4:21 am

Does the Famicom work with regular FC games? If yes then the adapter is at fault.
PAL games "should" work but eventually show bad graphics or have bad sound. But at least they should start.

As for the AV mod situation.
How do people judge this mod?
http://www.jsrc.jp/denshi/fcav2.html

I did it on my Famicom and so far it's working well.

Erai
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Re: Famicom AV Mod Situation in 2019

Post by Erai » Mon Feb 10, 2020 6:11 am

I don't have an FC game, soon i will have the N8 famicom everdrive but for now i can't test the famicom without an adapter. if i start the famicom without games i get a squashed black and white picture with a wide blurry line in the middle, so at least the composite is outputting ntsc.

Here some pictures of the mod https://imgur.com/a/ZM0TVrx

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Ben Boldt
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Re: Famicom AV Mod Situation in 2019

Post by Ben Boldt » Mon Feb 10, 2020 8:21 am

This thread has gotten pretty long, but we came up with what we thought was the best possible circuit, based very closely to the AV Famicom. I have put it into the wiki here:

https://wiki.nesdev.com/w/index.php/PPU ... deo_Output

I am not sure if this is the best spot for it, feel free to move it and let us know here.

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Ben Boldt
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Re: Famicom AV Mod Situation in 2019

Post by Ben Boldt » Mon Feb 10, 2020 6:48 pm

I am so absent-minded lidnariq, thanks a lot for your helpful corrections as usual with the typo fix and addition in the wiki.

lidnariq
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Re: Famicom AV Mod Situation in 2019

Post by lidnariq » Mon Feb 10, 2020 6:59 pm

Tangentially related, do you have any ideas at all what might be going on with the front-loader in this thread ?

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Ben Boldt
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Re: Famicom AV Mod Situation in 2019

Post by Ben Boldt » Mon Feb 10, 2020 11:25 pm

Very interesting. I don't often go into that section of the forum. I don't know how the jailbars are persisting so well. It would be helpful to swap the "RF" boxes (not sure I should call them that), or see the composite video directly through the AV mod circuit, eliminating the composite-to-rgb converter... It was a lot of reading but I didn't see that he ever did either of those things. Sounds like he doesn't have a TV with composite input so that makes things harder. He seems pretty savvy with soldering but swapping PPUs might not be a safe request. I don't think I could do it myself without expensive equipment and that is probably going to prohibit that. I would not dare put an NES board on a solder pot or use a heat gun on it; no doubt it would bubble and contort into a potato chip instantly.

I found that my famicom had some similar checkerboard that went away when I added the 560pF ceramic cap.

The NES with the bad jailbars was the one that had the blown caps? Maybe the PPU itself was damaged and causing this somehow. Could the electrolytics have leaked juice that is now making unintended parasitic contact somewhere? If that big cap vented, no telling what direction it sprayed, maybe the whole underside of the PPU got soaked.

soop
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Re: Famicom AV Mod Situation in 2019

Post by soop » Thu Feb 13, 2020 9:33 am

It looks like the visual component has been pretty much sorted now (I enjoyed reading the investigation by the way). But the other part of the AV mod is that the audio is separated, and it looks like everyone is pretty happy with the audio. I remember when I first did an AV mod, I found moving the capacitor around helped, and I ended up moving it away from pin 46 of the cart slot, and towards the RCA jack. Then I ended up adding a resistor towards the end, which, for that mod at least, eliminated the buzz, or at least got it down to a managable level.

The mod I just completed though still has a noticeable hum. It's not loud, you can't hear it when there's music playing, and it doesn't even bother me that much, but I thought I'd mention it in this thread since it's not something that's been mentioned yet.

My setup is a famicom with a completely replaced power/av circuit, the audio is taken from pin 46, the video from pin 21 of the PPU, and there's significant distance between the power rail and the other two circuits separated by a large ground plane. The two controllers are unplugged, so I know it's not the issue with the mic picking up noise.

What I've done so far is break out the audio just before the RCA jacks where I've broken the audio trace to two jacks. There's a slot for a capacitor right where it breaks out, and the first thing I did was swap out various capacitors, but none of them made any noticable difference. The next thing I did was try to add various resistors to the mix, and again, none of that did anything. So I watched a few videos about high/low pass filters, and admittedly, I don't really know what I'm doing at this point, but assuming I configured my high pass filter correctly it did nothing.

Specifically, the noise seems to be introduced at a previous point. My AV cable is quite long, about 2 meters, but I've made sure that is also isolated from any RF. The thing that makes me think it's connected to the PPU somehow is that the game I'm testing, Kirby's Adventure, has a sequence on the intro screen where music plays as Kirby is drawn, and then two paintbrush sprites colour him, and when each of the two sprites appears, there is noticeable audio distortion. So I think somehow, something is affecting it before my circuit. It might be to do with the small ground plane on the AV board succumbing to interference, but I'm just guessing. Has any reasearch been done on this?

*edit* just on a whim I tried taking ground from the nearest point to pin 46 I could find, the opposite pin. As expected, that made zero difference.

lidnariq
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Re: Famicom AV Mod Situation in 2019

Post by lidnariq » Thu Feb 13, 2020 12:14 pm

The audio path inside the Famicom/NES is pretty simple, and audio amplifiers are always high-impedance inputs. You should be able to follow along the audio path, tapping a wire to nodes as it goes along, and see where you pick up the interference.

Famicom schematic: http://nesdev.com/Ntd_8bit.jpg
Reverse-engineered front-loading NES schematic: viewtopic.php?p=203383#p203383
Reverse-engineered front-loading NES audio path: https://nesdev.com/NESAudio.gif

Erai
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Re: Famicom AV Mod Situation in 2019

Post by Erai » Tue Feb 18, 2020 6:46 am

Well the everdrive arrived and the famicom shows no video signal, i changed the transistor with an MJE 13003 NPN TO126 and still nothing, this is on paper a very simple mod, i don't know what to do besides changing transistors; help.

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Ben Boldt
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Re: Famicom AV Mod Situation in 2019

Post by Ben Boldt » Tue Feb 18, 2020 3:18 pm

Do you have audio? Not sure if the everdrive makes sounds its own though...

Erai
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Re: Famicom AV Mod Situation in 2019

Post by Erai » Wed Feb 19, 2020 2:08 am

The evedrive produces no audio, i tried to change transistor again, i went with a bc307b, now i get a stable white picture every time i open the famicom, i'm starting to think that the 1st one was busted and that the second one was out of specifications, this third one shouldn't be compatible either but at least showed me that the famicom produces a signal.

Before trying this last transistor i de-soldered two resistors, the 2.2k one and the 220 one near the original transistor, i bridged the two points where the 220 was, was that a necessary procedure ?

Erai
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Re: Famicom AV Mod Situation in 2019

Post by Erai » Wed Feb 19, 2020 3:51 am

Well apparently i just had to re-solder the 220 resistor back in, now it works, the 1st two transistors i got from a list of compatible ones were either busted or incompatibles, this working one was savaged from an old PCB.

EDIT: Everything works, audio and video, with the 2 ceramic caps under the pcb i get less jailbars, if anybody has a guide for improving the picture feel free to post it.

EDIT2: https://streamable.com/a48w8 The result, not the worst not the best, i'm currently using 2 104 caps (100nF) between the pin 20 and the pin 22 and the PPU, and between the pin 40 of the CPU and the mass point but apparently other guides call for 105 caps (1μF) would that improve the visuals ?

EDIT3: I will leave this here for other people that will end up here from a google search, go for 105 caps, it totally clears up the picture on some famicom apparently (mine too) on the other hand shielding does almost nothing, it eliminates a subtle noise similar to dot crawling but in my case nothing more.

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Ultron
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Re: Famicom AV Mod Situation in 2019

Post by Ultron » Sat Feb 29, 2020 6:41 am

Wow, didn't realize there were so many new replies on this thread.

Just so everyone knows, the transistor you use is not that important, as long as it is an PNP transistor. It is being used as a voltage buffer. A jellybean 2n3906 or equivalent works just as well as the 2SA937 that Nintendo uses.

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