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Correct Famicom AV mod
https://forums.nesdev.com/viewtopic.php?f=9&t=10554
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Author:  ThePunisher [ Sat Apr 29, 2017 11:59 am ]
Post subject:  Re: Correct Famicom AV mod

infiniteneslives wrote:
Well I've had this thread bookmarked forever, and finally got around to performing the twin famicom video circuit mod to my original famicom.

Hi, friend! I've recently started collecting parts for this specific twin-circuit, can you take a look, please, if it fits?

    1N4148 DO-35 switching signal Doide
    2SC1740 C1740
    A1015 2SA1015 0.15A/50V PNP TO-92 DIP
    Panasonic FM Series Capacitor Low Impedance (25v, 220uf)

I haven't done my homework regarding the resistors yet, can you suggest what kind suits best here?

Author:  infiniteneslives [ Sat Apr 29, 2017 12:05 pm ]
Post subject:  Re: Correct Famicom AV mod

ThePunisher wrote:
I haven't done my homework regarding the resistors yet, can you suggest what kind suits best here?


Your selections look fine to me at a glance. There's nothing special about the resistors.. Just get the suggested values in carbon film and you should be fine.

Author:  ThePunisher [ Sat Apr 29, 2017 12:19 pm ]
Post subject:  Re: Correct Famicom AV mod

infiniteneslives wrote:
ThePunisher wrote:
I haven't done my homework regarding the resistors yet, can you suggest what kind suits best here?


Your selections look fine to me at a glance. There's nothing special about the resistors.. Just get the suggested values in carbon film and you should be fine.

Thanks alot, friend! I will post a video of gameplay, once all the parts arrive, and I'm done with soldering.

Author:  ThePunisher [ Sat Apr 29, 2017 10:19 pm ]
Post subject:  Re: Correct Famicom AV mod

infiniteneslives wrote:
ThePunisher wrote:
I haven't done my homework regarding the resistors yet, can you suggest what kind suits best here?


Your selections look fine to me at a glance. There's nothing special about the resistors.. Just get the suggested values in carbon film and you should be fine.


Btw, the resistor values, described in the screenshot are regular ohm, right? Not kilo- or micro- ?

Author:  infiniteneslives [ Sat Apr 29, 2017 10:42 pm ]
Post subject:  Re: Correct Famicom AV mod

Correct.

Author:  ThePunisher [ Fri May 05, 2017 8:35 am ]
Post subject:  Re: Correct Famicom AV mod

infiniteneslives wrote:
Correct.

Hi, once more I need to consult with you:
    Where did you take the Ground? 80sFreak was very specific about it "Take ground from pin20 PPU. Don't ask why, just do it."
    Did you lift the leg of PPU or just cut the pcb route? I really want to keep PPU untouched.
    What kind of wire did you use? Shielded? Single vein?

Author:  infiniteneslives [ Fri May 05, 2017 9:01 am ]
Post subject:  Re: Correct Famicom AV mod

I wouldn't lift any pins from the PPU if it were my famicom. Maybe it would help remove some of the jail bars if I did though..?

You can see where I took ground in my pictures. I just took it from the most convenient location. You can also see I just used cheapo IDE style ribbon cable.

These choices prob aren't the best ones. They were just what I had and met my desires just fine. Feel free to experiment!

Author:  tepples [ Fri May 05, 2017 9:19 am ]
Post subject:  Re: Correct Famicom AV mod

My speculation about the use of pin 20 in particular:

PPU pins 40 and 20 are the PPU's power and ground pins, connected to the bypass capacitor nearest the PPU. I guess taking ground close to the PPU avoids ground loops and unwanted coupling.

Author:  soop [ Sun Jul 22, 2018 11:25 am ]
Post subject:  Re: Correct Famicom AV mod

Can I ask, the very specific values on two of the resistors - would an approximation work? Something like 75 ohm? I've never seen a 68 ohm resistor, but I could wire together some smaller resistors in series if that's the absolute desired resistance. I would have thought 75 would be within tolerance tbh.

Author:  lidnariq [ Sun Jul 22, 2018 12:08 pm ]
Post subject:  Re: Correct Famicom AV mod

68 and 82 are standard E24 size resistors. 75Ω isn't.

75Ω is actually a little too big, because the voltage follower T2 has some small finite effective output impedance, and using a 75Ω resistor would generate an unmatched transmission line.

Author:  Fisher [ Sun Jul 22, 2018 4:33 pm ]
Post subject:  Re: Correct Famicom AV mod

That's interesting.
Will I get a better video signal if I mod this circuit to be more like the suggested?
Attachment:
GRADIENTE PHANTOM SYSTEM vout.jpg
GRADIENTE PHANTOM SYSTEM vout.jpg [ 31.93 KiB | Viewed 1556 times ]

They're pretty similar...
PTG3 goes to the RF modulator.

Author:  soop [ Sun Jul 29, 2018 11:56 am ]
Post subject:  Re: Correct Famicom AV mod

Can someone help me out here? Everything seems to be good up until the capacitor. I get ~4v DC going into the anode, then the multimeter quickly runs down to 0v at the cathode.

This is the second circuit I've put together now without success, so I'm either repeating a mistake, or there's something wrong. Should I be getting 4v before that cap?

*Edit* I've just measured without the RCA jack connected to the TV, and I get voltage, then with it connected, same as before.

I'm thinking this implies that the resistance the other direction is insufficient?

*Edit 2* also, I've taken ground from the regulator. I'd still.expect to see a signal though


Ok, I'm dumb, that's how it's supposed to work. Hmm, no signal though. I'm reading (from memory) something like 0.06V AC at the cap and to the RCA jack. Could be a bad PPU since it's untested, but hard to tell without an oscilloscope.

Author:  Kangsteri [ Sun Jul 29, 2018 12:23 pm ]
Post subject:  Re: Correct Famicom AV mod

Seems like i need to try this out. Is there any recommended good point to add variable resistor for brightness adjustment?

Author:  niabi [ Tue Jul 31, 2018 2:23 pm ]
Post subject:  Re: Correct Famicom AV mod

infiniteneslives wrote:
Well I've had this thread bookmarked forever, and finally got around to performing the twin famicom video circuit mod to my original famicom. My primary motivation came from recently upgrading to a commodore 1702 monitor which lacks the RF tuner I made do with on my previous CRT.

Just wanted to share that this circuit worked great for me as well. For others looking to follow along, I also found some new transistors readily available from Digikey that worked well for me. I also used the 1N4148 as l_oliveira suggested.

Analog, video, and BJT's are far from my expertise, but I searched around and found the KSA1015 has comparable specs to the 2SA1015, and it's compliment KSA1815 looked comparable to the 2SC1740.

Also, just to shed some light on required audio modifications to go along with the video. No circuitry is needed, just need to tap from the cartridge audio output directly from the 60pin connector. So that ends up being cartridge pin 46.

Here's some pictures of my install. My goal was minimal intrusion to the console. I favor a modification that can be easily reversed with minimal signs that anything had been touched. Making the console appear that it was designed with AV out was not my goal.. The idea of taking a knife to my famicom makes me cry..

Thinking about picking up some tiny coax to replace my ribbon cables and cheapo RCA connectors, was all I had on had and good enough for now!



Hello this mod looks very clean congratulations! I am trying to do this mod to a Famiclone from the late 80s early 90s, I already have the circuit built based on the schematic that was posted by deadbody, but I have a question that hopefully you can help with, since the PCB on this clone is not 100% the same I need to ask you what are the 3 cables at the middle of the PCB? I assume its GND (the one with the red mark), 5V and (pin 21) AV OUT, correct?


Thanks!

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