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Author Topic: AV mod for Famicom(covered most revisions)  (Read 68276 times)
80sFREAK
Sharp C1
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Posts: 1191

Hardware freak


« on: July 18, 2012, 12:03:55 PM »

WARNING!!! Whatever you doing, you doing on your own risk and people, involved into this description, will not take any responsibility for any of your lost or damages.
 All photos and sketches of schematics posted by 80's FREAK are based on GPL(hello Da Bear LOL) or CC(thanks to famifan for hint)
 Whatever you soldering on PCB MUST have as short as possible pins/wires.

"Jailbars", that we have by default.

here will be photo of jailbars on revision 07 PCB with chipset revision E

here will be photo of jailbars on GPM-02

 Some boring theory first. Famicom based on NMOS/CMOS chips, which makes lots of noise on power lines and around. It's not  a bug, it's a feature of such a technology. However, we have to take over the thing. This sketch shows how chip looks like inside



 Video out is one of the longest wires due to design. /SYNC is second length, running parallel and very close to video out line. This explains why grounding pin 22 via capacitor(hello, jpx72) helps, (remember, this pin connected to power line direct or indirect and all noise from power line will be right next to video out line) but no need 220uF or so. Ceramic capacitor about 0.1-1.0uF is good enough.



 This for GPM-02 revision, but others looks almost the same. Notice, that hole for pin21 is empty - pin lifted.

 Next stop is remove EMI from surrounding traces on PCB. By some reasons capacitance between pin 21 and others quite big and also producing "jailbars".
I would add lifting/cutting the composite video pin, that made the biggest difference in eliminating jailbars on my system.
Only explanation for this is aging material of PCB or either bad layout of PCB.

 Shield is enough from bottom only - there is nothing above the PPU, but more easy just wrap around whole(half) chip as shown on photos below. I used self-adhesive cooper foil found in a home centre, a bit expensive stuff, but easy to use. Tape is just paper tape, picked in the same home centre around painting stuff(masking tape). Be sure to choose strong one, not delicate. Shield must be grounded on pin20 and only





A bit off-focus(not sure, if this old camera, which i picked for 100yen in a junk bin, can do super macro or not), but you can see "how to"



 To minimize EMI in output video amplifier have to be rebuilt around pin 20 and shield of AV cable have to be connected there.

 Time to choose schematic for video amplifier. For those, who confused at this step - below original, existing on PCB's schematics.

 Choose one. I'm after first diagram with 100R and 150R instead of 220R and 2K2. Special thanks to jpx72 for redrawing


 In the video circuit you don't need to use 220uF separating capacitor. 22uF or 33uF is far enough 47uF is common value and will work just fine. Ceramic capacitor should be best, but hard to get one with this big capacitance. For perfection, you can add in parallel ceramic capacitor 1.0-4.7uF. Some SMD ceramic capacitors around board needed. All pins MUST BE AS SHORT AS POSSIBLE.
 Don't need to use long wire for +5V - there is plenty spots on PCB to make it as short as possibe.
 Main difference between 07 revision and GPM-02 is where to power up video amplifier. The rest is same - 3 extra ceramic capacitors, bigger - better. One on pin 22 another two on power lines around PPU and CPU.

That's all. Happy modding. Will ad some photos and correct typos little by little.

F.A.Q.

Q. Do you ground the copper tape somehow?
A. Yes, you can see on this photo. A bit blury, but still visible.

Q.Why do i have to use values of components as stated in thread?
A. Take it axiom.

Q. Are you going to make video tutorial, how to mod Famicom step by step?
A. No.

Q. Can i use different transistor, because i might damage PCB or existing one, while desoldering?
A. Any low power general purpose PNP transistor will do the job. For example 2SA1015, 2N3906, 2SA733, SS8550 etc. Please keep in mind pinout.

added

Quote from: Ben Boldt@nesdev
Wrapping the PPU in copper foil, foil not connected to anything
Connected the foil to all 5 GND pins (pin 20 and 14, 15, 16, 17)
By doing this, there is definately no point to desolder PPU and cut pin 21.

Quote from: Ben Boldt@nesdev
One final copper foil test just to really put the nail in the coffin, since my PPU is in a socket and thus farther from the PCB, I put adhesive copper foil directly on the bottom of the PCB and under the whole mod circuit:
...
No visible difference. We can call this myth busted.
I LOL'd. But he trying really hard, kudos for that.

AGAIN. To reduce the noise circuit must be built around pind 20 and output cable's shield connected there as well. To make shield working, it must be grounded in one spot.

Quote from: lidnariq@nesdev
I think PCB layout is the biggest factor here... which may be why there's so much superstition. You're using basically the same amplifier as the original, but it's in a different place.
And we have WINNER!!!

« Last Edit: April 23, 2019, 06:32:53 AM by 80sFREAK » Logged

I don't buy, sell or trade at moment.
famifan
Twin Famicom
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Russian Federation Russian Federation
Posts: 934



« Reply #1 on: July 18, 2012, 06:38:41 PM »

All photos and sketches of schematics posted by 80's FREAK are based on GPL(hello Da Bear LOL)
GPL is suitable only for the code, and you need to distribute its text together with your code. For media content you need another license such as CC (creative commons).
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