Famicom av mod instructions?

[Da Bear]

As I recall, part of my research pointed to the lines being caused by the signals of PPU Pin 25 (A13) becoming commingled with the ground plane. If you draw a video signal directly from A13 (may have been Aq2), you will notice vertical bars that overlay exactly where the stripes fall. This video contamination is what I was, and am (when I get back to it) working to rectify.

To test this, wire your A/V board to everything but the video signal, and connect the lead from your amplifier (that normally goes to 2C02-p.21 'Video') to 2C02-p.25 'A13' and look at the video display from that signal. Compare it to the position of the lines, and you will see exactly what I mean in vivid detail. The A13 signal travels very close to GND in several places, so re-routing it is possible, but extensive.

(I didn't go that route, although I played with the idea, as it is not ideal. You'd need to life the traces and use shielded jumpers to connect the paths by hand in order to change it. I'm exploring other options...)

You may want to play around with this information...  Use it as you see fit: Perhaps you will find it useful, as the best way to fix this problem is to circumvent the cause, not merely deal with the symptoms.

How could I miss this post?

This weekend I'm borrowing a oscillioscope and I'm going to take a deeper look into this.

[bitmerge6502]

More discoveries, no answers
So, I swapped the my Famicom PPU with the NES PPU, no change.  I also completely desoldered the RF board completely & replaced with a breadboard power supply (regulated by a 7805, 5vdc). again, no difference in picture. the trouble has to be the mod or something else on the motherboard (possibly bad caps?).  One thing i had in mind was to add a RF modulator from another U.S.A. game system (PSX, DC, GENS2, N64, ...etc) to the Famicom board. At least that way, I can still play on CH 3/4 without the interference I get from the NTSC-J CH 95,96. I'm still trying to come up with any ideas what to do.

here's my mod:


this is what I get:


here's my board:

[jpx72]

@ bitmerge6502
Everything you said seems fine, there really shouldn't be any problem with your output. But there is. So I suspect the transistor. Can you replace it for something else?
Changing PPUs didn't made any difference so stick with the original one for the time being.

Try to connect PPU pin 21 directly to your TV without any components between them, you should get the signal too, it may be discolored but shouldn't be distorted (don't forget to connect GND). Let us know the result.

You can also play with the variable resistor on the Famicom PCB that is near the xtal. Mark the original position with a marker and turn it a bit, maybe you can get something from that.

Also try to change the 470uF cap for a 220uF one.

One step at a time, test in-between.

Double check if those resistors are really the ohmage you've written. Or try to replace them. This whole problem seems a "AV MOD" problem, not the Famicom's problem.

About the power input, as you said, you have 5V on the 7805 output so that's fine.

[Da Bear]

I'm not sure if it does any difference but try change the place of the resistor and capacitor. And doublecheck the values of the resistors.

Post Merge: February 24, 2012, 06:48:43 am
It could also be a problem with the capacitor. I remeber when I accidently scraped of the insulation of the capacitor and it touched ground, and had similar distortion in the picture. Took a while to figure out what was wrong.  

[bitmerge6502]

i used several types of transistors & pnp/ npn combos.  i also completely bypass the mod & connect video strait to pin21 (in fact, i do this everytime i make a change). the x-tal is the same as my nes (21.4772). so i'll mess with the timing circuit (check caps first) after i try to get something out of an usa rf modulator.

[famifan]

bitmerge6502, your famicom video distorsion is caused by non 75 Ohm output driver.

you can use my schematic from previous page, it is completely 75-Ohm driver.
Post Merge: February 24, 2012, 09:47:22 am

Try to connect PPU pin 21 directly to your TV without any components between them, you should get the signal too, it may be discolored but shouldn't be distorted (don't forget to connect GND). Let us know the result.

if you connect the open collector pin directly to 75 Ohm load like your TV AV-input you will get nothing. Signal level will be too low, right above the zero level.

[bitmerge6502]

thanks! i'll try it when i get home tonight. i also been thinking about using a 4050 buffer inline with  the video input & pin 21, or in place of the transistor altogether.
Post Merge: February 24, 2012, 08:33:21 pm
I have a bunch of these generic RF boxes laying around, so I soldered one directly to PPU PIN# 21.  And... It works (minus the "jail-bars")!  This means the signal coming from the Famicom board (at least ay pin 21, which I isolated anyway) is compatible with my TV (& others) & clock timing is correct. So I basically troubleshooted it down to the mod itself. This is what baffles me, because I tried several different mods from this forum & other websites. Mostly I have been working off a breadboard (except for the one int my previous picture). Some of the transistors I tried (mostly from my available supply) are: PNP = PN2907, 2N4403, MPSA77, MPSA70, 2M700, & the Famicom's  A973P. NPN= 2N2222, 2N3904, 2N4401.  It would seem one of these mods would work. I'm almost sure there's some small detail I'm overlooking. I have yet to experiment with matching the load to 75ohm TV input. I'm not completely sure how to test on the meter what the signal load is. I know video signal strength is measured in decibels (like a cable TV signal meter). I'm going to try famifan's mod & try to keep the lead wires as short as possible & solder them "component to component"  rather than use the breadboard.

Post Merge: February 24, 2012, 10:42:48 pm

Post Merge: February 24, 2012, 10:43:14 pm
GOTIT!
I rebuilt the mod using 1/8 watt resistor & got all the components as close together as I could. Then I added a 75ohmR between the cap & video out RCA. fired it up &... I got a really clean signal! the jail-bars are still there, but faint (or at least much fainter than the OEM NES top loader or YOBO or GEN NEX clones). The only problem: The picture rolls .
but then (on a hunch) I swapped the 220 uF cap for a different one (of the same type). The picture stopped rolling & was due to a bad capacitor (that what i get for salvaging & trashpicking my components ).

Well thanks everyone for your help! I have some more mod ideas I'll be posting soon along with a link to my restoration page when it's up. I'm glad I found this forum community. I always wanted a famicom since I first seen one in a magazine as a kid, now I can actually play & enjoy mine.

[jpx72]

This may be irrelevant, but I remember having problems with the picture too and the problem was in the AV cable that I used to connect the FC to TV. It had a 75ohmR hard-built inside the cinch connector. It was a AV cable from european NES.
AAAANYWAY congratulations!  Have many hours of FAMI FUN!

[Da Bear]

Had some fun this weekend  

With the help of a friend we hooked up an oscilloscope and tried to locate the noice. What we found was, without a gamecart and with power to the FC all the lines where 'fine'. Minimal powernoice and a nice and steady sine wave from the xtal all the way in to both PPU and CPU.

But when we inserted a game and powered on, hell broke lose inside the motherboard. VCC went from a thin line to at least 4 times bigger. A lot of noice! The xtal sine wave duplicated, and sometimes even tripled.

No idea where and what is causing this (yet), but I added some (a lot of) capacitors to the board to clean up the powerline. I have not checked the signals after, but they should be much more stable since there is only some very faint vertical lines remaining.

IGNORE THE WIRES BETWEEN PPU AND THE CONNECTOR!!! Tried to rewire the lines that goes near PPU pin #21 to figure out if there was any of them that was transmitting noise to pin #21. THERE WASN'T! The noise travels out in the powerline and then contaminates everything on the board.  I have NOT cut or lifted PIN #21 and I use Q1 in place to drive my AV out.

The size on the 'lyt' caps varies from 1uF to 10uF and the ceramic ones are 100pF to 100nF. And if I change all 'lyt' caps to maybe 47uF and the 100pF ones to 100nF I believe there shouldn't be much left of the vertical lines.



Any thoughts on this?

[famifan]

hice job!

i'd added ceramic capacitors on power lines of every IC too. 100nF+10nF

also, i'd retraced pin 25 (A13) as you mentioned above.
but i can't see the differences or improvements.

could you please capture the picture from "Darkiwng Duck" as i did?

[fredJ]

We don't have that game. You should use a more common game to compare with.

[famifan]

okay! please choose the another common game with the bright color palette and vast one tone areas.

[famiav]

Hello world!   I am a young French and I discovered this forum because I threw myself into changing my av Famicom (excuse me but I will surely make spelling mistakes ...)

I made the change with this method (http://jpx72.detailne.sk/modd_files/fc/avmod.htm), and it works very well, I find that the quality and the colors are even better than the Famicom AV official! Except the famous vertical bars ... So I tried putting capacitors between the pins of the PPU but it did not change anything ... I tried to read the pages of this topic but I did not understand everything, I thought I saw that disconnecting the pine 21 PPU directly, you can delete them?

If someone here could help me, that'd be great!  

Thank you!  

[famifan]

famiav, don't worry about spelling.

do you have AV-capture hardware for PC?
if yes, could you post the pictures with noticeable vertical bars?

[famiav]

famiav, don't worry about spelling.

do you have AV-capture hardware for PC?
if yes, could you post the pictures with noticeable vertical bars?

Yes, I have one. But the video input of my hardware only accepts PAL (or SECAM French), so my screenshots will be in black and white because Famicom's av out is NTSC ... However, I have found on the Internet a picture that is exactly what appears on my TV(with the same game) :



Can we do something? As I said, I tried to put capacitors, but no change! Thank you famifan to try to help me !