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Hi guys,

I’m trying to get an old French NES to work

I’m having those strange vertical interference, particularly visible on the blacks.



At first I thought there was something wrong with the av out module so I’ve replaced it entirely with the same result

I then changed all the cap on the main board... same

Then replace the cartridge connector... same.

Any idea ?

PS: I also tried to get the image directly from Pin 20 of the PPU and even though the image was quite dark it seems there was no such black bars.

Those bars are exactly aligned to the character cells, so you've got some kind of coupling (capacitive or power rail) between PPU A13 and the video signal.

They're often called "jailbars", if you want to ask other sources on the internet for help.

Getting rid of them involves rebuilding some portion of the analog path. Sometimes just replacing some capacitors helps; sometimes more is necessary.

Thanks for the fast reply.

So just to make sure I understand, I should follow PPU A13 trace and try to
Find something fishy ?

Any guess what I should try first ?

I actually had a similar problem on my top loader at one time, after changing the PSU I was using. I fixed it by hooking the RF cable up to my TV (even if I wasn't actually using it, as I was using composite output for video).
I think what was happening was that the extra connection to the TV helped the console being a little bit better "grounded".

Ok here’s an update for me :

I have tried MANY THINGS. Another PPU, another Q1. previously I had tried another Power box and changing all non ceramic caps

I think @lidnariq might be into something. I have disoldered PPU a13 and the jail bars were completely gone, (even though the image was logically badly broken. I have followed a13 to a resistor array i have changed but still no effect.


I’m not sure what more I can do but I feel I’m missing something really simple. Any idea ?

Thanks ,

Romi

This isn't something I've previously tried to debug; my two NESes don't seem to show jailbars. (I also don't play enough to really notice)

But to the best of my memory, the big thing is: replace the electrolytic capacitors.

In subsequent major revisions of the mainboard (NES-101 and AV Famicom), they accidentally ran the video trace parallel with some of the PPU control signals, so there the only solution is completely cutting the video trace and rebuilding the amplifier, but that's not true on the only European PAL (or SCART) model.

So you mean getting back from PPU pin 21 and rebuilding the entire line (transistor ans so on) ?

This console is à French nes. It seems to be a more recent one Nes-CPU-11 but as far as I can say the pcb design is exactly the same as all the other French NES...

I have tested the video directly out Q1 in composite and even though the jailbars are much less visible, they are already there.

Before q1 the signal is so week that that it s hard to see if the jailbars are there.

Could there be something before the PPU that would ultimately caused the PPU to display a slightly corrupted image like this ?


Thanks again for your help.

I did, but ... read the rest of my reply first.

As far as I know, all european NESes (both PAL and SCART) use the NES-CPU-10 and NES-CPU-11 boards with the 2A07/A and 2C07/A. The differences in the NES-CPU-xx revisions are all really subtle, and nothing that would cause this.

(This is in contrast to the NESN-CPU and HVCN-CPU mainboards, which are radically different)

That strongly implies you should replace the capacitors. Especially the electrolytic ones.

Have I missed some ?

Do you think I should change the Ceramic one too ? Some have a weird white stain I don't think I've seen before but I really thought Ceramic were immortal...




Thanks again for your help. I'm so frustrated with this one...

This guy removed the jailbars from a NES clone.
Hope this can help. The text is in portuguese.

Thanks, it looks interesting but this is a very different machine

Ones in the metal box that converts from PAL to SCART? There should be some big ones near the 7805.

We've actually only really documented the RF modulator seen on US front-loading NESes.

Conceivable, but I bet it's unlikely.

Well I have replaced the mitsumi box entirely so I know it s not the source of the issue.

I have started replacing the ceramic caps who looked the worst. But nothing

I’m really starting to despair.

The only major component I haven’t touched is the CPU. But it couldn’t do something like that right ?

Doubtful.

I mean, there's something causing coupling between the PPU A13 and the video signal, and given that it shows up almost immediately as the signal progresses, I've been assuming it's coupled through the power supply.

A simple enough test would be to use a capacitor to block the DC component from the +5V supply, and connect that directly to a composite video input. If you can see the video signal actually on the power supply itself, then I'm right...

Given the specific phase of the bright/dark pattern, it could be something wrong with the 74HCU04, maaaybe? Since that part not only operates the CIC but also is part of the audio path and generates NOT(PPU A13).


I don't suppose you have access to an oscilloscope?

Thanks man, that s a lot of good info. I will try replacing the 74HCU04.

I like your idea for the test. Where would you put this capacitor and what capacitance should I use ?

No oscilloscope but now I REALLY want one