Question regarding Famicom jail bar origin

Started by FlyingTomahawk, February 13, 2020, 03:56:18 pm

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FlyingTomahawk

I finally joined the Famicom AV mod world and i must say this stuff is very addictive. I know, i know, but better late then never right?
I have more fun tinkering than actually playing on that thing.

I modified one of my Famicoms using this mod version here

I do get a decent picture with very, very faint jail bars which is OK for now.

So i have read up on that jail bar issue and it seems that it originates from a trace near the video trace? Is that correct? As shown here in this picture



I haven't tried it yet but would it make sense to re-route that noise trace instead of working around it?
Wouldn't that reduce the jail bars?

I know that some say getting jail bars is like a hit or miss thing depending on your Famicom board.


P

February 13, 2020, 04:37:45 pm #1 Last Edit: February 13, 2020, 04:54:13 pm by P
Jailbars is supposedly a result of noise on the video signal due to shitty power traces on the Famicom.
What's that pin you marked as noise? Pin 25? That's an address pin, you mean it causes the noise?

Yes I now read that pin 25 (A13) an pin 24 (/READ) are especially bad for the noise.


I talked recently about AV modding situation here.
That circuit is now also on the Nesdev Wiki.

FlyingTomahawk

February 13, 2020, 07:20:46 pm #2 Last Edit: February 13, 2020, 07:48:55 pm by FlyingTomahawk
Thanks for your reply.
Do you know a source for the PPU pinout?

There is a few things i'd like to ask regarding that circuit diagram.

So it is not necessary to lift or cut that pin 21 leg is that correct?

The nesdev wiki says to remove the 2SA937 transistor and then reuse it?
Reuse how "exactly"? Solder the base, collector and emitter to the PPU legs directly?

On my current modified famicom I added two 1uf ceramic capacitors between pin 20 and 22 of the PPU and 40 to GND of the CPU. So i could/should replace the 20-22 capacitor to a 47uf tantalum?

Sorry for the questions, just started to scratch the surface of all this and some stuff needs more clarification for me to better understand.

P

Quote from: FlyingTomahawk on February 13, 2020, 07:20:46 pmThanks for your reply.
Do you know a source for the PPU pinout?
It's on the Nesdev Wiki page I linked.


Quote from: undefinedSo it is not necessary to lift or cut that pin 21 leg is that correct?
Correct, doing so may crack the pin off and you are suddenly without a video output pin. The noise reduction of cutting it (or cutting the trace to it to not risk the pin) seems to be questioned too.
Also you loose RF if you cut it or its trace.


Quote from: undefinedThe nesdev wiki says to remove the 2SA937 transistor and then reuse it?
Reuse how "exactly"? Solder the base, collector and emitter to the PPU legs directly?
Hmm I didn't notice you need to remove this. Maybe removing it is necessary to reduce jailbars, but if you do, you definitely loose RF. Whether you remove it or not, I believe you can just use a general purpose PNP transistor for the AV amplifier instead, such as a 2N3906.
I'm sorry but I have no idea of the exact details of using this. I guess shorter connections are preferred though.


Quote from: undefinedOn my current modified famicom I added two 1uf ceramic capacitors between pin 20 and 22 of the PPU and 40 to GND of the CPU. So i could/should replace the 20-22 capacitor to a 47uf tantalum?
Yes at least if you try this circuit a tantalum is advocated. 47 µF is the absolute maximum and 4.7 µF is the minimum. Bigger or smaller than that and picture quality suffers so it's best to pick a value somewhere in between, like 10, 15 or 22 µF.



Since you are interested in the details I suggests you to read through the "Famicom AV Mod Situation in 2019" Nesdev thread I linked to, and possibly ask there if something is unclear. I'm not fully understanding all this stuff so I can't really answer better than this.

FlyingTomahawk

Oh you helped more than enough thank you very much. Much appreciate it.

Did some more digging and more trial and error work on my test Famicom (HVC-CPU-GPM-02) and finally got the result I was hoping for. It does however require to lift that pin 21 leg and remove the stock 2SA937 transistor.

Once the transistor is out bend the "base" (B) leg so that it points or faces towards the PPU. The other two legs, collector and emitter stay as they are, facing straight down.
Then solder the transistor back in as it was and apply solder to the "collector" and "emitter" leg. The base hole should be empty.
The "base" leg should not be touching anything but the PCB at this point. Now solder a thin wire from the "base" leg to pin 21. Make sure you don't touch the collector or make any other contact. (Maybe it would be good to solder a wire to base leg before soldering the transistor back in? ) Also add two 1uF ceramic capacitors between pin 20 and 22 on the PPU and between pin 40 of the CPU to any GND on the PCB. The shorter the better I guess.

Sample pictures

HVC-CPU-GPM-02



HVC-CPU-07



I did this for two Famicom versions HVC-CPU-GPM-02 and HVC-CPU-07. You just need to be careful since the transistor is mounted the other way around. On HVC-CPU-07 "base" leg is most right and on HVC-CPU-GPM-02 most left. The jail bars are almost gone, the picture is 99.5% clear and that is good enough for me.

For the AV mod itself I used this circuit diagram for HVC-CPU-GPM-02 using the Video, Audio and GND points as shown here.



and this one for HVC-CPU-07

Those two and the 1uF ceramic capacitors and the pin 21 thing gave me best results.
Thanks again for the help and pointers.


P

What's that capacitor on audio though? Audio output should already be amplified line level.

FlyingTomahawk

February 14, 2020, 03:40:36 pm #6 Last Edit: February 14, 2020, 05:52:07 pm by FlyingTomahawk
To be honest I have no idea. I don't understand much about electronics and  Famicom tech. I can read circuit diagrams and I can follow instructions and that's about it. I don't know why you need a capacitor here or there or what it actually does. Same goes for PNP or NPN transistors.

Most diagrams I have seen out there say you should put a capacitor on each line, video and audio. I have more than enough to go around so I don't mind putting one there if it works.  ;D

One more thing. People say always you can use any other NPN transistor, that is correct as long the pinout is same. Otherwise you'll have to wire it up outside of the pcb and cannot solder it in. For example i tried to use a 2N3904 NPN transistor that I used also on my Mega Drive S-Video mod. And that one has a different leg pinout. E, B, C
The Famicom one 2SA937 has E, C, B. Just something to keep in mind.

P

Me neither really, I'm studying it whenever I have time for it, slowly but steadily.
I guess that capacitor is for filtering noise or something so that sound becomes more clear.

You need to learn about polarity for all components though. PNP (positive-negative-positive) and NPN (negative-positive-negative) is about the polarity for bipolar transistors. You can't use an NPN in a circuit designed for a PNP (2SA937 is PNP so you can't use a 2N3904 in the same circuit). Pinout is a different matter though and you need to check the datasheet for that and plan accordingly.

Other things to note is for example electrolytic capacitors usually have a stripe with a "0" on the "negative" leg, while tantalum capacitors has the "positive" leg marked. Ceramic capacitors have no polarity.