Famicom av mod instructions?

[lillin]

Also I love his idea of adding the capacitor to PPU - that's a signifficant improvement (reducing the brighter vertical bars)!!!

Empirically the bars are interference from the address bus, you don't need to put a giant-cap on your PPU to lose it.

EDIT: If you experience very bright image, try to experiment with the 2,2k ohm resistor connected to +5V. Try to exchange it with lower ohm resistor, but not lower than 500 ohm!

Any time someone has to experiment in a non-RF circuit there's something wrong. These "AV mods" are common-collector amplifiers so they simply buffer the voltage on the video pin. Your 2.2k /  220 R combo is setting the level too high in parallel with the TV's 75 ohm load. You only need to connect the emitter to the supply though a 300 ohm, and the output cap to the emitter.

I don't know the purpose for the 560p shunt cap, it's creating a low-pass filter with a Fc of 2.58 MHz... Since video is 6 MHz it's doing a bit of harm. I'd think it'd make it blurry...

PS: my old schematics suggested 10uF capacitor between pin21 and transistor. But this only worked on HVC-CPU-GMP-02. So I removed it from this one.

The capacitor is only necessary/only works when using a NPN transistor since the PPU doesn't source current, it sinks it. Using a PNP sources current for the PPU directly.

Any chance for a schematic?
thanks

[kyuusaku]

This is what the AV FC uses, simplified to remove over-engineering:

[jpx72]

Hmm thanks for the suggestions on removing the unnecessary parts from my schematics, but is it not too much for the transistor to handle if there is only 300 ohm resistor on +5V? I think it depends on transistor type though... The small cap is removing only the highest peaks on the output, the image will not be blurry, but the extreme sharpness will be reduced (barely visible).

Empirically the bars are interference from the address bus, you don't need to put a giant-cap on your PPU to lose it.

And what do you suggest to remove these bars? Because giant cap works like charm.

[133MHz]

Empirically the bars are interference from the address bus, you don't need to put a giant-cap on your PPU to lose it.

And what do you suggest to remove these bars? Because giant cap works like charm.

Completely isolating PPU pin 21 from the printed circuit board, using shielded (coax) cable to keep the video signal from being "contaminated" with computer noise.

[jpx72]

Completely isolating PPU pin 21 from the printed circuit board, using shielded (coax) cable to keep the video signal from being "contaminated" with computer noise.

Isolation not working, tried that. Shielded cable, maybe... Giant cap - 100% working (without isolation of pin 21 and with any cable). Maybe the signal is already contaminated when outputing from PPU because of the power adaptor and/or other components on PCB connected to PPU...

[kyuusaku]

Hmm thanks for the suggestions on removing the unnecessary parts from my schematics, but is it not too much for the transistor to handle if there is only 300 ohm resistor on +5V?

The transistor can handle a lot more current, over 10x.  Worst case the PPU level will be from 0.7V-1.7V = <17mA .

The small cap is removing only the highest peaks on the output, the image will not be blurry, but the extreme sharpness will be reduced (barely visible).

What's the point of it though? The reason it's not terribly noticeable is because the filter has such poor roll-off. It is affecting the high frequency components, particularly color information.

I'm not sure how to best solve the bar problem, my stock FC/AVFC don't have it. Some clones and AV-modded units I've had do. As best I can tell this has something to do with the board layout. The bars appear every 8 background pixels right? This corresponds to the toggling of PPU A7.

[jpx72]

The bars appear every 8 background pixels right? This corresponds to the toggling of PPU A7.

Oh I can't say if it's every 8 pixels, but probably yes. My point is that the solution IS adding the capacitor, on my FC with HVC-CPU-07 board - with added 820uF cap - the picture is nearly perfect, the bars are not visible at all. So a definite solution for people that know nothing about electronics at all.
Agreed to that small cap though, let's drop it

[lillin]

So you are puting the 820 cap instead of  the 220 right?

[jpx72]

NO, I am using 820uF instead of 1000uF cap which is placed between pin 20 and 40 of PPU (not on the schematics!) for getting rid of the white bars (a common issue of AV modded famicom).
On the schematics: I am not replacing the 220uF cap. BUT you can replace the  2,2k RESISTOR with something lower, like 0,5k (500ohms) when you have brightness-lowering issues. This depends on the type uf transistor you use.
Check my page for altered schematics and to get rid of the confusion. http://jpx72.detailne.sk/modd_files/fc/avmod.htm

[lillin]

Ah right, ok thanks When i first checked the site i stopped at the schematic and never went down to the bottom, my bad
Post Merge: September 23, 2010, 03:04:56 pm
Ok, so i just finished my mod and i am amazed how good the picture quality is.
Its clear, sharp, and best of all no ghost lines
To be honest i was expecting to have the ghost lines, as i did not add the 820uF cap( as i couldn't find one). So im guessing the ghost line problem varies from famicom to famicom, some have it, some don't.
One more thing to note is  that i used a 2N3906 transistor. If i have the time i'll probably try it with the 2sa937, but i don't think that it will have a big impact on the end result.
Bottom line, use a 300 ohm resistor and drop the 560pF cap. 

[jpx72]

Congratulations to your mod lillin!
Your transistor has slightly higher max frequency (250 instead of 140MHz) and  collector current (IC=200mA instead of 100mA) than the 2SA***. Have you added the 220ohm resistor between emittor and 220uF cap or just did the kyuusaku's schematics? And did you solder your video output on the pin #21 of the PPU?
What PCB model you have?

[lillin]

I did kyuusaku's  schematic.
And yes i soldered it to pin 21 on the ppu.
The board was a HVC-CPU-07.

[kyuusaku]

It almost doesn't matter which transistor you use in this case, in fact it's best to use 2N3906 because it's the most common PNP and cheapest (in the US, UK should use BC556 or something). The frequency also really doesn't matter (as long as you're not using a power transistor) because frequency depends on gain, and in this amplifier the gain is 1 (ie. no gain) and a video signal is only 6 MHz.

[Xious]

I wonder if the lines are a flaw inherent to the earlier PPU revisions...or just a weak signal path on the PCB. I note that they are quote noticeable on my C1, and on a couple modded FCs, but I don't think any of them are the HVC-GMP version. I need to look at how the composite video is set up on the Sharp AN-xx series to see how it differs from the HVC-101.

I guess the real question is: What causes the 'jair bars'  int he first place? I think it's a PPU design issue, but I guess it could be a signal strength or sync problem (?).Additionally, what is the inherent benefit of drawing video off the PPU directly, rather than after the 220 Ohm resistor? I would think the latter to be better, but I'm not sure.

[kyuusaku]

It's none of those those things. Reading comprehension.