Sega Master System 2 A/V Mod - Capacitor Question.

[Agent24]

Greetings!
I came across this forum after looking for information on how to mod an SMS2 for A/V out capability.

My friend said he'd seen some people doing it and wanted to know if I could do the mod on his console for him.

I Googled around and found several websites showing circuits similar but not the same: Some don't use resistors, some use a capacitor on the audio line and some don't. Some use audio from one IC, some take it from another.

What all seems to be consistent however is that they use 220uF capacitors on the video lines - This seems odd as every diagram shown in the CXA1145 Datasheet shows 470uF capacitors used.


And that brings me to another question:
What is the difference between the first example circuit and the other two?

The first shows all video outputs with two 75 ohm resistors, one in series with the signal and the other going to ground.
Similarly, the audio output has a 10K ohm resistor going to ground.

Neither of the other two circuits have these resistors.


Also, in the first circuit, the composite output from pin 20 has two connections: (C) and (C') - what is the use of (C') ?



Can anyone help answer these questions?

Thanks in advance.

[Agent24]

Well, after reading the datasheets of the CXA1145 clone ICs (Fujitsu's MB3514 and ITRI's ES71145) I think I get the idea of the different circuits.

The first one appears to be a test setup (explains the data on the page after it) and the other two circuits are the recommended circuits, one for NTSC and the other for PAL mode.


This theory seems to makes sense, however what still confuses me is why everyone seems to be using 220uF capacitors while the datasheet specifies 470uF.

Is it a size\cost issue? Does 220uF provide a better picture?

[albino_vulpix]

I modded mine with the 220uF capacitors and works perfectly. I've done one with 470uF as well, and didn't notice any difference. As for the audio, whether you need the 10uF capacitor depends where you tap the signal from. If you take it from the Sega chip, you will need it, but if you take it from the CXA1145, you won't (it's already in the console). I took my audio from the Sony chip.

[Agent24]

Thanks, however I gave up waiting and used a 220uF capacitor and like you everything seems to work just fine.

I took audio from Pin 8 of the CXA1145, but I did use a 10uF capacitor - as far as I can see there is no 10uF already in circuit, Pin 8 appears to be connected to nothing, unless I missed it.

[whiskthecat]

The capacitor is just to remove any DC bias from the video output since these consoles are using single ended power supplies. It's job is to take a signal from say 0 to 5v and center it at 0, so it will now be -2.5 to 2.5. As people have noticed the value is not super critical. An extremely large capacitor would have bad high frequency response, and an extremely small capacitor would have bad low frequency response. Most likely anything in the triple digit microfarad range will do just fine.

[Agent24]

Thanks for that, I had forgotten I'd even posted this until you replied just now!

Now that I know a bit more about electronics, I realise the DC blocking effect is obvious - and the reactance of the capacitor obviously also plays a part.

I assume one would want to pick a capacitor value that has a low reactance to the frequency of the video signal, so as not to attenuate it or otherwise disturb it too much?