MOD: Mega Drive 2 PAL 60Hz color fix with a 2 pin crystal

That's not pin 1B, it's U1B - U1 = the chip, B = the gate. Use any gate. You can clearly see the NOT gates in his diagram and yours from the datasheet.

 

Thank you. I have figured it out, now awaiting for components to arrive!

 

Just an FYI for anyone that cares, I managed to get a 10-pack of the 4-pin crystal oscillators for modding a Mega Drive 2, and since I clearly don't need all of them, I'm happy to sell them to anyone who needs one. I don't remember what I paid for them, but if you just pay a few dollars for postage, that should be fine. Otherwise, if you have access to a PIC, that method seems a lot tidier. I love my modded Mega Drive 2, but having to flip switches can be kind of a pain. My Saturn uses a PIC for region-switching (although it's still manual, but self-contained, as I prefer), and it's so much better.

 

CXA1645 is annoying only because SONY omitted the crystal oscillator driver. You have to apply pre-amplified clock. A clock from a 4 pin oscillator box or from the circuit posted on this thread "smoothed" with a R/C circuit (to round the edges of the clock a little bit) will give a image quality identical to the original design.

Using square clock as color carrier will generate a "1 pixel odd/even vertical stripes" pattern on the color and that might be undesirable for some.

 

Thank you all so much for useful info.

Tanked Thomas, whereabouts in NZ are you, I am keen to take those oscillators off your hands?

Cheers

 

can you post a schematic of this R/C circuit? also what else can be used as the oscillator driver? can it be achieved with some caps and resistors and transistors or will a hex inverter be the best bet? Wanting to do this with the 3.58MHz crystal to get rid of the jail bars which is the vertical stripes you mentioned above.

 

It's suggested on the datasheet. Basically something like this:


CLK----1k----+----||-----> to CXA pin 6
GND----||----+

cap to CXA pin 6 =100nf
cap to GND = 100pf

Not the best way to draw it, the circuit should look like a T with the 100p cap in the middle connected to GND.

 

Maybe post a link to the datasheet so I can have a better look but from what I'm seeing the clock is the output from the oscillator circuit then to a 1k resistor followed by a 100nf cap. What about making a driver with standard components like I mentioned above or is it simply better and easier with a hex inverter or pic (not programmed?).

 

1k with a 100pf cap in the middle connected to GND and 100NF going to CXA chip.

That is to be put AFTER the driver, to smooth the edges of the square wave you would have at the driver output.

Feeding square wave color carrier to the CXA chip will earn you horrible "jaggies" at the spots where different colors touch each other...

Why should I post a driver circuit when you can just buy a 4 pin oscillator box for your required video standard frequency ?

 

I looked around and not many places sell them and it's Chinese New Year so the only one I can find on ebay won't be able to ship for a few weeks, besides since these are becoming harder to obtain a circuit to build one using the crystal (easily found and is cheap) is a good alternative. Oh forgot to mention this is to be used on an NTSC with the Samsung chip (I have like 4 of these systems with this chip).

 

Auckland. I won't be selling them all but I'll sell some. Check my sales thread.

The ones I have are 3.578545MHz, so I'm not actually sure they're correct for this mod. They are the ones used in the two-switch Mega Drive 2 mod, and they work just fine, with colour in 60Hz (although all my TVs are very accepting of NTSC signals, even though they're PAL units). If they're the correct ones for correcting colour, then I'll need to keep a few for myself and then I can sell the rest. Whether or not they ARE correct, though, I'm not so sure. The details on this mod, especially for the Mega Drive 2 and ESPECIALLY with the hard-to-get 4-pin oscillators, are very hard to find, and it's bothering me since I'm in the middle of a switchless mod. I want the composite colour corrected before I finish, so I need to know if I need to order more parts now or not. If anyone has actual specific details, please feel free to chime in.

 

Correct color carrier frequencies:

3.575954Mhz = NTSC

3.575611Mhz = PAL-M (60hz Brazil)
3.582056Mhz = PAL-N (50hz Argentina, Paraguay and Uruguay)
4.433618Mhz = PAL-BDGHI (standard PAL, the letters only design the aerial transmission differences, aka RF modulators video signal is identical for all these)

 

So shouldn't the 3.579545MHz oscillator be the one that adds 60Hz support to PAL machines by outputting in NTSC instead?

 

If you want to use CVBS, that is. If you are planning to use RGB you don't need to worry much about the color carrier frequency.

 

I want to use both, mainly just because I like options (and not all TVs have both). Since we don't typically have TVs with SCART jacks here, it'll be a while before I get the cables for RGB (although I could probably make my own, but from what I understand, I need some sort of converter or upscaler for RGB SCART to anything else such as HDMI, DV-I or VGA). To that end, I'll be putting an s-video jack in, too, and I assume it'll have the same colour problem. Still, regardless, I'd like the options. Oh, and for now, I don't have any adapter for my Elgato HD other than the component one that accepts regular composite, so there's that, too.

ANYWAY, trying to stop this from being too long: I just want the option - how exactly do I build/integrate the circuit using the 4-pin oscillator?

 

Just power the 4 pin oscillator (it's basically the 2 pin crystal with the logic chip and everything already fit on a very nice and shinny metal box.

It's pins are counted exactly like a TTL chip:

4-----3
1-----2

Pin 1 is not connected, pin 2 is GND, pin 3 is signal output (usually a square wave) and pin 4 is +vcc (usually 5v but 3v oscillator boxes do exist)

You hook the oscillator box so it has power and connect the pin 3 to the resistor side of the T circuit I suggested before.

 

Thanks, that's tremendously helpful. I'm pretty tired right now (bed time at 5:30am, yay!) so I'll re-read it in the morning, but if I understand correctly, the "T circuit" you mentioned is an entirely new, extra circuit that I'll need to build, correct? I'm not familiar with the actual places of the pinouts you mentioned (CLK, which I assume stands for "clock", and the CXA chip), but I'll find them. A proper circuit diagram would be nice but it seems like you've put all the information needed into previous posts, so I'll have a look later today once I've had some sleep. Hopefully I can wire this all up on the same board to make things tidier.

By the way, very quick kind of off-topic question: is it okay to get rid of the metal RF shielding? I can probably get away with keeping the bottom part, although it'll need cutting, but without some serious hack job, the top piece just gets in the way of all my wires. I assume it's easily disposable, but I was just hoping it didn't help extract heat from the board for anything (since it runs right by/touches the heatsink).

 

The point of the T circuit is smooth a little tad the edges of the square waveform generated by the oscillator. The resulting waveform will be slightly triangular instead of having steep transitions which would result in very visible jaggies on places where two strong colors touch each other.

If you don't care about hooking the Mega Drive to ultra old TV sets with only the antenna input, it can surely go byebye. I used the hole for it's connector to fit an S-Video connector upside-down (so I don't need to drill holes on the PCB). So my Mega Drive has a S-Video connector fitted on the hole of the RF modulator. Super ultra duper upgrade.

 

So this is what the circuit looks like correct?

 

Yes, that is it.

If you're feeling adventurous, you could also put 330pf capacitors on the R,G and B signals (just the capacitors, no need to add resistors) in the same way as the 100pf cap on the drawing and see if that reduces the problem with jailbars.