Q about Genesis/MD, colour correction

[Mangledmon]

Hi guys,

I've got a tricky one that I trust some of the many clever people around here might know. On old PS1s, you need to use a special board thingo on the CXA1645M chip to get NTSC games to output a colour signal if you're using a PAL machine, kinda like how PAL Mega Drive games go in black and white when you jump to 60hz (it goes the other way too - force an NTSC Mega Drive/Genesis into 50hz and the output is b&w). From what I've read up for the PS1, you pull up pins 6 and 7 on the chip from the pcb, run a wire from pin 1 to pin 7, then run a wire from the colour board to pin 6, and it should then force the machine to output colour when it jumps to 60hz.

Now, I know the Mega Drive/Genesis uses a different chip (CXA1145), but I remember a colour mod doing the rounds that has a similar procedure to the above, but with a crystal and a few other bits in there. For the techies, would using a colour board solution on the CXA1145 as one would use on the old PS1's (SCPH1002 and SCPH5502) CXA1645M chip work? I realise this is probably a stupid question, but I thought if anyone knew this, you guys would

And no, I don't have SCART/RGB on my TV - one of the arse things about living in Australia: PAL sets, and no SCART on most sets

Thanks for listening... and being patient

-Mangledmon-

[ido8bit]

I seem to remember seeing a strange little converter board inside one of JAP Megadrives that caused it to output PAL in color, but I don't remember exactly how it did it.  I don't use composite video anyway.    

TVs with SCART RGB inputs are easy to find in Australia.  I'm in Melbourne and have several.  Most higher end Philips sets that predate component video have an RGB input, the same applies to Loewe, Grundig and some Sony models.  

[viletim!]

Mangledmon,

The little board is a crystal oscillator of some sort running at the colour-burst frequency (4.43Mhz). If you download the datasheets for both encoder cips it should be easy enough to work out how to conect it to the mega drive. In Australia, you can buy a little crystal osc. module from WES Components (code XO-110) in Sydney or Worldwide electronics in Perth.

Like ido8bit says, SCART TVs are easy to find in aus. All german stuff (Grundig, Loewe, Metz, etc) some chinese/turkish stuff (Orion, Teac, Grundig, etc). And most Akai/Samsung/LG TVs have a spot on the PCB for a SCART socket, you just have to solder one in (+ a few associated components).

[viletim!]

that should read:
...WES Components (code QXO-1100)...

[Mangledmon]

Thanks for the reply guys - I'll have a poke around.

Interesting to note vis a vi SCART connectors - when I bought my current CRT telly, I couldn't find much in my price range (then $1500 in 2001) that had what I wanted (component for my DVD player, flat screen, 68cm Funny how $1500 buys a helluva lot more now ). I did have a look around for SCART, but could only find it on the Euro TVs you guys mentioned... pity I couldn't afford Loewe sets, they were/are great sets. I have a Hitachi set, otherwise I'd look at throwing in a SCART socket.

Mind you, I'm finally looking to upgrade in a few years finally - I'm hoping to get SCART with the next one

I had a look at WES' site and found the product you mentioned. I've grabbed a datasheet on the CXA1145M, and I have to admit, most of my electronic skills revolve around pulling apart consoles and modding them, without knowing a whole lot about the electronics behind them. But, from what I can gather, would I lift pin 7 on the CXA1145N (NTSC/PAL in) and run the original trace for this from the PCB to the 'NC' pin on the crystal osc., then run the 'out' pin form the crystal to the lifted pin 7 on the CXA1145M, with 5v going to the '+' and ground going to the '-' pins on the crystal osc.? Sorry for being such a n00b - I've read through some of the technical discussions you guys have in other threads and you really sound like you know what you're doing... and I don't!!

Thanks again - you guys are a huge help!!

-Mangledmon-

[viletim!]

The encoder chip needs an oscilator input to generate the colour portion of the video signal. This frequency is 4.43MHz for PAL (50Hz) and 3.58MHz for NTSC (60Hz). The CXA1145 in your conosle can only produce PAL video because it's NTSC/PAL switch (pin 7) is connected to ground. What you want to do is disconnect the oscilator input to the encoder and connect your own...one that will stay at 4.43 MHz all the time.

I just had a look at the datasheet and rememberd that the CXA1145 has an on-chip oscillator...you just need to supply the crystal. So, either use a a crystal oscillator module: find pin 6 of the IC, trace the signal back through a capacitor and a 10k resistor and cut pcb trace at this point and insert your signal from the osc. module output.

Code Select Expand

.      / = cut
.            
.from big        10k        10n
.sega chip --/-+-RRR--+--+--CCC--- pin 6 of CXA1145
.              |      |  |  
. your         |      R  C
. 4.43MHz -----+   1k R  C 100p
. Oscillator          R  C
.                     |  |
.                     +--+
.                     |
.                    ---GND

Or you can use the on-chip oscilator by disconnecting everything from pins 5 and 6 and connecting a crystal, two caps and a resistor as shown in the datasheet.

Once you've done that the video will be PAL in 50Hz mode and PAL60 in 60Hz mode.

[ido8bit]

While a lot of Teac sets have SCART sockets (or space on the PCB for it) most don't have an RGB input, it is only wired for composite video.  The exception being those that have a Vestel chassis which are basically a Philips clone.

Does your Hitachi set have Teletext?  Its often quite easy to modify teletext sets to have an RGB input.  Tap into the RGB lines on the teletest module and then select teletext to enable your RGB input.

I wouldn't choose a set with a component input over an RGB input just for a DVD player.  It's easy enough to get a DVD player with an RGB output.  Aside from the obvious European players with SCART outputs, many of the noname chinese players have an option to output RGB even if it isn't completely obvious.  

I also wouldn't stress over getting a set with a dead flat screen.  Many so called flat CRTs are still curved internally, only the face plate is flat.  I've seen some flat screen sets with worse geometery errors than sets with conventional CRTs.  

I bought my Loewe set back in 1999.  I was also looking for a ~68cm set, but with RGB rather than component.  My budget was also around $1500.  I ended up spending around $1800 for a Profil 3272.  How times have changed.  I've seen those sets around used for around $80 (though that particular model does have a couple of weak points). Since then I've picked up a dead Calida 3572 (which was a $3000 set) for $8, I just need to get a flyback for it.  

[Mangledmon]

Viletim - you are a champion. I'm going to crack out my Mega Drive once I grab the parts needed and go for it.

id08bit - Sounds interesting - my Hitachi does Teletext, so I might poke around and see what I can find out!

-Mangledmon-

[blackevilweredragon]

How would I go upon this, with an NTSC Genesis model 1?  I installed a 50Hz switch, and I get a black and white picture too (TV also complains the Chroma isn't "Standard")...

[viletim!]

The genesis model 1 doesn't use the CXA1145?

[blackevilweredragon]

The genesis model 1 doesn't use the CXA1145?

I don't know at the moment..

But I just found out using a 32X, will give me NTSC-50..  Though, I didn't know that, because my HDTV would give me a black and white picture, as when my TV gets a 50Hz picture, it expects PAL, not NTSC, and there's no override (i have to use a regular TV)

[Guest]

*Bump*

Just thought I'd point out that I finally grabbed a few of those chips you suggested v-man, and the mod worked successfully!!!!!!!!!!!

I'm very excited - next I'm throwing an S-Video output on the back of my system as well. If anyone is interest, I'll throw up a tute with pics some time very soon

-Mangledmon-

[Skullmonkey]

*Bump*

Just thought I'd point out that I finally grabbed a few of those chips you suggested v-man, and the mod worked successfully!!!!!!!!!!!

I'm very excited - next I'm throwing an S-Video output on the back of my system as well. If anyone is interest, I'll throw up a tute with pics some time very soon

-Mangledmon-

I am very interested, looking to get this done in the next week or 2 myself, so this would be a great start.  Did you follow the quick diagram he drew and insert the crystal osc. module, or did you disconnect everything from pins 5 and 6 and just insert the plain colour crystal while using the internal osc. of the CXA1145? This is my preferance, as its hard to get the osc. modules where I am, but the plain crystals I can pick up locally.

Can't wait for the tute. This will complete my megadrive mod, as I've allready put in stereo AV out and Svideo out, it was really annoying that I would still get B&W when trying to switch my Pal Megadrive1 into NTSC.  (I also of course have the standard 50/60 & ENG/JAP switches). This is my 7 year olds megadrive, she loves the improved picture over RF.

I won't be doing mine, as I allways have a 32X on it, that takes care of the colour, but I will be adapting the Svideo/composite mod, so I can do it on the 32x  Hope to order 3 or 4 CXA1645M's next week on pay day, then I gotta wait for them to come from the US....

Sorry for the long post, hope you can put up a quick tute soon,

Cheers
Skullmonkey

[Mangledmon]

Hey,

Okay, all done - it's very wordy, but it's all in there just as I modded the machine. Click here to view it.

Cheers!

-Mangledmon-

[phreak97]

nice work, that's a simplified version of my mod i posted earlier.. i used a two legged crystal and caps and some more complex bs like that. i think i like yours better..
BUT. any work with clock signals or anything ac where the waveform on the line actually matters, should have wires as short as possible. i wouldnt be too suprised if your lag stopped after shortening the wires. long wires love picking up noise, and noise is bad. i did everything hanging off the cxa chip the first time, and mine only has noise cos i didnt tune the cap values up properly, and no lag on the colour between 50 and 60. also, did you try it on multiple tv's? the lag could be your tv thinking: "pal50 pal50 pal50 ntsc60 nts.. oops erm.. i mean pal60 pal60 pal60" or something along those lines.. i think i've seen tv's do that before.

overall.. nice mod, i think ill rip mine out and try yours to see if theres a cleaner signal.

[Nails]

What's the difference b/w a two and a four leg crystal?

Can you do Mangledmon's mod with a two leg?

phreak97 - you're saying it'd be better to lift pin 6, rather than cut the trace?

(Kloppy)

[Mangledmon]

Thanks for the feedback - this is the first time I've done a mod tute, so I'm glad it's generating a good response Thanks for the info RE: wiring - I wouldn't surprised if you're right about the lag due to the length of my wires too. BTW, is there a 5v point that can be tapped on the CXA-1145?

Nails: I *think* your getting confused. Using a 2-legged crystal means you still have to throw in a number of other components to generate the correct signal. The chip I used following viletim!'s suggestion has all the bits thrown into a single 4-legged chip of joy

Mind you, I might be wrong too, so I'll let one of the other, far more clever people respond to that

-Mangledmon-

[NFG]

In layman's terms, I think a 2-legged crystal just oscillates, you have to modify that oscillation to use it.  The four-legged ones provide a usable clock signal.