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PAL Video Mod

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Hi folks,

 

I searched the forum for a PAL video mod and found some threads about it but unfortunately not schematic or something.

Finally I got stuck on this page http://134.155.48.202/~wi01196/colude/retrobrot/

The result didn’t satisfy me because I was unable to trim the potentiometer to the correct values so I had a too bright picture, missing colors and some interference.

But at the end the page was very useful because it shows all necessary signals on the pal device.

I made the following modifications to the given schematic:

I replaced the trim potentiometers with some resistors:

Lum0: 2,2k ohm

Lum1: 1k ohm

Lum2: 470 ohm

Lum3: 220 ohm

Tie all endings together and connect it to a 1K ohm trim poti (here you can adjust the total brightness).

For the 2600 and 7800 Chroma output use a 10K poti instead of 1K and tie the end to a 1uF capacitor,

For the Sync signal I used a 470 ohm (a smaller value seems to make no sense) and connect it to a 1K poti.

If you put all together you have the composite video output.

It’s a good idea to use ResQsoft (http://home.earthlink.net/~resqsoft/7800_mod.htm) circuit for audio it gives a good balance of internal and external audio.

The result of the effort is a much better picture than RF.

Maybe I will try a separate luma/chroma output but for now I am satisfy.

One side note, I wonder about the used rom in my 7800, it’s an ordinary M27128 Eprom with a sticker (0.8.w/ASTRDS 7800TSC/PAL).

 

Matthias

post-3747-1096839693_thumb.jpg

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Do you have a dump of that EPROM? I'm pretty sure mine said 1.0 w/ Asteroids, so maybe you have a different version than what's out there.

 

Mitch

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Do you have a dump of that EPROM? I'm pretty sure mine said 1.0 w/ Asteroids, so maybe you have a different version than what's out there.

 

Mitch

Not yet but I could dump it (have to search my eprom burner in the cellar first).

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If you can find your EPROM burner that would be great. :)

It would be nice to compare it to the later release and see if there are any changes.

 

Mitch

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If you can find your EPROM burner that would be great. :)  

It would be nice to compare it to the later release and see if there are any changes.

 

Mitch

It took some time but I finally did it.

The rom is attached but it is the same than the one I have on my HD so (unfortunatly) nothing new :(

Maybe it is differend to the version you have...

a7800p.zip

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Today I decided to modify the circuit for s-video.

I have to say that I am very satisfied with it :)

All I did was to build the mentioned circuit (I build it one more time for my other 7800) and don't put the luma and chroma signal together.

Instead I used the luma circuit from Jay Tilton's Video Mod (http://home.earthlink.net/~resqsoft/).

That's it, runs perfect.

Here are before/after pictures:

post-3747-1105836469_thumb.jpg

post-3747-1105836470_thumb.jpg

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Cool! It looks like you have a great picture. :)

 

Incidentally, do you know who's site that is that you linked to in your first post? He used my ASCII PAL 7800 schematic with the A/V locations and didn't even give me credit. :(

 

Mitch

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Cool! It looks like you have a great picture. :)  

 

Incidentally, do you know who's site that is that you linked to in your first post? He used my ASCII PAL 7800 schematic with the A/V locations and didn't even give me credit. :(  

 

Mitch

Hi Mitch, I don't know the guy but you can mail him if you just press "kontakt" in the navigation (top right)..

The ASCII schmatic was very helpful thanks!

The mentioned site was the only place I found informations about the pal layout do you have some more links?

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Yeah, I noticed that but I wasn't sure how much English he understood so I didn't know if it would be worth emailing him.

 

And, sorry, I'm not aware of any other PAL 7800 A/V mods. How much different is that one from the NTSC one?

 

Mitch

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Yeah, I noticed that but I wasn't sure how much English he understood so I didn't know if it would be worth emailing him.

 

Let me know if you want help with a translation. ;)

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Yeah, I noticed that but I wasn't sure how much English he understood so I didn't know if it would be worth emailing him.

Ok, didn't thought about that.

I can mail him if you want.

 

And, sorry, I'm not aware of any other PAL 7800 A/V mods. How much different is that one from the NTSC one?

 

Mitch

The one at the retrobrot site is quite different it just use potentiometer (no fixed value resistors, capacitors, etc.).

The last one I did is basically the Jay Tilton mod.

What I changed was the resistor values for Lum0-3 and I used a 470 resistor and a 1k poti for sync.

post-3747-1105883729_thumb.jpg

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Thanks guys, I may take one of you up on your offer when I decide what I'm going to email him. :ponder:

 

Mitch

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Hey,

How would it look if you were to try to remove all the 2600 video resistors and the chroma (2600) resistor, and make it a 7800 only video? You could try cutting just one side of each resistor and looking at the results and if it isn't any better, you could "spot" solder them back in. They might be "loading" the video 7800 down. Rob

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