PAL Colecovision cvbs mod

[joyrex]

I bought a PAL colecovision (my first) a few weeks ago, and stumbled on some problems related to the rf-encoder. To fix this I wanted to install the Ben Heck composite mod, but since this is a PAL console, this will not work.

 

Instead I took an old MSX (Philips VG8020, common here), and removed the PAL encoder print which I mounted in the coleco. I connected the Y, R-Y, B-Y, VCC and VSS pins of the videochip to the PAL encoder and voila (I thought), I have a working composite out on my coleco.

 

Unfortunately, it works a bit; I have picture (in color), but there are two yellow bars going from top to bottom, and when reached the bottom the picture looses sync; stablelizing again when the bars start from top again.

 

I first thought it may be a ground-loop, but using shielded cables from video chip to pal encoder to cvbs connector, the problem remains.

 

Being a total noob when it comes to hardware, I ask for help. I attached the PAL encoder print, maybe that helps someone helping me

Edited March 25, 2008 by joyrex

[ten-four]

Sorry joyrex.

 

:!: Can't help you much, i am in the same boat too.

 

See this Link

[joyrex]

It works! The problem was with the additional circuitry that was already connected to the r-y, y and b-y signals. It was probably solveable by adding potentionmeters between the three signals and gnd, but I ended up cutting the 3 legs from the mainboard and connecting the signals from vdp directly to the pal encoder (adding the 3 resistors that were used in the VG8020, which are also recommended in the TMS service manual).

[ten-four]

It works! The problem was with the additional circuitry that was already connected to the r-y, y and b-y signals. It was probably solveable by adding potentionmeters between the three signals and gnd, but I ended up cutting the 3 legs from the mainboard and connecting the signals from vdp directly to the pal encoder (adding the 3 resistors that were used in the VG8020, which are also recommended in the TMS service manual).

 

 

Do you have any pics ?.

 

We do not have any Philips MSX VG 8020 here in Denmark.

 

Maybe you can find a non working one, with an intact encoder inside ?.

 

Let me know...

[Tony Montana]

It works! The problem was with the additional circuitry that was already connected to the r-y, y and b-y signals. It was probably solveable by adding potentionmeters between the three signals and gnd, but I ended up cutting the 3 legs from the mainboard and connecting the signals from vdp directly to the pal encoder (adding the 3 resistors that were used in the VG8020, which are also recommended in the TMS service manual).

 

That's excellent news man ! I have been quietly watching this thread in the hopes that you got it working.

 

Do you plan on releasing any info on this mod ? such as a tutorial, for us PAL colecovision owners ?

 

Top work !

[joyrex]

Here are some pictures of the mod I did. Note that I am not a hardware expert, and the pictures also include the butchering I did to find my way into getting it to work at all. Just ignore those; the concept is working, I just had a rough time fixing and creating other problems on the way (while learning some more basic electronics)

 

The first thing you can notice in the pictures is that I removed the videoram and replaced it with other videoram (that's the nice mushroom located where the original 4116 chips were located). During my examination, the videoram broke (or might already been broken); since I didn't have any 4116's, I replaced the complete circuitry with 2 mb81416 dram chips. That fixed my videoram problem.

 

The second thing you can notice is that I also replaced the videochip. This was one of the first things I did, basicly because I didn't know what the problem was, and because I don't have the right measurement equipment (or experience), somehow (don't ask why) I thought this was a good way of making sure that wasn't the problem.

 

After this I had a running coleco with the pal encoder from the VG8020 in it, though having picture (in color) I had a problem with 2 yellow bars going from top to bottom. I had various thoughts, but I couldn't solve it (and started this thread). I had a feeling it had to do with some circuitry still on the board that was causing incompatibilities with the signals the pal encoder should receive.

 

With this in my mind I thought it would be just a matter of cutting 3 tracks on the mainboard; the tracks that were connected to the r-y, b-y and y pins of the videochip. Unfortunately, I had a hard time locating all tracks, and I thought why not systematicly remove components that are connected to these pins, so I did that (yeah, I am a butcher). After a while, it seemed I removed all circuitry that was connected to the chip, but the picture was really unstable.

 

At that point I was still puzzled why it didn't work, but then it hit me; the VG8020 had some pull down resistors connected to the r-y, b-y and y pins. After I added those too it worked.

 

Conclusion; don't copy my work exactly My work proved that the VG8020 pal encoder is suitable for mounting into the Coleco, but my project had a lot of noise with other problems.

 

Though, it should be a matter of making sure the r-y, b-y and y pins aren't connected to the mainboard circuitry anymore (probably just making a few cuts on the mainboard), mounting the pal encoder print, connecting the input of that circuitry to the videochip and adding the 3 resistors (easiest is on that pal encoder print). Probably someone with more hw experience than me can redo the work, in a clean fashion

[ten-four]

 

That was a very big project you involved yourself in.

 

You replace the TMS9929 with TMS 9129, and then all the 4116's with two MB 81416-15 Dram chips.

 

Also you remove a PAL Encoder from an Philips MSX and replace it in the ColecoVision too.

 

I think you're much longer than i am.

 

I think i will take a break with the AV out for my self, it looks complicated.

 

My Grundig Tv. do have PAL/Secam/NTSC, but i have never try to change my AV-Tv. Output for NTSC yet. (If it can).

 

Thank you for your info, and i will not: "copy your work exactly".

[joyrex]

That was a very big project you involved yourself in.

 

I figured if I wouldn't try out stuff, I would never get picture out of the console The VG8020 was a great source for replacement parts.

 

Though, just before I started replacing the VDP I bought another Coleco (quite cheap); just to be sure I had one if this one would break. I want to keep that one in its original state though, so I have a reference machine for testing homebrew software. I keep this one on my desk for testing during development.

 

Maybe when I find another cheap Coleco, I redo the project (or try the rgb mod you did). With current experience and knowledge, that should be a lot easier.

[Vigo]

Very nice mod, but since the TMS9929 outputs a true YUV component signal, why not making a much sharper RGB mod instead?

 

http://msx.retro8bits.com/msxdocs/rgbmsx1.pdf

 

It's in Dutch, but it doesn't matter to use the circuit diagram.

 

EDIT: just saw that ten-four is attempting to do that mod...

Edited May 3, 2008 by Vigo

[joyrex]

Very nice mod, but since the TMS9929 outputs a true YUV component signal, why not making a much sharper RGB mod instead?

 

http://msx.retro8bits.com/msxdocs/rgbmsx1.pdf

 

It's in Dutch, but it doesn't matter to use the circuit diagram.

 

Fortunately I am dutch

 

I didn't know about this circuit until ten-four showed me his project

here. I think I even have the original magazine (cuc) in which this circuitry was published, but I totally forgot about it.

 

I won't continue on this Coleco, but once I have a new Coleco, I will try to do a rgb mod. I am not that experienced, and I don't want to break this working one

 

 

edit: ah, you already saw ten-four tried the mod.

Edited May 8, 2008 by joyrex