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RGB output circuitry in the CV


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#1 zCRP OFFLINE  

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Posted Tue Jul 24, 2018 12:37 AM

Hi,

 while reading about the VDP, there are mentions of the additional circuitry needed to get RGB video out of the TMS9928-9 chips. On the application manual there is also a sample circuit using (IIRC) LM318 chips. Now, if I look at French colecovision boards, it looks to me like the RGB conversion is done entirely with resistor, caps and transistors. Does anyone have a schematic of this circuit? Otherwise, I might try to draw one but I am not entirely confident about my electronics knowledge to be sure that I can get it right, so any suggestions to get me started would be helpful ;)

 

Thanks



#2 omf ONLINE  

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Posted Tue Jul 24, 2018 2:01 AM

this my be difficult, there are almost no inforations about the french rgb scart machine,  i have been looking for this information for some time because there are some display issues with mine, ghosting / lines gown the screen  (not video ram as its been replaced with 5v ram)

 

so, this i most welcome,  if i can help at all let me know

 

 

for electronic drawing, there are many ptions, you may try frizing or designspark pcb for easier ones, however none i have used have particulary good part librarys



#3 zCRP OFFLINE  

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Posted Tue Jul 24, 2018 9:39 AM

I am also experiencing ghosting, and a problem with ground (see my videos on another thread a few days back).

 

I am now in the process of learning to use EasyEDA, and then will try to create a schematic of the circuitry which goes from the B-Y, Y and R-Y output pins on the VDP to the DB15 video out connector. This should work fine as apparently there is only resistors, capacitors and transistors in this circuit, although a lot of them!

 

Do you have the french machine with separate RGB board or is it integrated in the main board?



#4 omf ONLINE  

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Posted Tue Jul 24, 2018 10:06 AM

it is the version that is all integrated into the single main board



#5 mobiusstriptech OFFLINE  

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Posted Thu Jul 26, 2018 8:27 AM

Why would you want to replicate the French RGB circuit? Most of the French RGB circuits on old consoles were cobbled together signals from composite video or the bare minimum effort to meet the standards. You can already purchase RGB boards or build them yourself and end up with a really nice quality picture.

#6 zCRP OFFLINE  

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Posted Thu Jul 26, 2018 9:03 AM

I believe that the french board takes the R-Y, Y and B-Y signal from TMS9929, it does not convert a composite signal, and the output is generally very nice.

Anyway, I do now want to replicate it. I just want to understand how it works in order to be able to repair it when/if necessary. Obviously, if I am unable to service the existing circuitry, which I hope will amount to replacing a few transistors, then I will have to hook up a new RGB conversion board.



#7 omf ONLINE  

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Posted Thu Jul 26, 2018 12:27 PM

im going to replace all the transisters on mine at some pont in case they are feeling sorry for themselves



#8 zCRP OFFLINE  

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Posted Thu Aug 2, 2018 2:42 PM

Here is a first draft of the schematic for the RGB circuitry. It is messy, and I had no idea what I am doing, but hopefully it can be useful. Any comments are appreciated.

Attached Files



#9 omf ONLINE  

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Posted Fri Aug 3, 2018 2:48 PM

looking hopeful :)

 

i noticed there are a few resistors that have question marks, is that because you couldn't read the writing on your board?

 

i always suspected that variable resistor was part of the video circuit now i know :)

 

any idea what it actually does?



#10 zCRP OFFLINE  

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Posted Sun Aug 5, 2018 2:02 PM

the components with question marks are indeed those where I could not read the actual name, mostly because it is written exactly below the component ;)

 

I have to admit, I drew up this whole stuff, but I have no idea what it all does. The reason I did it was to find appropriate points to probe and find out where a signal gets "strange", thus giving me some indication on which component to replace.

 

What I did leave out is all the circuitry that processes the signals from the expansion port... adding that would not be a big problem, but my eyesight is a bit taxed for now.

 

I have (in the past) socketed and replaced every single IC on this CV board, with the exception of the two 4066s. But now I have the feeling that they could be a significant source of trouble...



#11 omf ONLINE  

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Posted Mon Aug 6, 2018 5:51 AM

I also replaced all the IC's on my RGB scart board. i dont think the new chips are an issue as surely the machine wouldnt work at all if they were ?

i could test this because i kept all the chips that were removed i think apart from the actual bad chip (which was the most expensive part so i left it till last (the main system ram chips) :(  :thumbsdown: )

 

i will dig it out tonight and see if i can trace some of the ? resistors and things.

 

maybe put some resistor values on the drawing as well, could be useful in the event of the component blowing up or corroded etc?



#12 zCRP OFFLINE  

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Posted Sat Sep 1, 2018 5:35 PM

Please notice that the voltage indicators are inverted, where it says +12V it should be +5V and viceversa. Also, the "optional" video output connector labels are incorrect.

 

Anyway, I was finally able to solve the issues with this unit and now I have a perfectly working CV with brand new AC adapter (which I installed inside the CV) and crystal clear RGB output :-D






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