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Possible Bally RGB/YPbPr design


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Board is about 1.5 x 2.5 inches (not counting the nubby part):


So this WILL change once I do some actual measurements of the space limitations inside the Bally.

The nubby header area takes the pace of the RF mod outputs.

So you would desolder that header add a new header and then socket/solder this on top.

Those pins labeled "F" stand for float. So connect them just be aware they don't go anywhere. 


Although YPbPr is at the RF mod output, the video appears poor over there. These inputs will instead connect someplace else (TBD). 


The crux of this circuit is :


[this is just a small part of the entire circuit]

As you may or may not already know. The Bally suffers from the same offset during blank as the Coleco; however it has this issue on BOTH Pb and Pr. 

Which makes the needed fix a lot more complicated.


I am assuming that 2.5V is the middle of Pb and Pr; So instead of doing a sample and hold (like the TMS-RGB) this is just feeding the console's 2.5V as reference. 

There is a CPLD attached (I could have used something else but I stock these already). The CPLD will use sync and clock to figure out the blanking areas then switch from RY/BY and 2.5V as needed. 

The resistor dividers shown are just placeholders atm. I will have to do more testing to know what it needs to be; testing which will be easier when working off one of these 1st revision RGB boards. 

This cannot have extra pallets or anything like that; this board works with the analog signals not the digital data. 


TLDR if my thinking is correct this will correct for blank and put out RGB and YPbPr. 

If not correct, minor tweaks from this board should work.


Let me know if you have questions or comments. I don't think this will be my production board just used as a base for adjustments. 



Edited by the_crayon_king
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A more finalized design; I dunno why I went for that goofy T shape before. L shapes are way cooler,. 

This still 1.5X2.5 but now it includes S-Video and Composite as an option. 


There are multiple steps to the design; so the idea would be to test video at each stage; before moving to the next stage.

1. Component with no blanking correction (get the p-p correct) [resistor divider]. 

2. Then component with the blanking correction [LM1881/cpld/analog switch + VREF]. 

3. Component to RGB conversion [lmh1251/THS7374].

4. RGB to S-Video and Composite [BH7236AF]. 


If I make an error at one stage the following stages will be bad as well., If I can make it to stage 2 then everything else is simple. 



I bought enough parts to put two of these together. If all goes well I will make tweaks and order a batch.

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  • 2 weeks later...


This is YPbPR using the blanking correction thing (it has issues in its method atm).


As a comparison here is what I had 2 years ago just feeding the unaltered YPbPr signal to a TV:


Notice the smear of reddish/blueish over what is supposed to be white (on the bottom picture);

Even the top picture has a tinge of this issue (which will be corrected further). 

But the difference is significant so I can tell the basic idea is working. 



However, there are still issues in the design. Which I have corrected on the schematic.

Basically the design needed a voltage follower. I am building a DEV board around what I have learned.

The DEV board should work on the Coleco or the Bally; with the Coleco being able to use either TMS variant (PAL/NTSC).


I have made two different "blanking" circuits (each has a different trade off [one will surely work])


TLDR; this is a positive result; I need a different board to tweak from because I cannot work off of the one I designed (it was built for a pick and place not hand placing parts).

So I will have to make a board that's only purpose is to determine how to make the actual production boards. 

Edited by the_crayon_king
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