Sega STV - question for the techs

Okay, I have here a Sega STV main board that plays game carts perfectly and displays the colours correctly. However, it won't display the menu at all (except when it changes colour as part of some of the RAM tests), although I can tell that it's going through and running the various menu options just fine. IE it's a colour problem that seems to be affecting mainly the menu text. I know that the Sony CXA1044BP handles the final RGB output but I believe that the problem is before that - I suspect that PERHAPS one of the RAMs at IC33 or IC34 is faulty but can't be certain.

Does anything else handle the colour mixing before the Sony chip? Schematics would of course help immensely.

Thanks

What changes color the screen or the text?

The test menu is usually blank, BUT if I run the RAM test (I know what combo of Test and Service buttons to press as I have another board that will display the test menu) then when it gets to the test for what I assume is the colour RAM then the background colour starts changing (as it does on a good board) which serves to briefly show the text. Unfortunately it then goes blank again after revealing that all the RAMs tested up to that point are good, trouble is that I can't see the results of the test for the RAMs at IC33 and IC34 (and 37, but I believe that's the sound RAM so not worried about that right now).