No Granny (nor Baby PM) here ! but I know the vidiot (and deluxe boards).
Indeed, without board connected you should have a screen like this one (left side is not empty): 
2017-10-19_15h53_15 (resized).png
With connected board, you can see the XMIT/RCV datas:2017-10-19_15h49_32 (resized).png
And if you press again the TEST button, you get the color bar: 2017-10-19_15h55_11 (resized).png
If you get a partial video image, you can be quite sure that the TMS9928 (slave and master) works, but there is some default in the video RAM chips.
At the power up, there is a self test procedure (read the Vidiot dlx Theory of operation document - available at ipdb). The led flash depending on the test achieved. There is a video RAM test in this sequence, so it can be helpful to identify the issue.
As all chip are on socket, you can first test an interchange between the two TMS9928 - if this change nothing, this means that the video CPU are not faulty). You can also try to remove one or more RAM chip (U19..U24 and U29..U34) to try to identify which of them are deffective.
