So, I just got out my multi-meter and read your question again and realised I wasn’t quite sure on what some of that meant.
So I thought it would be best to double check before I make the situation worse!
With the positive probe when you say the DVI connector, do you mean the outer casing of the connector or to the relevant pin inside the connector itself based on the pinout image you linked to?
With the negative, do use it on the points on the bottom of the OSSC, or do I use it on the pins inside the DVI connector?
Apologies if this seems like a daft question, this is my first time using a multi-meter!