Twin Famicom discs writes garbled graphics, not carts (pics)

[Fixon]

Hi everybody!

I'm working on repairing a Twin Famicom (black/red) and I think the FDS writes the data from the discs incorrectly? The thing is, carts work great, but when I use discs, it feels like whatever the disc unit writes to the ram(?) gets messed up, but just small parts of the graphics? It's hard to explain but check out these images:

Most of the time, I get the result like the image on the left. But maybe 1 of 15 tries, I get a perfect picture like it should be displayed on the right.

So what do you think is going on here?

Things I've already done:
- Placed the FDS reader in another Twin Famicom and it works.
- Tried the game discs on another working Twin Famicom and they work.
- Swapped IC105 and IC108, no change.
- The Twin Famicom has previously been NESRGB modded but that's not causing the issue, still have the same problem over regular composite.

So I'm thinking it's gotta be RAM or some IC that the FDS unit writes to on the motherboard?
I couldn't find any schematics for the Twin Famicom so if anyone has these, I would really appreciate if someone could link to them

Here's how the PCB looks like:




Thanks!