Megadrive 2 - Nothing onscreen plus extra wiring....

I've just started to build up my retro console collection and I picked up a lot which included two Megadrive 2's.

One is fine, just needed a good clean, particularly the cartridge slot.

The other is puzzling me. It powers on, sends a signal to the TV but there is nothing onscreen and I've tried several known working cartridges but nothing. So far I've done the following:-

- Cleaned the cartridge slot
- Resoldered the points underneath the cartridge slot in case of a dry solder joint
- Had a look at the caps, all seem to be ok, no buldges
- No significant marks of interest on the board, not many marks at all to speak ok

However, I have observed some 'extra' wires - https://app.box.com/s/v1egcx0nmuvc245qkxsi - I've tried removing these wires but the machine produces the same results, no change.

Does anyone know why these wires would have been installed? And does anyone have any idea as to what may be causing the problem please?

 

The extra wires are a fix for dodgy power connector plug.

 

Interesting. The machine powers on ok regardless of whether the wires are connected or not.

Is it likely that this is related to the blank screen, or the wires were put in place because the ac port or the PSU previously used had a problem?

Is it worth desoldering an ac port from another Megadrive 2 to see if that solves the problem and then at least I know what I'm up against?