Thanks Manuel, I think it's a little more complicated than that.
Initially when I received the machine, there was indeed no video signal but I could hear the bios music playing along, though it was very faint due to blown caps.
After caps replacement, I cannot hear the bios music anymore. But I can hear the keyboard click, though it sounds like someone is shorting the speakers. Only F10 key sounds normal when you press it.
I haven't tried any carts yet as I'm not sure the machine wouldn't damage it.
Essentially now, I have no video and almost no sound 
Hello,
have you got the wiring pin out video for this MSX?
I need it for my Panasonic FS A1MkII and I dont see the service manual for anywhere.
Thanks a lot 
If you have the clicking sound, it probably means it's just the key click, so everything is working, except for your output image? Which brings you back to your 'no video signal' problem.
So it turns out that you were absolutely right, I finally dared putting a few carts to test the computer and indeed the sound is working perfectly and loud now too 
Controls are working fine. I'm not hearing the bios sound anymore though upon starting the machine without a cart...
But still no image 
from either RGB and Composite. Sounds comes ok on both RGB and Composite plugs.
I'm now testing the AC regulators, this is the one I have for +12V:
It seems to be the right type, I think it has been changed before though.
What reading am I supposed to get out of it on a multimeter? I'm getting 12V on one end and 60+v on the other end, doesn't make sense to me. But I'm not sure I'm testing it right. Will test again tomorrow.
I've refined my RGB21 cable but I can't any sound or picture out of it. My normal RGB cable gives sound out.
Is RGB21 my only option for a Japanese computer?
Thank you for your help in advance :)
If this is a standard 78xx regulator, there should be up to 22v on the left pin and stable 12v on the right pin. Middle pin is ground. If you have 60v on one of the pins, this may be very bad. I haven't seen such voltages in MSX. Are you measuring it correctly? I would replace this 7812 with a new one, but bigger size so it could dissipate the heat easier. And of course you need to find out where this 60v comes from. Such voltage can damage a lot of components.
Alexey
Thanks Alexey, I'm very puzzled as to why I get 60v on the board, perhaps I'm doing something wrong?
Here are the measurement I'm getting, just tested again this morning:
And here's my config on my multimeter, those 7812 regulator get AC power fed in correct?
Thanks Alexey, makes sense, pardon my ignorance
Here are the readings I'm getting, 12V on the left and between 5V and 18V on the right I cannot get a stable reading while in AC mode I could.
If I were to change this regulator, would 7812 +12v 100ma do the trick?