Do you think its possible to maybe downgrade the converters to the one the 303/505 used ? I actually like those a little better

The easiest mod I can see is to replace the cinch connectors with standard jacks.

this means change the RCA outs for phono jacks, right?

'cos that'd be a very nice mod to do

1/4" jack chassis sockets are pretty wide - the kind that fix to a panel with nuts. You won't get board mounting ones that fit the same board space as the existing rca jacks. Might be possible to use in-line (cable) mounting 1/4" jack sockets, they're a lot narrower and you can still bolt them to the back plate using the back cover (sleeve) as a nut. They'll stick out further into the 404 but there is space to spare that way.

Anyway, have figured out the extra feed from the mic pre - it's not a signal feed at all, just a reference voltage that stops the signal level jumping at the 4066 switch chip when it switches. So the diagram still stands for a general arrangement.

Missing bits. There are some spaces for stuff on the main board that're not fitted. An extra switch control on the 4066 (only 2 of its 4 switches are used as it is).
On the opposite side from the analog circuits, there are empty pcb holes for 2 connectors and a timing crystal.
What were they for?
Maybe the rest of the 4066 switch was supposed to disconnect the line-in like it does for the mic?
Maybe there was meant to be an s/pdif connector?

Cheers
Jim

maybe Roland left room for features in a later model
there'll probably be an upgrade at some point
it's very interesting that there's room for another crystal. makes me wonder what it would control


yeah phono jacks are pretty big, hey

There is a problem with 1/4" jacks. If you look at the photo I posted, there's a cable connector (white) right behind where the RCA jack sockets are (red). You could remove that connector completely and solder the cable wires directly underneath the jack board - there's a connector at the other end of the cable on the main board.