2 - Removing the power supply block (PCB 12 and 15)

This heavy block is hold by 5 screw to the main chassis. You need to remove the 5 screws show in yellow and loosen the 3 one show in orange.

You can hold the whole block by the sides of the band where connectors are, its in steel and offer pretty good grip and balance.
Then lift the whole block a little bit and lay it down, It will be hold by a bunch of wires but they should be soft enough to do it.

Now pull the power supply block to the side.

You don't need to disconnect any wire, but you may need to cut some plastic collars to gan more flexibility.
I needed it on one and not on the other, so no rules here, but my experience is cutting the collars didn't make the BS sound less good! 

3 - Removing PCB 7

Now come the scary part! Scary but not tricky if you go slow and careful.

PCB 7 is the big board between you and the tape mechanism. I opened my BS various time and close it after being stuck in front of that board, with the fear of arming any part. Well, they will be some electronic component out and some wires too, but be quiet and careful and it will be easy !

There's two kind of thing holding PCB 7: screws and 6 connectors.

As long  as you're careful, i found they're is no rule on the one you care first.

Removing the screw first may give you a wider access to the connectors…
Disconnecting the board, may prevent you from seeing it hanging by the wires…
Up to you…

Yellow arrows show you the screws, green ones the connector you can reach for now. The two other need the PCB to be removed. 

The 4 whites connectors need to be disconnected pulling them out perpendicularly to the PCB.
Dont pull them by the wire. The connector has a little edge you can use to play with a lever.

I did it by lifting them a bit with the nail or a little blade, and then slowly using a fat blade cutter as a lever between the edges.

-> Close up of the 2 connectors on the left.

-> Close up of the 2 connectors on the right.

Unplug the four connectors, unscrew the board and lay it down

When you're there, you still need to remove two connectors.

The first one is the one going to the playhead. It's a fat gray wire. It may be hold by plastic hooks glued to the board.
The two ones i worked on had the hook floating away, no longer sticking. If not, you can still remove the wire from the hooks.

Just lift the plug by the edges.

On the pictures, the black wires are just the ones from the previously unplugged wires on the left, hanging around.

Now there's still a group of wire going from a GREEN connector from the PCB right to the tape deck mechanism.

DONT TRY TO UNPLUG IT FROM THE PCB ! I believe it's not a plug.
You need to unplug it from the tape deck side. Carefully lifting it up, lets say, to the sky, assuming your BS is in a normal standing position.

Please note that all those wires may be fixed to the main chassis by any kind of piece, collar, strap… Removing them may help, as i did. but putting them back could be tricky hence not totally necessary (I mean, I manage to rebuild the BS without fixing them again…).

OK at this point, the PCB7 must be free to be removed easily.

Remove it.

Here is a picture of the PCB with the 6 connectors you'll need to plug back, highlighted.