To free the main PCBA, unplug all of its interconnect cables.  At one end of the assembly there are six connectors to unplug, shown here.

I always use a Sharpie^® to color code where the cables go, and which way.  This speeds up the reassembly.

Nearby, there's a nylon cable tie you need to cut, shown here.

With the cable tie removed, you see another connector to pull.

By the way, the blade of a screwdriver can help pry apart any tight connectors.

Along the front of the circuit board are two LED connectors to unplug.  One is shown here.

This photo shows six cables (including a little red one) that connect the main PCBA to the preamp tubes.

It also shows a couple more cables to unplug.

Once all the connectors are unplugged, the PCBA can be backed out of the 6505+ chassis and repairs can begin.