DISCLAIMER: Do this at your own risk! It has worked for me countless times over many years - these beasts seem pretty rugged. I'm assuming you've got some familiarity with the guts of an Intellivision, and can handle a soldering iron and the tools involved here.

• Soldering iron with a small, narrow tip, and larger, flatter tip (a basic unit will suffice)
• Desoldering braid (some say wick)
• Solder suitable for electronics
• Philips screwdriver
• Possibly 1/4" Hex Nut driver (some Master Components used hex-head screws internally)

These instructions are applicable to a "standard" Intellivision - i.e. any version except for the Intellivision II. An Intellivision II doesn't encase the logic board in a tin can, so it's a lot easier.

1. Open the Intellivision Console
   There are six "Phillips" - type screws to remove on the bottom of the unit. Remove all of them using the screwdriver.
   Then, putting the unit "right-side-up" again, remove the top cover. Slide the controllers through the top of the unit.
   
   NOTE: Use extra care when lifting the top off the INTV System III and Super Pro System units. These sport a power-on LED and the wires connecting the LED to the main unit are easily detached from the LED or the unit itself! That turns into another repair project.
   
   
2. Remove Controllers and Logic Board Assembly
   There are six more screws holding down the plastic where the controllers rest, as well as securing the main logic board to the base of the unit.
   On some units, rather than Phillips-type screws, 1/4" hex head screws were used. Remove the screws, then the plastic part.
   
   
   
   You will see a power conditioning circuit, which has some large capacitors on it. On the edge closest to the main board, two different connections to the logic board are made. One wire has a plastic connector that slides over two metal pins. Disconnect this. The other connection is a 5-pin ribbon-cable type of connector that plugs into a small socket.
   
   Disconnect with extreme care! On these older units, the connectors on this cable can become brittle and break away from the main logic board itself, or from the connector. If you do break one of these connections, you'll have to repair that, too, or your Intellivision will not work. See this page for tips on how to repair this problem should it arise.
   
   Once you've disconnected the two power cables, lift up the left side of the main logic board assembly and unplug the controllers.
   
   NOTE: When you reconnect the controllers, remember that the brown wire should be closest to the "back" of the unit.
   
   
3. Open the main Logic Board Assembly
   After you have disconnected the controllers and other connections, you should be able to lift the logic board unit out of the main system. Put the large flat solder tip into your soldering iron and heat it up. You will probably need to desolder 10 solder points as well as the large blob of solder near the cartridge port in order to open the unit. (OK, I confess - most times I just break it off by the cartridge port. :-) ) Usually, the top cover of the logic board unit is silver, and the bottom cover black. You will need to remove both if you plan to access the cartridge port.
   
   Note: Be careful not to let solder bleed onto the main logic board or to cause any inadvertent connections between different points on the logic board.
   
   Note: Although these units seem quite robust, you might want to take standard electrostatic discharge prevention measures to avoid any unintentional damage to the electronics in the unit. In my personal experience, these things are very tough, but ESD is a tricky beast best avoided.