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Once you've gotten the boards out and removed the metal shield enclosure from around the main CPU board (which isn't difficult; it just requires a hot soldering iron and a bit of patience), the guts of an Intellivision look something like this.  (Assuming you have either an original Intellivision, or the INTV System III – the boards inside the smaller Intellivision II unit will look different.)  Notice the 5-pin flexcable connecting the power supply to the CPU?  These things are notorious for going bad over time; the plastic eventually delaminates, allowing air to reach and oxidize the thin metal ribbons connecting the pins, and all of a sudden you've effectively got a bunch of metal-oxide resistors connecting your boards together, instead of wires.

This one was no exception.  It was a bit hard to get a clear photo of it with my camera, but if you look closely you can see how the outer ribbons are discolored.  Could this be at least partially responsible for the messed-up picture?  I wouldn't bet against it...  And even if it isn't causing problems now, it's only going to deteriorate further over time, so as long as I've got the console apart I may as well replace it now and be done with it.

Some new wiring will take care of this problem nicely.  If you need to do this to your console, use 24-gauge wire and be very careful to keep track of your connections!  The Intellivision power supply puts out +5V, +12V, +16V, and -3V... and there is nothing to protect the electronics on the CPU board from incorrect voltages if you hook it up wrong!  I recommend using a fine-point Sharpie marker to mark each pin before removing the flexcable, and once you've soldered in the new wires I recommend a healthy glob of silicone at each end to keep the wires secured.

OK, now it's time to add some A/V outputs.  Hold on to your hats, folks; this is where things get interesting.