I recently installed a video mod board from the Longhorn Engineer into an Atari 2600. This gives the system real composite and s-video output ports, letting me bypass the RF modulator. The result is a much cleaner picture, especially with the s-video connection. If you're curious about how it went, you can check my comments on the AtariAge forums.
However, in attempting to install it in my first 2600, I think I fried something. I got no video signal at all or any signs of life on the audio jacks. Since I'd already clipped off resistors and removed the RF output on the board, I couldn't easily test it to see if it worked without the mod board soldered in, so I called it a loss. However, that left me with a mostly good 2600 motherboard.
Now, this isn't the original classic 2600 with four or six switches on the front, it's a "2600 Junior" that was introduced late in the system's life. The board has a 1983 copyright notice on it. However, that's still old enough that everything was soldered on as through-hole parts. Most modern devices use surface-mount parts that are much smaller and only connect to one side of a PC board. Back in the 1980's, surface mount technology hadn't taken over, so most parts were soldered to holes drilled in PC boards. In general, it's much easier to hand-assemble a through-hole design, but if you're doing automated assembly, surface mount works much better.
So, like any good electronics hobbyist, I didn't just throw away the dead 2600 motherboard. Instead, I got out my iron and my desoldering pump, and I started taking off components that might be useful in the future.
First to go was the 28-pin cartridge port. This is useful for modifying an Atari Flashback 2 console, one of those $30 plug-and-play game systems sold a few Christmases ago. The designer included pads on the PC board that you can wire to a real cartridge port to play old Atari games. I got a second unit at a Goodwill, so I'll probably try this modification soon.
Also useful were the many switches. There were switches for power, color selection, TV channel selection, and left and right difficulty. The power jack is a standard 1/8" headphone jack, so that's useful. I was also able to take off the two joystick ports; they're DB-9 connectors, but I'll probably use them to hook an old Atari joystick up to an Arduino.
I got a couple of potentiometers from the video circuit. These were used to adjust the color values at the factory. There was a big 5V power regulator chip that I could use in a future circuit. I was able to remove the power LED too, as it had long leads to be near the panel display hole.
What's left on the board is mostly not worth saving. There are about a hundred resistors, but the leads are clipped, so they would be hard to reuse. There are a few more ICs, but they aren't very useful outside of repairing other 2600s. I could save a few capacitors or transistors, but I'll probably just hold onto the board and just remove parts as I need them.