Audio/Video (A/V) modding vintage game console can be easy to hard depending on how the video signal is processed, and for audio, it's usually very easy.
Today is how to make an A/V output from a Magnavox Odyssey 400 (and this also can be done on any Odyssey 100, 200 and 500) as this (and the others Odyssey) is very easy to mod as you don't need any electronic components, just a simple A/V cable (and a soldering iron, etc..).
First, you need to open up the console by turning the only screw in the back and after the back panel removed, you should see this:
And now for the Audio part, find the PIEZO speaker (upper left - up to the SOUND ON/OFF Switch) and solder the audio cable where the "engraved A (note that it's not there originally)" and the ground beside it - see next 3 pictures):
By soldering to this specific solder point, you can either use the audio out or both audio out and piezo speaker. I put the sound switch to OFF and by doing this, the piezo doesn't "chirp" and the audio out makes a "thund" sound. Now it's up to you if you only want to hear a "thund" sound only or "thund & chirp" sounds (OR...only chirp sound if you unplug the audio out cable).
Now before closing everything up, test your mod and you should get a clean picture (on the TV) like this:
Voila! So now your Odyssey 400 (or 100, 200 or 500) is ready to be played on any recent TV's!
PLEASE NOTE: I'm not responsible to any damage that you may inflict or cause on any game console you (or someone else) by trying to modify, use these information at your own risk.