page 2 of 5
For the A/V output modification, we turn once again to the simple one-transistor, emitter-follower amplifier that was used in the Intellivision modification:
|C1, C2, C4, C5||100uF, 25V aluminum electrolytic|
|C3||10uF, 25V aluminum electrolytic|
|C6||0.015uF 25V ceramic (optional)|
|Q1||2N3904 small-signal transistor|
|R1||1.6K 1/4W 5% resistor|
|R2||2K 1/4W 5% resistor|
|R3||10 ohm 1/4W 5% resistor|
|R4||75 ohm 1/4W 5% resistor|
You will also need: a piece of perfboard about 1.5" square; some shielded coax cable (about 24-30") and some regular hookup wire; a pair of panel-mountable RCA jacks (such as Radio Shack #274-346)
As before, C6 may or may not be required – personally, I haven't found it necessary on any of the consoles I've modified, but if the colors seem too strong and harsh on your TV, start with an 0.015uF capacitor and, if necessary, experiment a bit until you get the results you like. If the colors are too strong, increase the capacitance; if they're too weak (or the picture goes to black-and-white), reduce the capacitance.
Interestingly enough, while searching the web for information about the Odyssey2, I did find a site which claimed that you could just tap the audio and video from the board and run it straight out to some A/V jacks without any need for DC filtering or impedance matching. So, it's possible that this video-amp circuit isn't needed at all. But what the heck – this single-transistor amplifier is so cheap and simple to build, I figure there's really no reason not to err on the side of caution and put it in anyway.
Finding a place to tap GND and +5VDC off of the Odyssey2 board to power the video amp is easy. GND can be found all over the place, especially in the area of the board shown at left; just pick a good spot attached somewhere to the ground plane. +5V can be had at the decoupling capacitor attached to pin 16 of either of the two 16-pin TTL chips.
In retrospect, I think I could've also tapped power right off of the 3-pin connector that fed power and signal to the RF modulator – but at the time, I didn't have a schematic to follow, so it seemed best to pick off power from someplace I was sure of. 7400-series TTL chips in DIP packaging are always a good place to start looking for +5V and GND traces, since almost all of them (with very few exceptions) have their power pins in the same locations: if it's a 14-pin DIP, GND will be on pin 7, +5V on pin 14; for a 16-pin DIP, GND = pin 7, +5V = pin 16... and so on.
And speaking of the RF modulator – notice the empty space circled at left? That's where the RF modulator used to be. The Odyssey2's RF modulator is, conveniently enough, a separate off-board module, rather than one soldered onto the PCB. The unit inside this one was rather badly out of tune after all these years – neither of the channel settings gave a visible picture where they were supposed to; I had to hook it up to an old-school TV set with analog tuning knobs (remember those?) and fiddle with the fine-tuning ring to get any kind of picture. But since this modification makes the RF modulator obsolete anyway, why not just take it out entirely? Especially since it, too, is dead simple to remove; just remove two hex-cap screws, unplug its 3-pin Molex connector from the logic board, and it comes right out. As a bonus, the now-empty mounting posts provide a convenient place to mount the video-amp board.