Magnavox Odyssey 2 RGB

This is not the first time this has been done, and it’s probably not as polished as it could be, but it’s mostly an archive of what I learned doing this so either I, or anyone else can learn form this process. I wanted to take a stab at giving the Magnavox Odyssey 2 RGB because the existing RF only out is annoying, the Composite circuit works, but I have a RetroTink5X, and why shouldn’t I get to use it to the fullest. Besides, how hard could it be?

The first task was to assemble the key information:

• What version do I have?
• What has been done before?
• Are there any improvements that can be made there?
• Are there any extra features potentially to add?
• Can I get it to use the common Mini-DIN9 SCART cable with passive components in it?

I have two, slightly different, North American market Magnavox Odyssey 2 units. The differences are that one has modular silver controllers with somewhat shorter cables and uses a headphone jack for power, and the other has integrally wired black controllers with longer cables and a barrel jack for power (I just use a 12v DC power brick). Beyond that upper edge of the board where the controller ports are, I don’t think there’s anything significant to see between them.

I don’t know where all the information went from here, but it’s out there on the internet, scattered

The state of the art in Odyssey 2 video mods is actually pretty good. There’s the composite mod that is pretty well available and easy to build yourself. There’s also existing RGB mods that either break out to pins to wire to a connector yourself, or have a SCART connector on the board. I did find talk of an issue though.

At Nightfallcrew there is mention of two common versions of the RGB mod, one that is a bit older, and one that seems to take inspiration form the Odyssey 2 variant that had native RGB out (and an integral CRT). They describe how some software does not work correctly because the older mod doesn’t implement blanking the same as the intended, and after tracing around several different variants of the schematic it seems they’re right. The updated mod is from some french folks and I decided to base my mod, like theirs, on the Odyssey 2 with the integral CRT.

Let’s start small, this is what I based my SCART connector pinout on as it seemed to be what the official SCART cable was. I elected not to use this DIN8 connector used on that version of the Odyssey 2 because I’d have to make up a custom cable, ao I went with a regular SCART-to-SCART cable and used these pin numbers as the starting point.

And here is the other end of that cable. You can see some sensible connections, and also some ones that look pretty easy and as if they’re just for sending signals to the SCART display to tell it we’re present and how to use our signal. Let’s start with STATUS CVBS and STATUS RGB, now those go to pins on the SCART connector known as ‘function switch’ and ‘fast switch’ respectively. As I understand it function switch tells the TV there’s a signal present if there’s a high logic signal there (in this case high logic is supposed to be defined as 9.5-12v) and fast switch indicates that there is an RGB signal if there’s a high logic signal (in this case high is 1-3v) or a composite signal if there’s a low signal. Given the high impedance of the function switch signal, a single diode and a 1k resistor drop the voltage to just below the 12v max on that pin (if you assume the schematic is wrong and meant to read ‘+12’ and not ‘+2’. The fast switch line is supposed to have 75 ohm impedance, so feeding 5v through a 47 ohm resistor to that impedance would give us right around the max value of 3v.

This is how I implemented those function on my first design, why not do them exactly the same? Now that hose are sorted, what’s next?

Here’s an easy one, sound. There’s mono sound out of the Odyssey 2, and all that happens is a simple voltage divider before it exits the console.

And there it is, the audio output. You may notice, however, both of those schematics have something else hanging off that pin. Well….

It looks like that original schematic had a provision for an integral speaker with a single transistor amplifier to drive the speaker.

so I added that to my board design. I will have to guess at the speaker impedance, but I’m sure I will have something that could be used.

This next part is… complicated. This bottom section is the Sync line that went out the RGB connector in this console. Before I get to the details of how it makes the sync signal, let’s look at some other aspects of this design, like why does this have SCART out at all? It has its own CRT, what use would there be in giving it an output to another TV? Sure, it might be bigger, but there’s another critical thing that can be gleaned from this schematic. I’ll give you a hint, that signal that goes from near the end of the sync circuit and trails off to the left is feeding the input to the CRT driver circuitry. The circuitry that generates the signals to drive the horizontal and vertical deflection coils. And right near that is also where the RGB and Luminance signals come down from the video circuit up above, but what do we see them doing? They’re all combined together through some resistors into one signal! The blue and red have a potentiometer between them, but if you know anything about twiddling all the knobs on an old CRT television, you may recognize that one as the ‘tint’ pot that controls the ratio of red to blue. Now if all the signals are combined into one signal, that tells you the signal is monochrome, all the colors at once, this was a black and white picture tube. That tells us why there was a SCART out, because it was the only way to get color from this thing.

Now, this is the schematic for the North American Odyssey 2 with the modular controllers. I have here the same video section that I do from the one with the integral CRT. What you can probably tell is that the sync generation is much simpler on mine. From what I can tell, the other one uses a bipolar PROM with a precise set of bits in a specific order so that they can be clocked out and you will get a perfect video signal. In addition to that, according to the datasheet for the 8245 it can be set in either master or slave mode with pin 6 and in my console that pin is pulled high, whereas in the one with the CRT it is part of the circuitry attached to he vertical retrace signal so every frame it flips from master to slave and some… stuff is done. I’ll be honest, I kind of gave up at this point. These consoles are honestly pretty different and it’s amazing to me how much more intricate the circuitry is on one versus the other and yet they both run the same software.

I’m just going to say that I punted here and grabbed the composite sync circuit from here, another french site that seems to be similar to the ones selling their boards, but this one has a schematic. The problem is, where do I hook it up on my board? I don’t have the same connection points as on theirs.

This image helps, it indicates what the signal is supposed to look like before it goes into their circuit, and that’s a mostly high signal with low going pulses (presumably that’s during a line and it swaps in vertical sync). I took a guess and said that’s directly what’s coming off the video generator, especially because it’s called ‘comp sync’ on my schematic.

This is my version, since I grab it at a different place I didn’t run it through an inverter, maybe that was a mistake, I can hack on it when I get it though.

Now, if you’ve read my other articles, you may know how I feel about composite sync versus composite video. I figured I might as well have proper composite video coming out of this thing in addition to composite sync, if I absolutely needed it (spoiler: I did need it for full compatibility). To build this composite adapter circuit I ran through a lot of bad advice for simple converters and eventually landed on this one. It uses two transistors and has a pot to adjust the final amplitude of the signal which works just fine for me.

Here’s my schematic version of it in KiCAD. I did a little tracking and it appears this is the same circuit sold by RetroFixes, so if you don’t want to make your own, you can always buy one from them.

The last part is the hard part, how do I combine the video signals? Well, I could use one of the existing circuits out there, but they all seem to miss something. I can clearly see here that the Luminance signal (L) is ANDed with a signal from off the page that comes from P17 on the processor. That’s the only thing that pin does, so it must be important. If you look up at the schematic for the video section of our console we also have P17 being combined into the video signal (pin 10 is called either L or VIDEO). The schematic I stole the Csync circuit from includes the pin39 blanking the Nightfall crew post referenced, but it does not have P17, so how do I add it? Well, at this point the video is all digital logic so I should be able to combine it in like the Videopac schematics do, so I took a stab at that.

Using similar chips to the ones in the french schematic I made this, and that sums up my first pass at the Odyssey 2 RGB circuitry.

This was the board I had made, now I get to populate it and try it out. I will tell you right now I had to change just about everything on this board to get it to do what I wanted. Except the composite video circuit, that worked great and I kept it, no notes. I’m not going to go through these in chronological order of how I discovered the issues, but just start by itemizing the changes I had to make.

Here is the revised combination circuitry we were just talking about. Note the clanking circuitry in the upper left. I have changed the circuit from NAND-NOT to NOT-NAND to match the circuitry in my odyssey 2 schematic. I’m hoping this is right, because it works now. I did some of this work a couple months ago and left this for a while so I’m a little fuzzy on that particular fix.

The speaker didn’t work and I don’t really find the utility in using a period correct schematic for a speaker driver for a speaker I don’t have. Use whatever amp you like, I even have one you could use if you want a built in speaker. I have simplified the schematic to just the resistor divider, but I added the mono output on the genesis model 2 AV connector for compatibility’s sake.

Composite sync has been simplified. Technically the other circuit did work, exactly as needed to feed composite sync to the SCART connector (actually it needed to be inverted, just like the other schematic said), but that’s not what I need it for. I need it for driving the 5v level composite sync that some genesis model 2 scart cables are expecting on pin 5. That then gets fed through a resistor and capacitor inside the cable to attenuate it and feed it into the SCART port. The circuit from the french design would work if the cable I’m using doesn’t have anything inside, but it does so I need full strength 5v logic. I tried this with 74-series TTL gates, but I didn’t get a drive strength I liked, so I went full CMOS and am spending an entire 4049 to drive one signal that’s optional depending on your cable. I could have done this with something else, but this works and I had them on hand. I also added those attenuation components so you can optionally feed your straight scart connector with composite sync instead of composite video.

I don’t know that this was needed, but I copied the circuit inside the SCART cable I’m using and rebuilt it directly on the SCART connector. This has the advantage that I know it works with the RetroTink5x and also it means the single 12v requirement on the board has been removed so now it’s 5v only.

Now here is the annoying thing. The circuit in the french schematic works if I feed the SCART port directly. Well, it works as long as the SCART TV has proper video termination, which means an internal 75 ohm resistor pulling the signal to ground. If it does not the screen gets very bright all the way to white because the inputs have voltage being applied, but it’s not being dissipated anywhere. My fix for this was 75 ohm pull-down resistors on all the video lines. After that the video on the CRT looked great. The problem was, that was only when I wasn’t using the MiniDIN-9 cable, that cable has series resistors and capacitors in it which attenuates the signal and means that the picture basically doesn’t get through. Because of this I have to work backwards and terminate, re-amplify, and output the video signal again at the levels expected by the cable I want to use. I used the same setup as for the 3DO to determine the proper values to terminate the video to before the amplifier so the video out was at the proper level. I also added back the contents of the inside of that cable so I could then re-attenuate that signal so I could output it from a SCART cable directly if I wanted to.

Oh yeah, one more thing, this SCART cable nonsense. I bought a single SCART-to-SCART cable for this project because I wanted to have that as a second video output because why not. Turns out the cables roll over some pins from input to output. I think this is so TVs can output sound and can take in sound and video on the same port or something? I don’t really have a use for this other than to point out that you need to be careful if you’re applying a pinout for a male connector plugging into a thing, or a female connector intending to have one of these cables plugged into it. Oh, and some of them also roll over 17 and 18. And none of the grounds are connected internally. The more you know…

This is the first version I got working. Note all the removed parts, the external board, and there’s a lot of cut traces to separate the RGB lines for the two different connectors. The 4049 is glued to an open spot on the top of the main board which explains the wires running under there. I got to this point and realized two things:

• There’s nowhere sensible to mount this board in the case and have the connectors accessible
• I did this all on the more scuffed up one that doesn’t look as nice.

Cue the A-team montage of doing it all again, but without the need for experimentation.

There it is all mounted up. In order to have this all clear I had to leave off the RF shielding and remove the RF modulator, but honestly no great loss there. I have included all the parts needed to go straight out without that specific video cable, but just didn’t connect them.

Since I just got done with some triple bypasses on some model 1 Genesis-es let me point out some things about the model 2 AV port.

• Ground is on the shell of the connector, you need to remember to wire this as otherwise you literally don’t have a complete circuit
• Csync is 5v level, some cables use that and attenuate it for the SCART composite pin
• Composite is just straight, regular strength, composite video, some cables use that and don’t touch it for the SCART composite pin (so you need BOTH for full compatibility)
• Audio Left, Right, and Mono are not internally connected in most accessories so you need to have signals on all 3 of those pins to ensure whatever peripheral will work on this connector.
• 5v is used for the SCART detection signals, but I believe with all these pins populated you could install a genesis model 2 RF modulator and be back exactly where you started.

Pretty crisp

I will also warn you that there is a bit of an ideological difference in some RGB cables from others. Retrosix seems to think that the console should have its signals set up so that a passive cable will work whereas lots of other folks want to enable use of existing consoles and fix up the signals coming out of them for use with SCART or other RGB monitors. Just be aware of what’s in your cables and what your console expects to be in your cables.

I have my KiCAD stuff hosted here, the board is alright, but it could be better optimized. If I committed to surface mount it could be nice and small, but to be honest I did two of these and I’m not planning on doing others so feel free to make my design, or hack it up to be just what you want, or whatever.

Final note, this guy is really good at RGB stuff, maybe look through what he did as well.