This is my collection area for Tandy Color Computer 2 Projects. I've only got one at the moment. It's a composite video mod for the CoCo 2. The CoCo was designed in a time when it was perfectly reasonable to assume that all televisions had an analog RF input. Of course that's no longer the case, so something has to be done to get a CoCo 2 to work on a modern monitor or television.

My CoCo 2 has a vertically oriented RF modulator. So, I designed a replacement PCB for the inside of the RF modulator to convert it to a composite out and sound out RCA port. I desoldered the MC1372 chip from inside the old modulator board, since I'm repurposing that chip.

I'm not an expert, but it appears that the video graphics display chip of the CoCo2 (the MC6847) outputs a luminance and two chrominance channels (called PHI-A and PHI-B). Even though one of these is a luminance channel, and in theory could drive a black and white composite signal, it isn't at the right voltage levels or bias, and so it must both be conditioned and mixed with the chrominance channels. That's where the MC1372 chip comes in. Originally intended as an RF signal generator, it has a useful "test mode" that allows it to output a mixed composite (or CVBS) signal that can then be conditioned to drive a television monitor that accepts CVBS, which most TVs still accept (though some very modern ones don't have separate inputs for it -- they accept the CVBS channel on the green RGB input).

Basically the way you accomplish this is to put a diode between pins 13 and 14 of the MC1372, which is where the "tank" circuit is supposed to go, which traditionally was used to "tune" the output to match the expected frequency rate for analog channel 3 or 4 on a traditional television. With the tank circuit bipassed, the chip produces a plain composite signal with colors mixed in appropriately. So, then you simply use some transistors to get the signal into the right level and bias, and tune the bias level with a potentiometer. I also included a potentiometer to tune the output level of the sound signal.

With some right angle headers, you can simply plug this board in where the original RF modulator can went. I made the PCB the same size as the original PCB in the RF can, so hopefully I'll be able to put it back in the can, drill another hole for the sound RCA jack, drill a clean hole in the back of the CoCo 2 case right where the "channel select" hole was, and it should be a nice clean little mod.

As an FYI, there also exists a VGA mod that can be purchased for the CoCo 2, which uses a FPGA and plugs right into the socket where the video display generator chip (the MC6847) sits. This would probably be the cleanest signal you can get out of a CoCo 2.

I may decide to build my own VGA or HDMI board, though if I do, I may use the outputs of the original MC6847 rather than "co-opting" it like the the CoCoVGA does. I've wanted to learn how to program FPGAs, and I found this little reference to an approach for how to coax a digital signal out of the analog outputs of the MC6847. Using that approach, I think it would be possible to use the AM26LS32 to feed a FPGA (probably along with a few other signals from the MC6847 to be able to detect sync signals) to paint an in-memory video buffer, and then figure out some way to push that buffer out an HDMI (TMDS) port.