[Coco] [Color Computer] RF to TV connection

[Arthur Flexser]

Wasn't it harder to get a decent picture with a CoCo 2?  This article may
have predated the CoCo 2.

Art

On Sat, Apr 13, 2013 at 6:17 PM, Neil Morrison <neilsmorr at gmail.com> wrote:

>       HARDWARE
>
>       BY MARTIN H. GOODMAN
>       COLOR-MONITOR DRIVER
>
>
>       At last you have at your disposal an extraordinarily simple circuit
> that does an excellent job of driving your color-composite monitor. This
> circuit represents a significant improvement over previously introduced
> versions for two reasons.
>       First, it uses only the +5 volt line to power it, unlike several
> others that require +12 volts. This makes it considerably safer in the
> event of a short or goof in your construction, thus reducing the danger to
> the expensive chips on your board. Second, it is much simpler to build
> because it requires only four components.
>       As an added advantage, it does not interfere with your existing RF
> output, as some of the earlier circuits do. You can also combine it with my
> monochrome monitor driver circuit (see page 36, HOT Coco, July 1983) and
> interface it to your CoCo in a plug-in fashion.
>       Theory
>       Your Color Computer produces a color-composite video signal (the
> kind required to drive color monitors) by mixing the outputs of the VDG
> chip with a video-mixer integrated circuit (IC), MC1372. The output of this
> chip (pin 12) then feeds into your ASTEC RF modulator box. This box does
> several things. It acts as a miniature television station to produce a
> signal that an ordinary TV set can receive. This allows CoCo owners to use
> their TV sets as monitors.
>
>      Lower power requirements and ease of construction
>       improve this circuit
>       board's performance.
>
>       Unfortunately, the box adds significant noise to the signal. For a
> crisper image that is free of the Moire patterns of RF interference that
> often plague the Color Computer, it is necessary to use a color-composite
> monitor.
>       Color-composite monitors currently cost between $250 and $350.
> Manufacturers are also introducing TV sets that have a built-in option
> allowing you to use them as color monitors. This option actually costs the
> manufacturer very little. RCA and Panasonic have sets that include this
> option in the $300 to $400 price range.
>       My circuit taps into the output of the video-mixer IC and runs it
> into a dual emitter follower buffer. The buffer reduces the DC bias on the
> signal as well. The circuit, which appears in Fig. 1, is simple to
> construct.
>       Construction Hints
>       You can lay out this circuit on a scrap of perfboard or add it to
> the board on which you built the monochrome monitor driver. To supply it
> properly, you can get the required +5 volts, ground, and color-composite
> input from pins 11, 5, and 12 of the video-mixer chip. This chip is MC1372,
> a 14-pin DIP chip located near the 6847 chip.  The chip is called U12 on D
> and E-board CoCos and U6 on new-board CoCos and TDP 100s. On new-board
> computers, you can spot this chip because it has a 56K-ohm resistor sitting
> on top of it soldered to pins 2 and 12. On older boards (D and E), it is
> located under the RF shield.
>       You can mount the board on top of the ASTEC RF box and run three
> wires from it to the MC1372 chip. You can then solder them directly to the
> chip or attach them via an adapter plug composed of a socket soldered to a
> header. In the latter case, you can configure your project so that it
> becomes a plug-in installation. You can run the output through a short
> piece of coax to an RCA phono socket. You can then snake this out through
> existing holes in the back of the case.
>       Notes
>       As with the monochrome monitor driver, you must supply sound using a
> separate circuit. Refer to my article on the monochrome monitor driver for
> hints on how to do this.
>       I must also warn readers that opening your CoCo's case will void any
> existing warranty. Also, Radio Shack reserves the right to refuse to repair
> any board that has been soldered.
>
>       Address correspondence to Marty Goodman, 1529 Addison St., Berkeley,
> CA 94703.
>
>
>
>
> --
> Coco mailing list
> Coco at maltedmedia.com
> http://five.pairlist.net/mailman/listinfo/coco
>
>