CoCo2 Composite A/V modulator replacements

Not long after I did my first CoCo project (a composite A/V board) I happened to buy a CoCo2 that had an interesting difference from the usual models.  It had factory composite and sound output in place of the RF modulator! What a surprise!

20141120_190514Tandy factory original composite/sound board

After a little research, I discovered this was a variant that had been intended for educational sales, where a composite monitor was prefered to TV sets as a display.  I was very interested in this as I had just been working on something similar, but this was clearly a different circuit that Tandy used.  It’s also interesting to note that this ‘educational’ model uses the T1 variant (XC80652P) of the MC6847 VDG.

I decided to reverse-engineered the circuit and design some modulator replacements of my own based on this design.  Here is an early prototype of that effort.

Modulator replacements, first prototype

The board is a direct replacement for the RF modulator, requires no modifications to the case, and uses the channel switch to select between B&W / COLOR.

Output appears to be very good. I would say considerably better than the simple reference circuit in the MC6847 data sheet, which was all I’d been able to find previously.

Further working with this circuit, I decided to develop versions for both major modulator types, the ‘vertical’ modulator found in early American produced CoCo2s, and the ‘flat’ type found in Korean models.  Here are some pictures taken during development.

After getting the design more or less finalized, I produced a number of these and sold them to people wanting something a bit better than RF output on their CoCo2s.  Well over 100 boards were distributed.

Type A’s (Korean), in production

Final versions

Identifying modulator types

This portion is to help identify which composite video board style you will need to replace the RF modulator in different CoCo 2 computers.

Type A
I’ll call the first one the “Type A” modulator.  This is found on later, Korean made CoCo 2’s.  It lays flat against the motherboard next to the channel switch.  If your modulator looks like this, you need a Type A Composite board.

Type A, Korean motherboards, ‘flat’

Type B
I’ll call the second the “Type B” modulator. This is found on earlier, US made CoCo 2’s.
It attaches to the motherboard by one of it’s edges, and has the channel switch in the RF box. If your modulator looks like this, you need a Type B Composite board.

Type B, American motherboards, ‘vertical’

Installing the board, Type A

Here you will find some basic instructions for installing the Type A composite video board.  This is how I do it, of course it’s not the only way, and all the usual disclaimers apply.

Removing the RF modulator

The first step after removing the motherboard from the computer, and removing any shielding on the bottom, is to remove the RF modulator box.

You will need a temperature controlled soldering iron for this, as it takes quite a bit of heat.  Somewhere around 350º C is appropriate, otherwise the large ground plane under the modulator sinks too much heat to get the solder for the box ground connections liquid in a timely manner, or heat the signal pins enough to melt the solder at the top of the pin where they are connected to the modulator pcb.

First the modulator pins carrying the various signals should be removed.
Special care needs to be taken not to damage the pads these pins are soldered to.  I’ve found the safest way is to heat the pin from the motherboard bottom while applying moderate pressure to the end of the pin.

When it gets hot enough, you’ll be able to push it up into the modulator box. At this point there is enough of the pin protruding on the top side so that it can be grasped with a pair of need-nosed pliers or the like, and pulled (with moderate force again) while it’s heated again from the bottom until free. Repeat for the other 7 pins.

Heat the large solder connections where the RF box is connected to the ground plane, using a solder sucking device of some sort to remove the bulk of the solder.  Use soldering wick to get what that won’t until the sheet metal tabs of the box are free.  A couple of them will need to be straightened to fit through the slots in the board where they were originally bent to hold them in place prior to soldering.

The modulator should now be easily lifted/gently pried free.

Whew…  The hard part is done!

Preparing the motherboard for installation

Use solder wick to clean up the pads for the pins, and also the rearmost slot in the ground plane where the modulator box was soldered.  This is where the ground lug on the new board will attach. You’ll want to be sure that slot is completely open (free of solder).

Next, insert the board (pins and ground lug), make sure it’s fully seated and level.  Solder one of the pins to hold it in place until you are sure you’ve got it positioned as you want it (down all the way and parallel to the motherboard).

Solder the rest of the pins, and then the ground lug.  The ground lug is soldered to the board, so you’ll want to apply enough heat long enough to solder it to the motherboard, but not so long that it melts the connection on the Composite board; not really an issue as it can’t move around at that point anyway, just something to be aware of.

All that’s left now is the power leads.  The WHITE wire connected to P2 is the AC power to the board, and will be attached to the anode of one of the large power diodes on the motherboard.  This should be ~ 8.4 VAC,  you can use a voltmeter to verify.  The RED wire connected to P1 supplies DC voltage to the board, and will be attached to the cathode of one of the large power diodes.   It should read ~ +9.9 VDC.

Type A installed.jpg

Type A, installed

Once the power leads are attached, you just need to trim the pins sticking out of the bottom of the motherboard (the ground lug is pre-cut and should not need trimming).

I will have already set the potentiometers while connected to a TV and an oscilloscope for testing, before sending the board. They can be adjusted if needed on your particular setup, but try it first. 🙂

And that’s it! Put it all back together and enjoy Composite video on your CoCo2.

Notes for installing the Type B composite board

Installing the Type B composite boards is essentially the same, except for replacing one resistor on the motherboard.  Remove the 10K resistor at R13 (shown in the pic below), and replace with a 1K resistor.  This should improve your artifact colors.  If you installed the board some time ago, before I added this note, you may want to consider swapping the resistor.  Especially if artifact colors are not quite as expected.


Location of R13, next to the MC6847 VDG

Also, watch the power connections carefully. The diodes are reversed in their orientation on the motherboard.

Type B Power Connections.png

Power connections, Type B

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