Someone who installed one of my composite boards (Type B) was asking me about a problem he was having with a vertical yellow bar on the left side of the screen in the hires (artifact) mode of the CoCo 2 he was using.
This is a very common problem with Color Computer 2 machines. I have fixed quite a few of them at this point. The most likely culprits are the 555 timer itself, or one of the other components in the artifact circuit (schematic shown below).
In CoCo2 machines, a 555 timer IC is used to generate a pulse which pulls øA (coming from the MC6847) a few hundred mv lower, using Q2. When phase A is pulled lower like this during the period between HSYNC and active video, it causes the MC1372 to generate a color burst signal.
When a display decoding the video sees a color burst in this portion of the signal, it will display artifacts as intended (by Tandy) in the B&W hires mode. The MC6847 in normal operation uses the same method of generating color burst in the MC1372 while in alpha and semi graphics mode by internally pulling phase B low.
Since the hires graphics mode was intended to be black and white when Motorola designed the chipset, color burst in this mode is not supported. Tandy chose to use phase A to generate the burst, because using phase B would result in green and magenta artifacts rather than red and blue.
Here is the circuit found on all Color Computer 2s that does this.
The yellow bar is caused when the duration of the output pulse from the 555 timer is held too long, extending into the horizontal scan line information (active video).
The above image is an oscilloscope shot of the start of a line of video (yellow), and the output from pin 3 of the 555 timer on the CoCo motherboard (blue). This is the pulse that generates the color burst. C26 on the motherboard can be adjusted to control the duration of the pulse.
As you can see, the pulse starts at the beginning of the HSYNC pulse, and in this case extends slightly into the active video region. I pulled C26 and it tested at 930pF, this was the cap in place for the image at the top of the page. Schematics call for 1,000pF at C26. Over time components can “drift” in value or degrade. A possible simple fix is to adjust C26 to change the pulse duration.
Here is a shot with C26 changed to a tested 1,012pF. As you can see, the pulse now extends further into the active video.
I corrected the problem by lowering the value of C26. You can see the results with 667pF (measured) @ C26. The pulse now times out before the start of active video. As you can see from the screenshot below, the yellow bar is gone!
If you have this problem, try replacing C26 with a 680 or 820pF ceramic cap, it may fix the problem.
If the simple fix of replacing C26 with a slightly lower value doesn’t correct the problem, you’ll have to resort to replacing other components in the “color burst” circuit. Likely candidates for replacement are the 555 timer IC itself, the 3904 transistor (Q2), or the diode D13.
When replacing D13, use a germanium switching diode, not silicon.