This neat diagram from Wikipedia, shows in graphical form, how 8 colours can be produced from the three primary colours (Red, Green and Blue). As the image shows, on an 8 colour RGB system, Cyan is generated by combining Blue and Green, the obvious conclusion therefore, was that there was a problem with the logic driving the Green output from the 80 Column board.
To "prove" that theory, I tried generating various display conditions using the control codes listed in the 80 Column board manual. Entering the appropriate control codes at the keyboard, I confirmed that there was definitely a problem with the colour generation, but it was difficult to diagnose where the problem lay. I could produce a Green background, so the Green output was working, but I needed to get a better idea of exactly what was working and what wasn't.
I knocked up a "rough and ready" FDX BASIC program to run through the available colours to see what was working correctly and what wasn't. The small program shown should set the background to one colour and the foreground to the next colour in the available palette, clear the screen, print the current settings and move onto the next colour when any key is pressed.	10 FOR X = 0 TO 7
	20 LET Y = X + 1
	30 PAPER X
	40 INK Y
	50 CLS
	60 PRINT "Paper colour = "; X
	70 PRINT "Ink Colour = "; Y
	80 IF INKEY$ = "" THEN GOTO 80
	90 NEXT X
A quicker way to set a particular attribute is to use the <control> or <escape> codes listed in the 80 Column card section of the FDX Technical Manual on the Manuals page.

The table above summarises the results of the quick test that I ran under FDX BASIC and shows how the foreground colour did not change from Blue and the four colours (Red, Yellow, Magenta and White) reliant on the red component to to generate the correct background colour are incorrect. The implication being that there were two faults :

• Fixed foreground colour (Blue)

• No output from the Red component when setting the background colour

The schematic diagram for the 80 Column board shows that the RGB outputs from connector J8 are on pins 7, 8 and 9 respectively. Tracing these back through the board shows that the signals are either "active high" or "active low" depending on the position of links LS, LR, LG, and LB at location 4E on the board. The "high and "low" colour signals are generated by the 74LS175 Quad Flip-Flop at board location 5E, who's inputs in turn come from the output of the 74LS157 Quad 2 input multiplexer at board location 7E. Finally, the RGB signals are conditioned by the SN7406 inverter buffer/driver at board location 3E and outputs from the inverters on this chip are fed to connector J8.

The voltages measured on the output of the SN7406 confirmed that there was a problem with the Red channel, since this chip was installed in a socket, rather than being soldered onto the PCB, replacement of this chip was an easy option. I ordered up a new 7406 but unfortunately, found that Red was still not being produced by the 80 Column board. Swapping back to the good 80 Column board, I found that the issue with the Red output was now present on this card too. Further investigation showed an intermittent fault on the cable between the output from the FDX RGB video socket and the monitor - aaargh! Unsurprisingly, fixing the faulty cable restored the Red output on both of the 80 Column boards.

This resolved the background colour problem, but the fixed colour Blue foreground fault was still present.

Foreground Colour Problem