I bought this board off of e-bay as broken. It was a mess for sure. Ken and I got it going tonight, but neither of us knows anything about the 2 blue and pink (?) diodes. We don't know what kind they are and they are not on the schematic. The traces don't match up with other boards and I would have had to remove the data chip to trace them out. They are in front of the data chip and to the right of that little green capacitor, but you can easily see that in the pics.
You can see in the pics that I've replaced C7, but up by the light pen, there are a couple of large capacitors. I'm going to send pics of those too if you want them. Ken thinks this may be one of the very last boards that was sold.
Here are the eight pictures that he included in his email. After the pictures are some comments that Allen got from Michael.
Allen followed up his original email to me with this telling me that he did get a reply from Michael about that board.
I looked at your photos. These are my guesses. Let me know if you figure out their purpose, a very curious, possibly rare motherboard variation.
Looks like the common ends (anodes) are wired to pin 11 of the custom data chip, which is grounded.
Looks like the cathode ends are wired to pins 9 and 10, related to the 7M clocks. You can confirm this with an ohmmeter. Could these be zener diodes used to shape those two clocks??
One of those black caps near the light pen connector is wired to pull up resistor R19 at +5v. I suspect these two caps, looking at their physical size, may be there to help maintain that end of the board at +5v?
Note: Bally schematic ERROR. Cap C48 is wired from +5V to gnd, NOT +5v to +10v.
Does anyone know anything about these modifications?