I just got this in today, and haven't even turned it on.
It came with a composite mod board, and another mystery to me board.
The board was mounted under the shield with the ram, and has a 74ls12 and 74ls393 on it.
It has Vcc,GND, and 3 other signals running to the board labeled HS, VCK, HSP.
HS goes to the 6847 pin 38,
VCK goes to 6847 pin 33,
HSP goes to the 6883 pin 9, which is also removed from the socket.
CLK is the clock signal
HSP is output to the 6883 HS pin.
HS is input from the HS pin on the VDG and has the following function on the 6883.
"Horizontal Synchronization. Detects the falling edge of the VDG "HS" pulse in order to initiate eight dynamic RAM refresh cycles.
The secondary function is to reset up to 4 least significant bits of the internal video address counter."
So, it's intercepting the HS signal from the VDG and changing it somehow. It looks likes it's been upgrade to 64k. (Caps clipped)
The ram chips are Tandy branded 8040665 chips.
So... I'm going to wager that what that is is a hack to get the SAM chip to output the correct refresh cycle for a 64K upgrade without having to upgrade the original Color BASIC 1.0 ROM. Here's a kind of confusing article that talks about the initialization issue, and takes a different path to solving it. I have vague memories of circuits like this being sold out of ads in Rainbow Magazine in kits for upgrading original "Rev D." CoCos, I assume that's what you have here.
