I have built my Gigatron from open-source Gerbers and schematics.
The first problem which I faced was fault in the gamepad subsystem. It was not the obvious problem,
but I have resolved it.
Symptoms: gamepad did not work, I could only see some random cursor movements right after starting the computer,
but it did not react to the buttons as expected.
Efforts: many hours spent with multimeter and multi-channel oscilloscope.
Resolution: the problem was with 74HC595 chip. These chips are manufactured by many different vendors.
The first vendor which I used was Siemens, the second was HGSEMI. They both did not work.
Finally, I took NES Semiconductor's 74HCT595 (Note: HCT, not HC) and now all works fine, including PS/2 adapter with
all keyboards which I have, even USB ones.
Explanation: Some 595 chips have 10 MHz resolution, some have 20 MHz, some have 25 MHz, it is noted in the corresponding datasheets.
For the Gigatron to work correctly with gamepad and keyboard, one should use at least 20 MHz chip.
Result: up and working Gigatron.
Last problem: Video image is shown with cut-off areas outside of VGA sync, like this:
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