In the Omega Home Computer, pin 14 of the keyboard connector is the kana led control signal coming from pin 6 (IOB7) of the YM2149.
But in the JFF, pin 14 of the keyboard connector has been left unconnected as well as pin 6 of the YM2149.
The MSX Technical Data Book mentions the "Kana Lamp" (only for Japanese computers) that can be turned on by using IOB7 (bit 7 of PSG port B). It must be actually driven low for that.
The Omega keyboard has pin 14 of the keyboard connector on the keyboard side connected as an input to a 74LS07 buffer that drives the kana led through a 470 Ohm resistor.
When using a JFF computer with an Omega keyboard it has been observed that the kana led flashes from time to time when some combination of keys are pressed simultaneously (like down / right arrow keys for example).
It seems that the floating input to the buffer causes sometimes the buffer input signal to change state unexpectedly, which causes the led to light up when the resulting buffered signal is LOW.
On page 2 of Fairchild Semiconductor Application Note 363 "Designing with TTL" [1] there is a section titled "Tying all unused inputs to a solid logic level" where it explains that TTL inputs usually float to HIGH, but are susceptible to induced noise that can end up changing the state of floating input.
The recommendation there is to tie the unused inputs to VCC through a resistor.
In the case of the JFF, one possible solution could be to connect pin 14 of the keyboard connector back to pin 6 of the YM2149 (and enable driving the Kana Lamp as if it was a Japanese computer). Another solution could be to connect pin 14 of the keyboard connector to VCC via a 1K resistor and leave the Kana led always off.
[1] https://www.onsemi.cn/download/application-notes/pdf/an-363.pdf
In the Omega Home Computer, pin 14 of the keyboard connector is the kana led control signal coming from pin 6 (IOB7) of the YM2149.
But in the JFF, pin 14 of the keyboard connector has been left unconnected as well as pin 6 of the YM2149.
The MSX Technical Data Book mentions the "Kana Lamp" (only for Japanese computers) that can be turned on by using IOB7 (bit 7 of PSG port B). It must be actually driven low for that.
The Omega keyboard has pin 14 of the keyboard connector on the keyboard side connected as an input to a 74LS07 buffer that drives the kana led through a 470 Ohm resistor.
When using a JFF computer with an Omega keyboard it has been observed that the kana led flashes from time to time when some combination of keys are pressed simultaneously (like down / right arrow keys for example).
It seems that the floating input to the buffer causes sometimes the buffer input signal to change state unexpectedly, which causes the led to light up when the resulting buffered signal is LOW.
On page 2 of Fairchild Semiconductor Application Note 363 "Designing with TTL" [1] there is a section titled "Tying all unused inputs to a solid logic level" where it explains that TTL inputs usually float to HIGH, but are susceptible to induced noise that can end up changing the state of floating input.
The recommendation there is to tie the unused inputs to VCC through a resistor.
In the case of the JFF, one possible solution could be to connect pin 14 of the keyboard connector back to pin 6 of the YM2149 (and enable driving the Kana Lamp as if it was a Japanese computer). Another solution could be to connect pin 14 of the keyboard connector to VCC via a 1K resistor and leave the Kana led always off.
[1] https://www.onsemi.cn/download/application-notes/pdf/an-363.pdf