OK it almost entirely solved and I can fully use this great instrument at last !
Remaining dead leds were due to... absence of resistor (I forgot 2 of them while changing from 1k to 10k
) and bad solder joints. I was even able to remove the jumper wire as the extra solder made the connection effective again.
There is only one thing remaining but it is far from vital :
When I power on the x0xb0x, "prev" and "next" leds are not lit. IC4 pins 3 ("prev" led) and 4 ("next" led) provide 0v.
But if I press "next" in Pattern Edit Mode then "next" led light up and stays this was (5v on IC4 pin 4).
And if I press "prev", nothing happens, and "next" led is still on. IC4 pin 3 never provides 5v when "prev" pressed.
I checked connections between IC4 pin 4, resistor and led, and between led and ground.
It tested the led which is working.
Weird, isn't it ?
Anyway, thanks a lot for your help, I would never have done it without it ! Now my x0xb0x is starting to play with my sequencer and my MB6582 and I think they get along pretty well...