HP-65 1603S03504 - display not working at all

[albertofenini]

Hello everyone
From this unit we had recovered the logic board to fix another unit
but the led board has never worked
Basically no sign of life on the leds
We have used a working logic board to check the basics :
Vgg, Vbsw, Vss and Vcc are all there and correct
The coils are all closed
On the anode driver Vbsw is present and correct
On the cathode driver Vbsw and Vcc are present and correct
We have then checked the following waves both at the logic board and on the anode or cathode driver where needed
Disp0 : flat at 0 volt
Disp1 : flat at 6 volts
Disp2 : flat at 0 volts
Disp3 : flat at 0 volts
Disp4 : flat at 6 volts
RCD : flat at 0 volts
Step is equal at anode and at cathode and is flat at 6 volts
On the anode driver all the pins for the segments are flat at Vbsw

What could it possibly be ? And what could be possibly check ?
Pictures, not much to be seen can be found here : HP-65 1603S03504 display not working at all
Thanks for any help or suggestions you may want to share !