My guess is that the clock source must be OK, otherwise the blinking function would not work or the display would blink at a "wrong" rate.
I think I would probe all pins around both the LD110 and LD111 with a voltmeter and an oscilloscope, and check for correct supply voltages, reference voltage, "illegal" logic levels, proper multiplex signals etc ... I know the 3403 insides are "densely packed" and the probing may be difficult, but it may give very useful clues.
... And of course please keep us posted !
Jo?l
