If you find the display is missing segments, then it will usually be either an MPS-A42 transistor or the 4543 CMOS IC.
This only applies if you are missing the same segment on all digits. If you are only missing one segment on one digit, then it will usually be the display glass that is faulty.
MPS-A42 transistors:
a segment (top) - Q13
b segment (top right) - Q14
c segment (bottom right) - Q15
d segment (bottom) - Q16
e segment (bottom left) - Q17
f segment (top left) - Q18
g segment (centre) - Q19
