So I have a circuit using the 4510 and 4511 chips as well as 3 seven segment displays on a PCB (designed and created). What it is meant to do is count up to 9.99 in 1's, 10's and 100's by cascading and manual input. It can also count up and down as well as being able to reset. I have used Push-to-make switches, push-to-break switches and a rocker switch (display ON/OFF). When powering the circuit everything works as expected - when I count 2 out of the 3 seven segment displays can show up to a 9. However, the 10's seven segment display only counts to 8 and then goes straight to 0, thus cancelling the cascading effect onto the 100's seven segment. It displays all digits on the seven segment display except 9! However this only happens when counting up as when counting down it seems to be displaying a 9 on the display.
I have no idea how to troubleshoot this problem - I have replaced the chip and have also checked the continuity of the track and the same thing happens. Any ideas on how to solve this problem (I know I haven't explained this well)?
All help appreciated.