Evel Knievel
Bally, 1977Looking for some help. I have installed a new rectifier and MPU (NVRAM Weebly) on a Bally Evel Knievel. Game boots up and plays fully. The only thing is that all of the scoring displays are only showing 4, 5, 6, or 7. It is all of the scoring displays (even the credit one). Does anyone have an idea on what is wrong?
You've likely got a bad connection on one of the number select signals from the MPU board to the displays.
There are four signals that combined together form a binary coded decimal (BCD) number for the displays. The four signals on the MPU board are marked PA4 to PA7 and come out on connector J1 pin 25 to pin 28 respectively.
In the table below, the four digits in the left column are the logic BCD numbers represented by the PA4 to PA7 signals - "0" means zero volts, "1" means 5 volts.
You may notice that in order to display numbers 0, 1, 2, 3 and 8, 9 requires the PA6 signal to be a logic low "0". So it means this signal is not reaching the displays properly. Most likely it's a bad wire crimp terminal connection at MPU connector J1 pin 27 which you need to check/reterminate (yellow-black wire).
PPPP
AAAA
7654 - Display Digit
=============
0000 - 0
0001 - 1
0010 - 2
0011 - 3
0100 - 4
0101 - 5
0110 - 6
0111 - 7
1000 - 8
1001 - 9
1010 --
1011 --
1100 --
1101 --
1110 --
1111 --
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Thorndale, PA