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Williams Pinbot - Missing Characters and Segments on Displays


SNESNESCUBE64

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Seems like I am doing a lot of display repairs as of late. This time it was for Pinbot by Williams. In this instance, it was missing the middle character for player 2 and player 4. This one is fairly easy to handle looking at the schematics. The other issue is that the credit digits are missing segments 'B' and 'D'.

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The first thing to tackle is the missing segments. each display unit uses it's own driver IC for turning on the digits. So it would be just finding out what is in common. Looking at the schematics, I can see that the middle digit is STB13 and STB13'. 
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With this identified, I can see that they each converge to a single NOR gate at U4. Odds are U4 is my problem with that. They used a CD4001. I've had the worst luck with late 70s and early 80s 4000 series CMOS ICs, so I didn't doubt that it was bad.
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After replacing the IC, I found that the two characters were back. So onto the next issue, the missing segments on the credit section. As previously mentioned, I was specifically missing segments 'B' and 'D'. The first thing to check in this circuit are the resistors that go in-between the driver and the display. In this case, they are 10k 1/2W resistors R21 and R23.
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Testing them in circuit, I found that they were open, meaning that they would not pass any current through. After replacing those two resistors, I found that all displays were working as they should.

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