System 6 display issue

craigmack

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Hi guys,

Got an interesting issue on this Williams system 6 display I am working on. Overall the display works like it should. All segments light when they should, they are bright etc.

The issue is when a 0 is displayed, there is a slight bleed on the master displays digits and player 1 and player 2's digits in the g segment. So when the 0 is on it looks slightly like an 8. Is this resistor opening up or something else? When the game is supposed to show an 8 it works perfectly so I am pretty sure its not an IC. If it was the resistor which should I change?

Thanks!
 
Are you seeing it *only* on that display, if you have others connected, or does it do that across all of them? Also, does it do it regardless of which location/cable it's connected to?

You want to isolate the issue to that particular display, that particular position/cable, etc...

It's not uncommon to have a bad/flaky display cause issues with the other displays, and that may be the case here, especially if other displays show zero issues (regardless of where they're connected), and any issues only show up when this particular display is connected..

Assuming it's just that display - make sure you don't have any cold solder joints on the display connector, make sure there's no short between any of the pins, and otherwise check the pins going into the glass as well for any poor solder connections/shorts.

Beyond that, there's not much you can do - probably just a display starting to fail. Check the resistors on the master display board; if any of them start heating up significantly when this display is connected, don't use it.
 
I see the issue on the master display, player one and player 2.

I have tested all of the resistors and they were all between 9.7 and 10.2Kohms, so within spec.

I am thinking its the UDN chip, which are hard to find and expensive...

I am going to try a different master display tonight and see what happens.
 
Had a similar issue with a sys7 game. More extra segments on the same displays.

A UDN7180 and some 4049s that I don't remember changing, and another display driver board, changed the symptom not a bit.

Found a clue on Clay's sys9-11 page..

Replaced the 74154 and a 6821 on the mpu and no further display issues.
 

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Interesting, I am not seeing that section on Clay's guide but I will look again...

If it is the same on this other Master display then I will see how those chips relate on the System 6 MPU (hopefully the same) and swap them out...
 
The UDN's are easy to check (diode test), and I've found the test to be pretty reliable. If the issue is happening with *only* the master display board connected (nothing connected to players 1 through 4), then you're on the right track.

If the issues with the master display board display and any others only comes up when this "problem" display is connected, regardless of which location it's connected to, it's the source of your issues.
 
Interesting, I am not seeing that section on Clay's guide but I will look again...

It wasn't really explained in the text, rather just dropped in near the end of part 3m. http://www.pinrepair.com/sys11/index3.htm

"Here's the credit display on a F-14. Note the left most
character is scrambled. This was not a master display board
problem, but was rather a CPU board PIA and a 74154 chip
problem (they were shorted internally)."
s11f14dp.jpg


Had to go back and study the sys3-7 page some more...

Lots of info on testing UDN chips...
"Important Note: Testing transistors (or chips) using the methods below does not give 100% proof that the component is good or bad! It's probably about 95% accurate, but it is not 100% accurate. It does show with 100% accuracy if there is a short in the chip."

Seems that most display driver issues were with segments missing or extra segments fully lit. "Locked on segments can also be a problem with a UDN7180A chip (shorted internally)."

Found one reference to the 6821 PIA chip on the CPU board at IC18, and a couple examples of issues with IC6(74154)..... "Remember a bad 74154 chip will effect digits on both players 1 & 3, or players 2 & 4."
 
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