Centipede Upright Player 2 Trackball Issue - Repaired

davefinn

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Centipede Upright Player 2 Trackball Issue - Repaired

Had a strange issue with an upright (not a cocktail) Centipede when it was player 2 turn. The archer would go all the way up on its own at the start of the turn, could not be moved down but would go left and right. Player's 1 trackball control was normal.

Looking at the schematics, there are provisions for joystick inputs for P1 & P2 as well as the normal trackball inputs. Using a Fluke and 6502 pod, I read memory location 0C03 (memory location for P1 & P2 joystick inputs) and sure enough it came back with FE (instead of FF) showing that P2 Up bit was active (with no controls attached).

Found C49 was almost a dead short which caused the game to think a P2 joystick was being held constantly in the up direction. Replaced C49 and now player 2's trackball controls are working as they should.

Just wanted to document and share...
 
Yes, those little caps on the control inputs often go bad. They become partially resistive, which causes the control signals to appear stuck on. They act like a resistor, and end up pulling the signal down, looking like the control has been activated. It's a common problem on Atari boards.
 
Did C49 look physically damaged? :confused:
Aren't these 50V parts? I wonder if just age made it "short"?


I've seen them not physically damaged. They just fail, and if you measure them with a DMM, they measure a few Kohms or so, which is enough to pull the signal far enough to make it not look high or low (as the associated pullup resistors are 10K).

They are 50V parts, so it is a bit of a mystery why they fail. They're always the glass-encased type, FWIW, but I can't say what causes the failure, as it does seem strange. Maybe they're just old, or maybe it's static damage, who knows.
 
Well, the cap is there to help protect from ESD (and filter noise etc). Still, 25V cap is not the best choice.

TRIVIA: http://incompliancemag.com/article/...itors-for-electrostatic-discharge-protection/

I wonder if the CP is well grounded? Did Atari specify another ground wire at the trakball housing? LOL - now I'm paranoid and will have to check my upright when I get home tonight. :eek:

LOL There is a dedicated ground wire in the CP harness specifically for the control panel itself... ;)
 
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