Defender rug pattern fix

crazygamer

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Just fixed 3 Defender mpu's all with the same problem. The problem was the 2 cycle rug pattern in repeat mode and the fix was 3G(153) @ pin7 on all 3 mpu's.

Spread the word, the fix is out. ha,ha..aaa..


crazygamer
 
Somebody got lucky. Just wait for the 4th board......

J/K. Congrats. The 74153 chips are one of the more common reasons for the addressing errors on Williams boards. Unfortunately, they are not the only reason.

ken
 
Real lucky indeed but I'm not out of the woods yet. One board has a vertical color ram error and another is stuck in audits.
 
The vertical issues can be tested by taking pin 23 of Decoder 4 (3K) high (+5V). This will force the cocktail player 2 mode. The screen should invert. If it is the 74166 chips the screen will clear up. These are the 8 chips from 1O to 4O and 1P to 4P. The O bank controls Player 1 mode (normal) and the P bank controls the inverted (Player 2 mode). There is a way to tell which one it is by where the lines are in the screen, but I usually just start at 1O and work my way down the line, unless I can trap the bad one with the logic probe or logic comparator.

ken
 
Last edited:
Just fixed 3 Defender mpu's all with the same problem. The problem was the 2 cycle rug pattern in repeat mode and the fix was 3G(153) @ pin7 on all 3 mpu's.
When my Defender was doing rug resets, I replaced the same chip for the same reason - pin 7 was stuck low. I can understand these muxes being a common point of failure since they're constantly switching; but it's curious that the same chip and channel has failed in all our cases.

On a side note, with a decode issue like this the RAM portion of the rug test obviously fails. But in order to call the drawing routines to display the failure message, it starts using RAM for the CPU stack - bad idea! Once it calls the routine to clear the screen (due to the palette settings it's blue), the stack gets corrupted, the CPU goes off into the weeds upon returning, and the watchdog reset asserts.

LeChuck
 
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