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Scrambled video, ROM issues
Repair of: Dig Dug(Namco (Atari license) 1982)
Repair Type: PCB
Status: Fixed
Contributed by: ieure
Symptom:
Powers up to scrambled video and watchdogs.
   
Solution:
Game appears to be booting, but video output is non-functional. Probing /CSYNC shows it stuck low. Tracing sync back from the test lug, found a broken leg on R86. Fixed, but still no video.
 
/CSYNC originates at LS368 @4E, with /CMPSYNC output on pin 3 and input coming from pin 2. Continuity from the sync pin to 4E's output was good, so started probing with a scope. Signal on /VSYNC and /HSYNC test lugs looked good, but output on 4E was not correct — H sync was present, but there was no V sync added.
 
Per the schematic, input to 4E comes from a LS00 @6H, LS74 @6C, and LS86 @4A. 4A is not populated on this PCB, and 6C's outputs connect directly to 4E's inputs. 4A is driven by /VSYNC and the 4V line from the 07xx custom @4C. Both these inputs appear good, but output lacks V sync, so the LS74 is suspect. Comparing in-circuit against known good part with a logic comparitor confirms a fault. Socketed and replaced the part, and video is now present.
 
While there is video, the board is watchdogging. It gets through most of the power-on tests, prints "RAM OK," and resets, which is a good indication that there is a ROM problem. Put it on the Fluke and ran ROM tests, which returned the correct signatures, but at the wrong locations. Visual comparison against another board showed that the -205 and -206 ROMs had been installed in the wrong sockets. Swapped them into the correct locations, and the board boots and plays.

Parts Replaced

 
The following parts were replaced:
Part Name 
Part Location 
Original Part Number 
Price Paid 
Substitution Parts 
Reason For Replacement 
 
ExpandFlip-flop4E74LS74$0.00 Defective
    Sum=$0.00  
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