Repair Log


When this PCB was first fired up, it had a graphic glitch that seemed to affect the playfield only. The score area at the top of the screen, as well as the "extra men" area below, were fine. The glitch had two "states". The first, featured a blue "mask" covering the playfield. Every other column was blue. Items normally green, became red/orange stripes. The glitch would "disolve" into the second state, where green was restored, but the blue and red sprites would become affected. "DigDug" had red pinstripes and "Pooka" had blue pinstripes.





After posting the problem on several newsgroups and message boards, a fellow RGVAC'er suggested either a shift register or a multiplexor was at fault. Since they are relatively cheap (thanks Mouser!), I decided to take the shotgun approach and replace them all, starting with the four 74LS157 chips (no luck).

May 5, 2001 - Next, were three 74LS298 chips, two of which appear in the Playfield generator circuit, and one 74LS257 (still no luck). Back to the drawing board...

As seen in this picture:



There was a after-market repair made, using a piece of yellow wire running from C65 to C42, out of curiosity, I removed one end and checked continuity. It checked out fine, so I removed the wire since it didn't seem to do anything.

There was also a small ceramic capacitor joining C43 to pin 13 of the 74LS393 nearby. The schematics I had made no mention of this so, again out of curiosity, I removed one end - no change, so off it came.

May 7, 2001 - Six 2114 and two 2148 RAM chips are on there way from who else, Bob Roberts...let's hope this does the trick.

May 10, 2001 - I placed one of the 2148 RAMs on top of the one located at position 7A ("piggybacking" it, being careful to line the pins up) - not only did the glitch NOT go away, the board now exhibited a weird "ghosting" effect where some of the graphics appeared twice. I then piggybacked the replacement on the chip at location 9B and the problem went away. Off came the chip at 9B, and in went a socket and the new ram - SUCCESS!! DigDug now looks like it should!



Above, the bad ram (top - missing it's legs as a result of it's removal from the board) and the good replacement (bottom).






Click Here to return to MyArcade.