Apr 082017
 

Picked up this faulty Knuckle Bash PCB on Ebay:

The graphics were totally wrong:

The board use a custom marked ‘GP9001’ @U13 which acts like a GPU generating all the parts of graphics (and video timing signals too)

Package is QFP 208 pin so it’s a common issue to find lifted pins of this kind of ICs after some time.Under a microscope it seemed OK :

But actually a lot pins were detached from pad when I probed them with a needle.Reflowing the IC restored all graphics but colors were wrong:

The palette is generated by two 2K x 8-bit static RAMs and all the 16 bits of color data are latched by two 74HCT273:

Probing the two RAMs revealed a stuck bit (D7, pin 17) on the one @U7 (comparison with an healthy signal on the left of the below picture)

This data bit is latched by pin 13 of the 74HCT273 @U9, piggybacking it restored correct colors:

Chip failed in some gates when tested out-of-circuit:

Job done.

 Posted by at 11:35 pm