The Board

This WPC-89 MPU came out of a Judge Dredd. The game had stopped booting entirely and was showing intermittent direct switch issues before it gave up — two classic symptoms that point straight to MPU corrosion damage.

Diagnosis

On WPC-89 MPUs, battery corrosion is one of the most common failure points. The onboard batteries leak if not replaced often, and the corrosion spreads across the board under the batteries, taking out the switch matrix components and traces underneath. In this case the corrosion had only reached U16, one of the LM339 comparator ICs responsible for the direct switch inputs, which explained the switch issues prior to the full boot failure.

The RAM was also suspect and usually causes a no boot or lockup after booting. Only after restoring factory settings would the MPU boot for a few seconds, show “FACTORY SETTINGS RESTORED”, then presumably freeze when the MPU attempted to write to the faulty SRAM. Testing confirmed U8, the 62256 SRAM that stores game settings and audits, had failed. WPC games can use either 62256 or 6264 interchangeably, so rather than replacing it like-for-like, a 6264 was substituted. All you need to do is swap jumper settings to do this modification (W3 out, R93 in 1.5k ohm 1/4W)

Repair

  • Cleaned and abated battery corrosion from the topside of the board.
  • Replaced U16 (LM339) damaged by corrosion. Ran various jumper wires to fix damaged traces and pads from the corrosion.
  • Replaced U8 (62256 → 6264 SRAM), and changed jumper settings so the board can use 6264 correctly.

Result

After the repair, the board booted cleanly and all direct switches tested correctly. I am beginning to see more and more SRAM failure on WPC games, especially 62256s.

Battery corrosion is by far the most common issue on WPC boards. Don’t let your batteries get to the point of leaking, check them every few months and replace at least them every few years!