Jan 132016
 

Had on the bench this Marchen Maze PCB (Namco System 1 hardware) :

Marchen_Maze_PCB

On the power up I was greeted by this screen:

D6_error

The RAM @D6 is a 6264 located on CPU board (fault was surely on this board since I succesfully swapped a good one) used in pair with another one @E6 , both are adressed by the near custom ’48’ which is a sprite address generator:

custom48_D6_E6_RAM

For first I went to replace both RAMs (sometimes the board booted showing an error also on the one @E6) but this didn’t cure the problem as well swapping a good known custom ’48’ had no effect.Probing these RAMs revealed some address lines stuck high.Looking at the schematics of Pac-Mania (which runs on same hardware) I figured out that adress lines from custom to RAMs are driven by two 74LS365 @H6 and @L6.

 

object_control_circuitry

Parts mounted on the PCB were from Fujitsu which means high chance of failure:

74LS365@H6_@L6

After a quick test with my HP10529A logic comparator (which confirmed me troubles on all outputs) I decided to remove them.They both failed in my programmer:

74LS365@H6_@L6_failed

This cleared the error on startup so board successfully booted into game but sprites were all blocky:

blocky_sprites

Judging from the kind of fault on screen, this, more than an addressing issue, had to have something to do with data.Always looking at schematics I noticed that data bits  from the custom ’48’ and RAMs are routed to two 74LS377 @A3 and @A4 :

object_datal_BUS

and from these to the custom ’39’ sprites generator:

object_datal_BUS_2

74LS377@A3_@A4

Piggybacking the two 74LS377 I could partially restore sprites :

sprites_restored

This lead me to remove and replace both.The desoldered ones failed when tested out of circuit :

74LS377@A3_A4_failed

74LS377@A3_@A4_reworking

Sprites back again in all its glory and board 100% fixed!

sprites_100%

 Posted by at 7:21 pm