Saturday, 7 August 2021

1943 : Midway Kaisen repair log

Got for repair a 1943 : Midway Kaisen PCB (the japanese release of 1943 - The Battle of Midway).The board, a two stack one, was in good condition :

But according to the owner the PCB lost its sound after he connected it backwards.I had confirm of the issue as soon as I powered the board up :


Using my audio probe I was able to figure out that the lack of sound had a digital nature.Indeed, probing the Z80 audio CPU revealed the /INT line (pin 16) was asserted all the time :

This means a maskable interrupt to the CPU was triggered by an external I/O device.This causes execution to jump to a specific interrupt vector (which is some code at a fixed location) so the CPU was not properly running.

The Z80 CPU was socketed by factory so this was a good chance to fire up my Fluke 9010A troubleshooter and perform some tests.Looking at the sound memory map in MAME source the Z80 RAM lies from 0xc000 to 0xc7ff of the address space of the CPU :

On the board the device is, indeed, a 2K x 8-bit static RAM, specifically a Toshiba TMM2015 which is well known prone to failure :



I set up the Fluke 9010A for a RAM long test which failed at initial address :



This meant the RAM chip was likely bad so I removed it :

The pulled chip failed, indeed, the out-of-circuit testing :

 I fitted a socket and a good RAM chip :

This restored the sound and fixed the board completely.Another successful repair.

No comments:

Post a Comment