Oct 132016

Got this Wonder Boy (on Sega System 1A board revision) for a repair:


On boot I was greeted by this static garbage screen:


Main Z80 CPU had /HALT line asserted and no activity on data bus.I dumped the program ROMs and they were good.So I thought this was a good chance to use my Fluke 901A troubleshooter.According to memory map in MAME source the WORK RAM  lies from 0xc000 to 0xcfff of the Z80 address space:

AM_RANGE(0xc000, 0xcfff) AM_RAM AM_SHARE("ram")

When I ran a long RAM test I got an R/W error at the beginning :


The WORK RAM is in form of two 2016 chips @IC135 and IC136 (so each one is 2K or 0x7ff in hex) :


So I performed a test from 0xc800 to 0xcff which was successfully accomplished:


This meant the chip lying from 0xc000 to 0xc7ff had to be bad (the  74LS245 between the data bus of Z80 and RAMs were good since bus is shared among the two RAMs).I hooked up the Fluke probe and looping a write of some values I could identify which RAM chip lies in which address range.The one @IC135 lies from 0xc000 to 0xc7ff  while IC136 from 0xc800  to 0xcff.As further proof, I wrote random values in some memory locations from 0xc000 to 0xc7ff  and reading them back they didn’t match the written ones meaning the memory locations were faulty.So confident I removed the RAM @IC135:


It failed miserably when tested out-of-circuit:


Fitted a socket and a good chip and board sprang to life with no issue.Another classic game preserved.



 Posted by at 12:00 am

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