Oct 192015
 

I got this Ghox PCB from my friend ‘robotype’ for a repair:

Ghox_PCB

On the power up I was greeted by this static screen:

boot

Address/data bus of 68000 main CPU was inactive.As usual I went to loot at MAME source (that is a bit like a bible for us repairers) and I found this interesting comment:

// Ghox 68K reads data from MCU shared RAM and writes it to main RAM.

I could identify the HD647180 MCU RAM in a 6116 @U14 :

RSCN0566

So I went to probe it with my analog oscilloscope and found weak signal on some address lines (good one on left picture, bad on right) :

address_lines_comparison

I piggbacked this RAM and board booted showing this error message:

palette_RAM_error

After replaced the MCU RAM (which actually failed the out-of-circuit test), I traced the palette RAMs in two 6116 chips @U41 and U42 :

palette_RAMs

One of them had already been replaced so I went to piggyback the other one @U41 and board successfully booted:

fixed

Obviously the chip failed miserably the test on my programmer:

6116@U41_failed

End of job.

 Posted by at 11:21 pm