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Dead Gaplus

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Lurch666 View Drop Down
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    Posted: 16 Jan 2020 at 5:26pm
As part of a deal for a spares pacland to help fix mine I also got a Gaplus board which was dead.

Blank screen and not much sign of activity on the board. The main reset was working but there was no clock and very little signs of life anywhere on this board.
All I could find was some clock signals near the crystal. None of the three CPUs had a clock signal so that's why I was not getting anything displayed.

Anyway we have the schematics for Gaplus but it's for the midway version that has different circuitry for generating the signal that goes into the CUS33 (that looked like the custom that generates the various clock and video signals).
Unfortunately the part of the schematics that show which pin of the CUS33 that the signal goes into is unreadable so all I knew was that I needed to find out where the crystal signal was failing to reach the custom.
Looking at pacland I could see that the namco custom CUS27 (that is paclands signal generator) just gets fed the waveform from the crystal direct but on this board there was some circuitry in-between.
After trying to trace where everything went I just started probing the chips near the crystal looking for discrepancies.
I found a couple of possibilities but these chips had already been socketed by what I assumed was a previous fix attempt. I changed these chips anyway just in case but no joy.
One that had not been changed was a 74LS163 at 5D that had a signal on one pin (can't remember which one) but no outputs so I swapped that one out and still got nothing.

I was rechecking and when I touched my probe on a 74LS74 next to the 163 I had just changed I got a white screen but with no sync so I couldn't see if anything was happening but after a while it went black so it looked like something was happening.
I changed the voltage to the PCB a little and tried again and it booted up fine.
Sound,controls and graphics seem fine so it looks like I've been lucky.

So a dead Gaplus could be revived if you check around all the chips near the crystal-for me it was the 74LS163.
You rang......?
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