Space Invaders (Taito 3-layer boardset) - weird boot fault

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Space Invaders (Taito 3-layer boardset) - weird boot fault

I've repaired many Space Invaders boardsets over the years but this one has me stumped.

It's an original Taito 3-layer boardset.

As I have other known working boardsets I've been able to swap out the boards and know for certain that that it's the CPU board at fault.

On power up the display is partly drawn (top line displaying the scores, bottom line showing the credit text) then it constantly refreshes and redraws the same screen. It doesn't matter which game I try - Space Invaders, Super Earth Invasion, Space Invaders Part II, etc. All display the same fault.

I've tried my Fluke on it and the RAM checks out as does the reading of the ROMs on the ROM board.

I also tried Scott Brasington's multigame and that doesn't report any errors - I cannot of course play games from the multigame menu as that just causes another reset.

The CPU is fine.

I've 'scoped out all of the chips on the board but can find no faults. No stuck inputs, outputs or dodgy outputs. I've even compared all of the signals against a known working CPU board.

There's no shorts or track breaks.

Voltages are fine.

I've even tried piggy-backing known good logic chips on the ones on the board - a technique that is far from perfect but worth a try. No luck there either.

So I'm pretty foxed by this. Seems like it's some kind of interrupt fault but cannot see any issues along those lines.

Any ideas please?

Thanks
 
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Does that Multigame work on the Taito 3 stack board sets? It mentions in the docs that it works on the normal two L shaped board set but nothing about the Taito boards. If it does work on the Taito boards that would be a great upgrade for my machine.

Robert
 
I would disable the reset while the screen was redrawing to see if the watchdog is giving problems to try to figure out why the game had problems
 
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