The Invaders – Zaccaria

Byadmin

Totally dead, no screen display (blank white screen).  For this I used the Fluke Z80-S2650 adapter to check out the ram and rom, and located two errors. The rom mapping was incorrect, with a large range of locations returning random values. Examining the board (since I have no schematics), all of the chip select lines meet up at IC41, a 74LS156. Replacing this put the memory map back to normal (it seems to map all of the RAM and ROM areas). Having sorted the memory map, the Fluke revealed a couple of bit errors in the video ram. Replacing the two 2102 ram chips sorted the problem.

IC positions are marked with the bit number they occupy.

Oh, and the blank white screen – the chip select for the character rom was high, when someone had replaced one of the video ram chips before, they must have damaged the track. Adding a wire link returned everything to normal.

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