Nintendo Mario Bros arcade pcb repair #1

Board appeared to play blind – sounds were produced, but no video.  Logic probe quickly revealed there was no video sync signal coming from the video board.

The sync signal is mostly produced by a couple of LS123 chips – there was no output from them – but also no input to them either.  I followed the path upstream until getting to the clock circuit, so much like the Popeye repair the video clock circuit didn’t seem to be active.

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In this case the crystal oscillator and LS04 tested fine – the problem chip was the upstream S74 which had failed.  When replaced the board was 100%.

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