BryanMcPhail.com

Professional software development, amateur BMW tinkering, old arcade game stuff

By

Konami Turtles In Time arcade pcb repair

Board was stuck in a very fast watchdog reset loop, to the point where I thought the reset circuit itself may not be working.  The watchdog reset can be disabled by shorting the pad marked JP near the edge connector.

With this done it was clear the CPU was executing just a few cycles then halting.  When in halt mode all data and address lines are meant to be released by the CPU, but it seemed D0-D3 were still active.  This could have been quite difficult to track down as the data bus connects to many components on the board, but the schematics showed the LS253 @ 19C connected to D0-D3 specifically (most other connections are D0-D7) and indeed a logic probe showed it had active outputs when it should not have.

So what was happening was the CPU would reset and try to read the reset and stack vector from ROM, but ended up getting values with the low four bits corrupted.  When the LS253 was replaced, everything worked fine.

20180807_074528

20180807_074247

20180807_074237

 

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>